1FA392
Examensarbete 30 hp November 2017
Neutron Irradiation of Concrete at TSL
a Comparison of Nuclide Specific Measurments with FLUKA Simulations.
Christer Åström
Teknisk- naturvetenskaplig fakultet UTH-enheten
Besöksadress:
Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0
Postadress:
Box 536 751 21 Uppsala
Telefon:
018 – 471 30 03
Telefax:
018 – 471 30 00
Hemsida:
http://www.teknat.uu.se/student
Abstract
Neutron irradiation of Concrete Core at TSL
Christer Åström
This thesis studies the possibility of using the Monte Carlo simulation program FLUKA to determine the neutron induced radioactivity of concrete walls at the The Svedberg Laboratory (TSL) in Uppsala. If a simulation of the activation would produce reliable results, it would be a useful complement to measurements for the
decommissioning and clearance of the buildings of the facility. An experiment was performed in which a concrete core was taken from one of the non-activated walls in the facility. The core was cut into samples and irradiated with a neutron beam. The samples were then measured in a gamma-ray spectroscopy setup, by which the produced radioactive nuclides were identified and their activities determined. The same setup was then simulated in FLUKA. A comparison of the simulations and the measurements shows that the average activity for all nuclides obtained with FLUKA is similar to the measured one, however with large differences for some nuclides. The average ratio of the simulated and measured activities or all nuclides is 1.07 with a standard deviation of 0.55. The obtained results may be useful for future radiological clearance work at TSL.
Handledare: Johan Nyberg och Mattias Lantz
Sammanfattning
Vid The Svedberg-laboratoriet (TSL) i Uppsala har man under ca 60 ˚ar anv¨ant en par- tikelaccelerator, Gustaf Werner cyklotronen, f¨or att accelerera bl.a. protoner med ener- gier upp till 180 MeV. Protonerna har anv¨ants vid olika k¨arnfysikexperiment samt f¨or str˚alningsterapi av cancersjuka patienter. En annan m¨ojlighet som finns p˚a TSL ¨ar att placera ett str˚alm˚al av wolfram framf¨or protonstr˚alen vilket producerar neutroner som kan fokuseras till en neutronstr˚ale. Denna str˚ale har en energif¨ordelning liknande neutronerna som tr¨affar jordytan efter att de har skapats i kollisioner mellan kosmisk str˚alning och atomk¨arnor i atmosf¨aren. Neutronstr˚alen kan anv¨andes t.ex. f¨or testning av elektronik- komponenter.
Vid anv¨andningen av en partikelaccelerator blir ¨aven byggnaderna, som till st¨orsta delen best˚ar av betong, bestr˚alade. N¨ar neutroner och protoner tr¨affar nukliderna (atomk¨arnorna) som finns i betongen kan flera olika k¨arnreaktioner ske, vilket leder till att betongen blir radioaktiv. N¨ar de radioaktiva nukliderna s¨onderfaller s¨ands olika typer av str˚alning ut fr˚an byggnadens v¨aggar, golv och tak.
I framtiden ¨ar det t¨ankt att anl¨aggningen ska avvecklas samt att lokalerna ska friklassas f¨or att anv¨andas inom andra icke radiologiska verksamheter. D˚a ¨ar det viktigt att det inte
¨ar f¨or h¨og str˚alning i lokalerna, vilket inneb¨ar att man m˚aste ta reda p˚a vilka radioaktiva nuklider som finns i lokalen samt deras aktivitet. Detta kan g¨oras genom att man tar ett stort antal borrprov och analyserar dem med hj¨alp av gammaspektroskopi, med vilken man kan m¨ata fotonernas energi och antal vid de s¨onderfall som sker genom emission av gammastr˚alning. Denna process kr¨aver dock mycket tid och arbete. F¨or att minska antalet borrprov skulle ett alternativ kunna vara att komplettera m¨atningarna med simuleringar.
Det som kr¨avs f¨or att man ska kunna g¨ora detta ¨ar att man har ett simuleringsprogram med vilket man kan ber¨akna aktiveringen tillr¨ackligt noggrant f¨or att metoden ska godk¨annas av Str˚als¨akerhetsmyndigheten som ett komplement till m¨atningarna.
Huvudfr˚agest¨allningarna i detta arbete ¨ar f¨oljande: kan man med hj¨alp av simuleringspro- grammet FLUKA best¨amma vilka radioaktiva nuklider som finns i betongen samt deras aktivitet, och ¨ar de simulerade v¨ardena tillr¨ackligt exakta f¨or att kunna anv¨andas som ett komplement till m¨atningarna?
Arbetet utf¨ordes p˚a f¨oljande s¨att. En icke aktiverad borrk¨arna av betong togs fr˚an en v¨agg i anl¨aggningen. Betongens kemiska sammans¨attning analyserades. Borrk¨arnan s˚agades upp i skivor och bestr˚alades med neutronstr˚alen vid TSL. Efter bestr˚alningen gjordes totalt 37 gammaspektroskopim¨atningar av skivorna, av vilka 13 analyserades i detta arbete med avseende p˚a de radioaktiva nuklidernas aktiviteter. En simulering av neutronbestr˚alningen gjordes med FLUKA. Simuleringen genererade data som direkt kunde j¨amf¨oras med de
experimentella resultaten.
De erh˚allna resultaten visar att f¨or alla experimentellt observerade nuklider ¨ar medelv¨ardet av de simulerade och uppm¨atta aktiviterna ungef¨ar lika stora: f¨orh˚allandet mellan de si- mulerade och uppm¨atta aktiviteterna ¨ar 1.07. Standardavvikelsen av detta f¨orh˚allande ¨ar 0.55, vilket fr¨amst beror p˚a att det ¨ar stora variationer av aktivitetsf¨orh˚allandet f¨or de olika nukliderna. Huvudslutsatsen som kan dras av detta arbete ¨ar att FLUKA b¨or kunna anv¨andas som ett komplement vid friklassning av lokalerna vid TSL.
Executive summary
The particle accelerator at TSL in Uppsala will need to be decommissioned and radiolog- ically declassified in the future. To do the declassification of a facility generates a lot of work in mapping the radioactivity in the walls by drilling out concrete cores and measuring these. Depending on uncertainties as much as 20 drill samples per square meter may be needed. This is why an alternative mapping method would be advantageous. This thesis has studied one of these alternatives, simulation of induced radioactivity in concrete from neutrons with the Monte Carlo program FLUKA. The purpose was to compare the sim- ulation of neutron irradiation of a concrete core at the facility with an actual irradiation experiment with the same setup.
An non irradiated core was drilled out of a facility wall and sawed into smaller discs before being irradiated by neutrons for 66.5 hours. These disc samples were then measured with gamma-spectroscopy to determine the induced radioactivity. The same setup was then built up in FLUKA and simulated, and then the results was compared. 37 measurements were done but only 13 were analyzed due to lack of time. From these analyses ratios between FLUKA and the measured values was produced for each nuclide identified and the average ratio for these were 1.07 with a standard deviation of 0.51 %. This means that FLUKA predicts on average 7% more activity than the measurements show, but the variation is significant. The nuclide specific ratios however have significantly lower variation at 21 %.
The conclusions drawn from this study are that FLUKA give a relatively close result to reality but with a rather high variation. This may be improved by further analysis of the remaining measurements. Neither the affect of the homogeneity of the concrete nor FLUKA’s ability of predicting long lived nuclides has been explored in this study which would be needed for future use of FLUKA. A preliminary conclusion is however that simulations made in FLUKA can be used when mapping the activity of neutron irradiated concrete as a complement to measurements to lower the amount of measurements needed significantly.
Contents
1 Introduction 1
2 Physics and Methods 2
2.1 Neutron activation and radioactive decay . . . 2
2.2 Mean Life Time, Decay Constant and Half Life . . . 4
2.3 Photon Interaction with Matter . . . 4
2.4 Gamma-Ray Spectroscopy . . . 6
2.5 The Monte Carlo Method . . . 7
2.5.1 General Principles . . . 7
2.5.2 The FLUKA Monte Carlo Code . . . 8
2.6 Previous Studies . . . 8
2.6.1 Tegmyr . . . 8
2.6.2 La Torre. . . 9
3 Experiment 10 3.1 Neutron Irradiation Experiment. . . 10
3.2 Measurements of Gamma-Ray Spectra . . . 11
3.2.1 Detector and Electronics . . . 11
3.3 Analysis of Gamma-Ray Spectra . . . 12
3.3.1 Energy and Efficiency Calibrations . . . 12
3.3.2 Spectrum analysis . . . 14
3.3.3 Calculation of Activities . . . 16
3.4 Chemical Analysis of the Samples. . . 17
4 Monte Carlo Simulations with FLUKA 18 4.1 Simulation of ANITA Neutron Beam . . . 18
4.2 Simulation of Concrete Core Irradiation . . . 18
5 Data processing and Results 21 5.1 Data processing with Python Scripts . . . 21
5.2 Overview of the Results . . . 21
5.2.1 Radiological Declassification of Building . . . 24
5.2.2 High Energy Gamma rays . . . 24
5.3 Results from Samples . . . 26
5.3.1 Sample 000 . . . 26
5.3.2 Sample 013 . . . 27
5.3.3 Sample 024 . . . 27
5.3.4 Sample 277 . . . 29
5.3.5 Sample 685 . . . 29
5.4 Profile . . . 29
6 Discussion 32
7 Conclusion and Further Studies 34
References 35
Appendices 37
A Measurement Plan . . . 37 B Input for Flair/FLUKA . . . 39 C Result from all Measurements Analyzed . . . 45
1 Introduction
The Svedberg Laboratory (TSL) in Uppsala has a particle accelerator, the Gustaf Werner cyclotron, which has been used to produce a number of different types of particle beams, for example protons with energies up to 180 MeV and secondary neutrons. Many different experiments have been carried out since the accelerator started operating in the beginning of the 1950s [1]. This has resulted in activation of many of the premises of the facility, foremost by secondary neutrons created in the nuclear reactions induced by the primary beam when it hits the material of the cyclotron and the equipment along the beam lines.
In the future, when the facility is to be decommissioned, the premises must be radiologically cleared, i.e. determined free from harmful levels of radioactivity. To do so, the demands from the Swedish Radiation Safety Authority (SSM) must be fulfilled. If it was possible to simulate the activation of the concrete walls the workload could possibly be significantly lower. This, however, means that the accuracy of the simulations must be determined, which is what the work presented in this thesis aims to do. The main question posed at the start of this work was the following: is the simulation program FLUKA able to simulate neutron irradiation of concrete at a high enough accuracy that it can complement measurements and substitute them to a certain degree?
The content of this report is the following. In section2, the relevant physics and methods will be explained as well as a highlight of similar studies. In section3the practical part of the study will be covered. In this section the experimental setup will be explained as well as the analyses of the results. In section4 the FLUKA simulations will be explained. The results with comparisons of the experimental and simulated results are given in section 5, followed by a discussion of the results in section6 and conclusions in section7.
2 Physics and Methods
2.1 Neutron activation and radioactive decay
The basis of this study is that neutrons, when they interact with matter, can produce radioactive nuclei. This may lead to long term problems due to buildup of radioactivity in the premises and in the equipment of the facility. In this work, the radioactivity produced in a sample taken from a wall of concrete was studied. Concrete contains many different elements such as oxygen, aluminum, silicon, calcium and iron. Each element may consist of a number of isotopes, with different number of neutrons. For example, iron with 26 protons, has four stable isotopes with 28, 30, 31 and 32 neutrons respectively. An atomic nucleus with a specific number of protons and neutrons is called a nuclide. Each nuclide has a different probability, called cross section, to interact with the secondary neutrons through nuclear reactions. Many of the reaction products are radioactive and will undergo radioactive decay, for example β− or β+ decay.
An illustration of one type of neutron induced nuclear reaction, a so-called neutron capture reaction, followed by β decay, is shown in Fig.1.
Here, an example of a neutron capture reaction on27Al, with 13 protons and 14 neutrons, is given. When a neutron is captured by 27Al 28Al, with 13 protons and 15 neutrons, is produced in a highly excited state. This state decays in a very short time (typically less than 10−12 s) by emission of prompt γ rays until it reaches the ground state of 28Al. The ground state of 28Al is not stable, but will undergo β− decay, to an excited state in the daughter nucleus 28Si. This excited state decays by emission of a γ ray with a specific energy to reach the stable ground state of28Si. The energy of the γ ray emitted after the β− decay can be measured by γ-ray spectroscopy. The whole reaction and decay process can be written in the following way:
27
13Al + n →2813Al + γ1 → 2814Si + e−+ γ1+ γ2+ νe (1) Most β decays lead to an emission of one or several γ rays of specific energies. These γ rays which can be used for identification of the radioactive nuclides that decays inside the sample of concrete. For the detection of the γ rays, a suitable detector is needed, see section2.3.
The cross section for neutron capture reactions on some nuclides may be very large for neutrons with low energies, down to the so-called thermal energies at about 0.025 eV. In the present work the neutrons have a broad energy distribution up to 180 MeV, which,
Figure 1: Illustration of a neutron capture reaction [2]
.
in addition to neutron capture, will produce a large number of different types of nuclear reactions. In such reactions neutrons, protons or heavier nuclei may be emitted from the initial nuclide when it is hit by a neutron. This leads to production of many different final nuclides starting from a neutron plus the initial nuclide. The relative intensity of the produced final nuclides depends on the reaction cross section, which strongly depends on the neutron energy and on the identity of the initial nuclide.
2.2 Mean Life Time, Decay Constant and Half Life
Atomic nuclei can either be stable or unstable. A stable nucleus will stay the same unless an outside force affects it, while an unstable nucleus will decay spontaneously at some point.
Due to the probabilistic nature of radioactive decay, the time when a specific nucleus decays cannot be determined. Instead, the mean life time τ and its inverse, the decay constant λ, which is the probability to decay per unit time, are used. Each nuclide has a specific mean life time and decay constant. For instance, the mean lifetime of55Fe is about 4 years, which means that on average a 55Fe nucleus decays after this time. Instead of the mean life time, the half life t1/2 is often used, defined as in Eq.2.
t
1/2= τ ln(2) (2)
The half life of a nuclide can be used to calculate how many nuclei that are left (that have not decayed) in a sample after a certain amount of time as well as how many nuclides that existed at an earlier time. The following equations can be used to calculate the number of nuclei Ntand their activity At at time t:
A = λN, N
t= N
0e
−ln2 t/t1/2, A
t= A
0e
−ln2 t/t1/2(3)
Here, N0 and A0 are the number of nuclei and activity, respectively, at time 0. An illus- tration of the half life is shown in Fig.2.
2.3 Photon Interaction with Matter
When a γ ray interacts with matter the photon may transfer part or all of its energy to the atoms through different processes. There are three processes of relevance for γ- ray spectroscopy. In the photoelectric effect the photon is absorbed by the atom and an
Figure 2: Illustration of the half life of a nuclide [3]. The half life t1/2is the time it takes on average for half of the nuclides in a sample to decay.
electron is emitted [4]. The energy of the emerging electron, which usually comes from the K or L shells, equals the energy of the absorbed photon minus the binding energy for the electron. The second process is Compton scattering where the photon interacts inelastically with an atomic electron. A new photon with lower energy is emitted and the electron is scattered with an energy that depends on the scattering angle of the emitted photon [5].
The third process is called pair production and may occur for photons with energies above 1022 keV when the photon interacts with the electromagnetic field of a nearby atomic nucleus. The result is that the photon is transformed into an electron and a positron, that have a total kinetic energy of the photon energy minus the energy 1022 keV, corresponding to the combined mass of the electron and positron. The positron will interact with another electron and annihilate, resulting in two photons with 511 keV energy [6].
These three processes have probabilities that vary with energy and nucleus charge Z. Thus the probability for interaction with matter is higher for heavy materials, such as lead, compared with lighter elements such as hydrogen or carbon [5].
2.4 Gamma-Ray Spectroscopy
The physical processes described in the previous section have to be taken into account when γ rays are to be measured. This brief description describes the detection of γ rays in a detector based on a semiconductor crystal made of high-purity germanium (HPGe).
A γ-ray interaction that occurs in the sensitive volume of the crystal, transforms all or part of its energy to an electron or to an electron-positron pair. These particles are called primary particles and typically have kinetic energies in the range from keV to MeV. They will loose their energy by multiple collisions with other electrons in the detector and so on until a large number of electrons have obtained kinetic energies in the eV range. In a semiconductor detector, these low-energy electrons are excited from the valence band to the conduction band to form electrons and holes, also known as charge carriers. An electric field is applied across the crystal to form a depletion region, which is free of charge carriers. When a γ ray interacts in this region a large number of electron-hole pairs are created. The electric field is arranged in such a way that the electrons and holes move to the two opposite electrodes at which a net charge signal is produced. The size of the charge signal is proportional to the absorbed energy and the signal is large enough to be processed by the electronics attached to the detector. The last chain of the electronics is a Multi Channel Analyzer, which stores the energy signals from the detector in a γ-ray spectrum that can be displayed on a computer and saved for further analyses.
The main advantage of an HPGe detector is its superior energy resolution, which makes it the best choice for measurements and analyses of complex γ-ray spectra containing close lying peaks due to detected γ rays with similar energies. The energy resolution of a typical HPGe detector, given as the full width at half maximum (FWHM) of a γ-ray peak in the spectrum at 1332 keV, is in the range from 1.7 keV to 2.3 keV.
Germanium has a relatively large atomic number and the HPGe crystals can be made with large volumes. This is important in order to have a high efficiency for detection of γ rays with energies up to several MeV.
An important effect that may need to be considered in γ-ray spectroscopy is so-called true coincidence summing (TCS). If, during the decay of a nuclide, a cascade of more than one γ ray is emitted simultaneously (in true coincidence), or within a time much shorter than the signal processing time of the spectroscopy system, there is a probability that more than one of them hit the detector at the same time and deposit some energy in it. The detector then records the sum of the deposited energy, which is why the effect is called true coincidence summing. The probability for TCS to occur depends mainly on how many γ rays there are in the cascade (larger number gives larger probability of TCS) and on the solid angle subtended by the radioactive sample with respect to the detector (larger solid
angle gives larger TCS effects). Another type of summing in γ-ray spectroscopy systems is so-called pile-up, which occurs when γ rays from different decays (not in coincidence) hit the detector within a time that is shorter than the signal processing time. Pile-up also leads to a summing of the energy and the probability of it to occur depends mainly on the detector count rate (number of detected γ rays per time unit).
Another effect that is observed in γ-ray spectra are peaks due to single and double escape, which are a consequence of pair production. When the positron, which was created in the pair production, annihilates with an electron, two photons with energy 511 keV are created. This will result in three peaks in the spectrum [7]:
• Full-energy (FE) peak: both 511 keV γ rays are fully absorbed in the detector. The energy of this peak corresponds to the energy of the incident γ ray.
• Single escape (SE) peak: one of the 511 keV γ rays escape from the detector without depositing any of its energy in it while the other one is fully absorbed in the detector.
The energy of this peak corresponds to the energy of the incident γ ray minus 511 keV.
• Double escape (DE) peak: both 511 keV γ rays escape from the detector without depositing any of their energy in it. The energy of this peak corresponds to the energy of the incident γ ray minus 1022 keV.
2.5 The Monte Carlo Method
2.5.1 General Principles
Monte Carlo simulations make use of pseudo-random numbers in a certain distribution based on probability. This probability can either be the same for all values or follow a certain curve based on some physical property, such as absorption cross sections. An example of these curves can be seen in Fig.3where a Gaussian curve, or normal distribution is illustrated. This is a distribution often used in statistics and natural science. Since the Monte Carlo method uses random numbers you get a new result every time you make the simulation. The probability distribution makes it so that when the number of simulated events, N, goes towards infinity the result converges towards a certain value that with high probability is the ”correct” value, provided that the probability distribution describes the simulated property in a correct way. In this figure the x-axis represent the variation around the mean value of 0.When working with nuclides the Monte Carlo method is suitable since cross sections, angular distributions and other parameters reflect different probabilities for what will happen. Half lives, cross sections and energy distributions are all physical
quantities that involve probabilities and uncertainties which means that the Monte Carlo method is needed and provides the most reliable results.
Figure 3: A Gaussian distribution.
2.5.2 The FLUKA Monte Carlo Code
The program FLUKA [8, 9] is a Monte Carlo simulation program with a history of over 50 years originally focusing on radiation shielding, but it has gone through a development towards a multipurpose multi-particle code for simulations in a vast variety of fields. It has been developed and enhanced in phases, primarily at CERN, for instance during the design of the Superconducting Super Collider (SSC) and the Large Hadron Collider (LHC) and similar projects which needed the possibility to simulate high-energy interactions, strong magnetic fields and high- and low-energy neutron interactions [10]. The latter is of particular interest in this thesis.
2.6 Previous Studies
2.6.1 Tegmyr
In the draft of the master’s thesis of Oscar Tegmyr, a study was made of how well FLUKA can simulate the radioactivity produced in an irradiation of a sample of concrete with 171
MeV protons from the cyclotron at TSL [11]. The study took place in 2016 and was made in a similar fashion as this study. A concrete core sample was drilled out of a non-irradiated wall and cut into discs. The discs were then lined up in the beam path from the Gustaf Werner cyclotron and irradiated with protons. A simulation of the setup was made with FLUKA and the results were analyzed similarly to this study. The results of the study by Tegmyr show that the ratio of the simulated and measured activities have large variations between the samples. The weighted average ratio for the analyzed samples were 0.798 with a standard deviation of 0.745.
2.6.2 La Torre
In Francesco Paolo La Torre’s thesis ”Study of induced radioactivity in proton accelerator facilities” [12] the neutron induced activation of concrete near a 600 MeV proton accelerator at CERN has been examined. In this study, 30 years of operations were simulated in FLUKA and multiple concrete cores were extracted from the wall and analyzed. This study was made 24 years after the last irradiation had been done in the facility meaning that only long lived nuclides could be measured. A total of 18 different samples were measured and the result gave a weighted average ratio of 0.854 with a standard deviation of 0.572.
3 Experiment
3.1 Neutron Irradiation Experiment
A neutron beam with an ANITA spectrum (see section4) was used to irradiate a cylindrical concrete core sample with a length of about 700 mm and a radius of about 37 mm. The core was taken from a wall in room 17-1113 (Blue hall). The walls in this room have been exposed to irradiation with scattered neutrons. However, the induced radioactivity of the core due to this irradiation is known from previous measurements to be negligible. The core was then cut into slices as shown schematically in Fig. 4. Some of these slices are the samples that were used in the analysis done in the present work. The intensity of the irradiation beam was on average 3.6 × 107 particles per second for 66 hours and 33 minutes and hit evenly on the face of the core. The setup of the irradiation experiment is shown in Fig.5where the concrete core is to the left of the image at a standardized [13] distance of 62 cm from the collimator to the right. After the irradiation, the samples were measured in a γ-ray spectroscopy setup (see section 3.2). In order to get good statistics both for nuclides with short half lives (minutes to hours) and long half lives (days to a few years), at least two measurements were made for each of the thin samples. The thick samples with ID numbers 129, 203, 285, 390, 491 and 583 (see Fig. 4) were not measured. The first measurement was done about 20 minutes after the end of the irradiation while the last one was done 23 days later. A non-irradiated sample (so-called zero sample), used for background subtraction of the γ-ray spectra (see section3.3.2) was also measured several times during the measurement period. A list of all measurements is shown in AppendixA.
Figure 4: A schematic picture of the concrete core with the samples and their ID numbers, which approx- imately give the distance in mm between the sample front faces oriented towards the incoming neutron beam.
Figure 5: The setup for the irradiation experiment performed at TSL. The ANITA neutron beam enters into the room through the collimator in the hole of the green wall on the right hand side of the image. The irradiated concrete core is the grey cylinder on the left hand side.
3.2 Measurements of Gamma-Ray Spectra
3.2.1 Detector and Electronics
For identification of the produced nuclides and for determination of their activities, the following setup was used for the measurement of the γ rays emitted by the radioactive samples. The γ-ray detector used was an Ortec coaxial closed-end p-type detector (serial number 42-TP31662A). The HPGe crystal of the detector had a diameter of about 73 mm and length of about 81 mm. The energy signal from the preamplifier of the detector was sent to an Ortec 92X Spectrum MASTER multi channel analyzer (MCA) with a built- in spectroscopy amplifier and a 16384 channel ADC. The MCA was connected to a data acquisition PC, running the Ortec MAESTRO MCA software package.
To reduce the detection of background γ rays emitted from natural radioactivity in the measurement room, the HPGe crystal of the detector was placed inside a lead cave, see Fig.6. The samples were placed in a sample holder made of plastic. This assured a known, reproducible and stable positioning of the samples in front of the HPGe crystal.
Figure 6: Low-background γ-ray spectroscopy setup used for the measurements at TSL. The HPGe crystal is placed inside the lead cave shown on the left hand side and the liquid nitrogen dewar of the detector on the right hand side of the image.
3.3 Analysis of Gamma-Ray Spectra
3.3.1 Energy and Efficiency Calibrations
Energy and efficiency calibrations of the γ-ray spectroscopy setup was performed in the energy range from 53 keV to 1408 keV using standard γ-ray point sources of152Eu, 60Co and 133Ba. The measured energy resolution of the detector, given as the FWHM, was about 1.8 keV at 1332 keV. For the efficiency calibration the sources were placed 205 mm from the HPGe crystal. This efficiency curve obtained from the calibration can be seen in Fig.7.
In the measurements of the irradiated samples, the samples were placed as close as possible to the HPGe crystal. The front-to-front distance between the sample and the crystal was typically about 8 mm. This was done to increase the number of counts in the peaks observed in the spectra. This, however, increased the TCS effects due to the increased probability of summing of cascading γ rays in the closed geometry. The efficiency of the setup for this geometry was determined in the following way.
Two special calibration sources were prepared at TSL by mixing a small amount of60Co and
152Eu with concrete powder. The powder was molded into cylindrical discs with roughly the same dimensions as the irradiated samples. The activity of the concrete sources was determined by a measurement in which they were placed 205 mm from the end-cap of the HPGe detector. By using the efficiency of the setup measured with the point sources at this distance, as explained above, the activity of the concrete calibration source could be
Figure 7: Absolute peak efficiency, absas a function of γ-ray energy for a measurement with point-sources of60Co,133Ba and152Eu placed at a front-to-front distance of 205 mm between the source and the HPGe crystal. The black dotted curve is a fit of the data points.
accurately determined. When calculating the activities of the concrete calibration sources, a correction was made for their non-point source geometry and for the self absorption of γ rays.
Measurements were then done with the concrete calibration sources placed at a front-to- front distance of about 8 mm between the source and the HPGe crystal, i.e. at the same distance as used for the measurements of the irradiated concrete samples. The results of these measurements are shown in Fig. 8. The solid curve in the figure was obtained by fitting the following function to the data points:
abs=
(−7.247 × 10−6Eγ2+ 0.00138 Eγ+ 0.00153 Eγ ≤ 120 keV
−0.01785 ln(Eγ) + 0.149 Eγ > 120 keV (4) The two green curves in Fig.8have values of ±20 % from the black curve. As seen in Fig.8 there is a much larger scattering of the data points than in Fig.7. The scattering is due to the TCS effects. In the irradiated samples, the decay of a large number of different nuclides were observed. The γ-ray detection of most of them is also influenced by TCS. It was not possible within the time limit of the present work to do proper corrections of the TCS effect
for the observed nuclides. Instead an estimated uncertainty of ±20 % was assumed for the efficiencies. The measured efficiencies only cover energies up to 1408 keV. The efficiencies at higher energies are extrapolated using the function in Eq. 4, which for these energies may lead to larger uncertainties than the estimated ±20 % (see section5.2.2).
Figure 8: Absolute peak efficiency, abs as a function of γ-ray energy for a measurement with the concrete sample sources placed at a front-to-front distance of 8 mm between the source and the HPGe crystal. Blue and red data points are for152Eu and60Co, respectively. The black curve is a fit of the data points to the function given in Eq.4and the green curves have values of ±20 % from the black curve.
3.3.2 Spectrum analysis
The analysis of the measured γ-ray spectra was done with the program GF3 of the Rad- ware package [14]. With this program it is possible to examine the spectra and fit the observed peaks. The program produces an output file with the following data for the fitted peaks: energy, FWHM, net counts (also called peak area) including the uncertainties of all parameters.
With the software GF3 it is possible to examine the γ-ray spectra and fit the observed peaks.
The program produces an output file with the following data for the fitted peaks: energy, FWHM, net counts (also called peak area) including the uncertainties of all parameters.
To exclude peaks in the spectra due to γ rays from natural radioactivity in the surround- ing area and in the samples themselves, background subtracted spectra were used in the analyses. This is needed since concrete is naturally radioactive but it’s only the induced
radioactivity that is desired. Such spectra were produced by subtracting for each channel in the spectrum a fraction of a measured zero-sample spectrum. The fraction used was the ratio of live times of the irradiated sample and the zero-sample spectra. An error spectrum was also produced and used for weighting of the least-square peak fitting in GF3. This is required to obtain correct uncertainties of the fitted peaks when background subtracted spectra are analyzed.
An example of a γ-ray spectrum is shown in Fig. 9. The data of fitted peaks with a net area greater than three times its uncertainty were saved for further analysis. The peak identification, assigning them to the decay of specific nuclides, was done with the help of the nuclear decay database ENSDF available at the National Nuclear Data Center (NNDC) [15]. In the assignment procedure the half lives of the nuclides were taken into account. This was done by rejecting peak assignments for short lived nuclides on samples that was measured a long time after the end of the irradiation, since they would have decayed by then. The consistency of the relative γ-ray intensities of the decay of each nuclide was also checked. Peaks due to TCS, SE and DE were also identified and excluded from further analysis.
Figure 9: Example spectrum from GF3[14]. Energy in keV on X-axis and number of counts on the Y-axis.
3.3.3 Calculation of Activities
When all peaks were identified the activity from each was calculated based on the following equations:
A
t= N
t
ltabsI
γ(5)
A = A
tt
rtln(2)
t
1/2(1 − e
−trtln(2)/t1/2) (6)
Here At is the activity at the mid-point of the measurement time. N is the net-area of the measured peak, tlt is the time of the measurement, abs is the absolute peak efficiency and Iγ the relative intensity of the γ ray as obtained from ENSDF. Eq. 5 [7] can be used to calculate the activity but doesn’t take the activity reduction during measurement into account. This is corrected by Eq. 6 where A is the corrected activity and trt is the real time.
The final activity for each nuclide was obtained by calculating the weighted average of the activity values for each of the observed peaks. The following formulas, based on the maximum likelihood method [16] as seen in Eq. 7, were used to calculate the weighted average Aw and its uncertainty σAw:
f (A
1, ...., A
n|µ
A, σ
A) = Π 1 σ
Ai√
2π e
−(Ai−µA)2/2σAi2(7)
A
w=
P
Aiσ2
Ai
P
1σ2Ai
(8)
σ
Aw=
s 1 P
1σ2Ai
(9)
3.4 Chemical Analysis of the Samples
The chemical composition of the concrete core was determined in the following way. A small sample of the concrete was sent to the company Swerea for analysis [17]. Five small areas of this sample were analyzed by the company. The obtained results are shown in Tab. 1. As seen from the results in the table, the chemical composition has rather large variations between the five areas. An optical inspection of the samples shows that pieces of stones are mixed in with the concrete. Thus, it may be that the actual variation of the chemical composition of the irradiated concrete samples is considerably larger than what is shown in Tab.1. In the present work, it was, however, not possible to do a better analysis of the chemical composition. The mean values of the abundances given in the table were used in the FLUKA simulations.
Table 1: Results of the chemical analysis of five small areas of a sample of the concrete core.
Area O Na Mg Al Si P S
Abundance in relative weight [%[
1 46.97 2.27 1.64 6.28 30.78 0.02 0.13
2 46.47 1.07 1.66 6.20 30.36 0.08 0.13
3 47.18 1.68 1.67 5.49 31.53 0.09 0.15
4 46.86 1.01 2.02 4.90 31.21 0.02 0.18
5 46.70 1.59 2.01 5.69 30.74 0.04 0.11
Mean value 46.84 1.52 1.80 5.71 30.92 0.05 0.14 Standard deviation 0.27 0.51 0.20 0.56 0.45 0.03 0.03
Area Cl K Ca Ti Mn Fe
Abundance in relative weight [%[
1 0.04 1.83 8.06 0.15 0.20 1.61
2 0.12 3.85 7.44 0.25 0.15 2.22
3 0.06 1.76 7.78 0.33 0.20 2.06
4 0.06 1.79 9.59 0.25 0.21 1.89
5 0.11 1.91 7.38 0.30 0.16 3.25
Mean value 0.08 2.23 8.05 0.26 0.18 2.21 Standard deviation 0.03 0.91 0.90 0.07 0.03 0.63
4 Monte Carlo Simulations with FLUKA
4.1 Simulation of ANITA Neutron Beam
At TSL in Uppsala the neutron beam is produced by bombarding a thick target of tungsten with 180 MeV protons from the Gustaf Werner Cyclotron. The proton interacts with the tungsten nuclides which releases neutrons scattering in all directions. The neutrons going in a certain direction are then selected with a collimator before hitting the experimental target. This neutron beam is called ANITA; Atmospheric-like Neutrons from thIck TArget.
The energy distribution of the neutrons in this beam are needed for the simulations in this project and there have been a simulation and an experimental confirmation of this energy distribution called the ANITA-spectrum [18]. The spectrum provided in this project was only defined down to 1 MeV which is far to high since we can assume neutrons are produced and scattered all the way down to thermal neutrons at 0.025 eV. Furthermore neutron count provided is only for neutrons with energies above 10 MeV meaning lower energy neutrons is needed to be mapped. The spectrum was also a fitted curve meaning the exact values measured and simulated were not available. Because of this the neutron beam setup had to be simulated in FLUKA. The resulting spectrum can be seen in Fig.10in a logarithmic scale plotted along with the old spectra previously mentioned. The results differ at multiple points but the effect of these discrepancies has not been investigated.
4.2 Simulation of Concrete Core Irradiation
To receive a result from the Monte Carlo simulation of FLUKA the graphical user interface Flair [19] was used which provides an easy-to-use tool for creating the simulation setup.
The simulation made in FLUKA was done to replicate the reality as much as possible. In FLUKA you use Cards (the word stems from the time when punched cards were used) in which all inputs for the simulation are defined. The input file used in this study can be seen in AppendixB. Input cards of high interest in this study are listed and explained below:
• IRRPROFIle - set the irradiation time and the flux of the neutron beam.
• RADDECAY - is used so decay chains and build-up of isotopes are taken into account.
• RESNUCLEi - is used to score (estimate quantity) of all nuclides in each sample that is produced from interactions.
• DCYSCORE - sets the different timestamps from DCYTIMES in RESNUCLEi.
1e-10 1e-09 1e-08 1e-07 1e-06 1e-05 0.0001
1e-05 0.0001 0.001 0.01 0.1 1 10 100
anita_plot_v08a.pdf, 2017-08-17T15:36:26+02:00
Absolute Spectral Fluence [n/cm2/MeV/proton]
En [MeV]
FLUKA sumulations of ANITA spectrum A. Prokoviev 2009
C. Åström 2017
Figure 10: The simulated ANITA-spectrum used as source for the simulation of the irradiation experiment.
The blue line is the experimentally defined ANITA-spectrum.
• DCYTIMES - sets the different timestamps used in DCYSCORE.
The simulation produces results from RESNUCLEi in an output file which gives us the activity from each nuclide in every sample at the time of the measurement done in the experiment. This means that the result from FLUKA can be directly compared to the results from the measurements.
The simulation was done with 2.5 × 1010particles being simulated which was chosen from the simplistic reasoning that it gave a reasonable simulation time, while providing a sig- nificantly lower statistical uncertainty than the results from the γ-ray spectroscopy. The accuracy of a Monte Carlo simulation is proportional to the number of simulated events:
σ ∝
√1N
This can be shown using the Central Limit Theorem [20] and means that using 100 times more primary histories (particles simulated) will increase simulation time by 100 times but only increase the accuracy by 10 times. Thus increasing the number of primary histories isn’t justified.
5 Data processing and Results
5.1 Data processing with Python Scripts
The large amount of data received from the spectrum analysis and FLUKA simulation made it impossible to perform the comparison of the measured and simulated data manually.
Several Python [21] scripts were used to accomplish this. One of the Python scripts, which was written by O. Tegmyr [11], was used for extracting data from the FLUKA output files concerning the nuclides produced in the simulation. A Python script, which was used to compare the identified peaks in the γ-ray spectra with the data in the ENSDF database, was written. The database was provided by O. Tegmyr as well. Since multiple nuclides may have peaks lying close in energy, a careful manual inspection of the results produced by this script was done. Three more scripts were written: a script that was used to join the data from the spectrum analysis with the simulated data in a spreadsheet file, a script that calculated the measured activities using Eq. 5-9, and a script that produced nicely formatted tables of the results.
5.2 Overview of the Results
Nuclides that were not observed experimentally but were produced by FLUKA with an activity larger than 0.1 % of the total activity are shown in Tab. 2. All of these nuclides decayed either by emission of no γ rays, too low relative γ-ray intensities to be observed in the measurement or by emission of only the 511 keV annihilation radiation, which cannot be used for unique nuclide identification.
Tab. 3 summarizes the comparison of the measured and simulated results for five of the analyzed concrete samples (000, 013, 024, 277 and 685). In the table, the ratio of the activity predicted by FLUKA (AF) and the measured activity (Aw from Eq. 8) is given for all of the nuclides that were observed experimentally in these samples. How the ratios were calculated can be seen in Eq. 10 with RA being the ratio while AF and Aw are the activity from FLUKA and the measurement respectively. The last row in the table gives the weighted average, Aw,tot and its standard deviation sAw, tot for all nuclides for each sample. Details of all results obtained are given in AppendixC
RA= AF
Aw (10)
Each of these samples were measured at least twice, with some samples being measured up
to four times. Sample 000 for instance was measured four times and sample 013 two times.
This affects the uncertainties of the activities - the statistical uncertainties are reduced if a nuclide is observed in more than one sample.
Table 2: Nuclides not observed in the measure- ments but produced in the FLUKA simulation with an activity larger than 0.1 % of the total activity.
Nuclide Gamma-ray energy [keV]
3H -
11C 511
18F 511
31Si -
32P -
33P -
35S -
37Ar -
45Ti 511
49V -
55Fe -
As can be seen in Tab. 3, the activity ratios have large variations with values between 0.1 and 2.5. However, the average activity ratio for all nuclides and all five samples is 1.073 with a standard deviation of 0.550 (51 %). Thus, the FLUKA simulations performed in this work give a good estimate of the average activity of all nuclides in the five samples.
The results in Tab. 3 show that the activity ratios have large variations between the differ- ent nuclides, but for the same nuclide there is a significantly smaller deviation throughout the five samples. For a specific nuclide the av- erage activity ratio for the five samples has a standard deviation of between 0.7 % and 70 % with an average at about 24 % compared to the total standard deviation of 51 %. Due to its short half live,41Ar could only be observed in samples 000 and 024, for which measurements
were performed only a few hours after the end of the irradiation (see AppendixC) and38Cl could only be observed in sample 024 for the same reason. The nuclides with the small- est activity ratios in most of the samples are 24Na, 28Mg, 28Al and 42K. For all of these, FLUKA gives a relatively high activity, but the small ratios indicate that the activities are actually even higher. 28Al can be observed in samples measured a long time after its half life of 2.2 min. This is due to it being a daughter nuclide of28Mg with the half life 20.9 h.
Table 3: Activity ratios, RA, of FLUKA and measured activities for the experimentally observed nuclides in the five analyzed samples. A value shown as a hyphen means that no activity was obtained from the measurements. The values on the last row are the weighted average, Aw, tot and its standard deviation sAw, tot. The weighted average of the activity ratio for all samples is 1.073 and its standard deviation is 0.550.
Sample 000 Sample 013 Sample 024 Sample 277 Sample 685 Nuclide t1/2 RA ± Unc RA ± Unc RA ± Unc RA ± Unc RA± Unc
7Be 53.2 d 1.670 ±0.196 1.583 ±0.056 1.571 ±0.039 1.799 ±0.030 1.838 ±0.260
22Na 2.60 y 1.404 ±0.167 1.408 ±0.141 1.415 ±0.110 1.654 ±0.162 1.322 ±0.188
24Na 15.0 h 0.350 ±0.035 0.399 ±0.057 0.211 ±0.009 0.210 ±0.030 0.103 ±0.015
28Mg 20.9 h 0.333 ±0.027 0.360 ±0.045 0.415 ±0.052 0.310 ±0.042 0.278 ±0.058
28Al 2.25 m 0.289 ±0.043 0.318 ±0.064 0.307 ±0.055 0.279 ±0.061 0.293 ±0.076
38Cl 37.2 m - - 0.354 ±0.068 - -
41Ar 110 m 0.611 ±0.197 - 0.605 ±0.129 - -
42K 12.4 h 0.198 ±0.026 0.347 ±0.071 0.338 ±0.069 0.144 ±0.030 0.036 ±0.009
43K 22.3 h 0.877 ±0.057 0.864 ±0.072 0.797 ±0.062 0.896 ±0.093 0.858 ±0.151
47Ca 4.54 d 1.047 ±0.092 0.947 ±0.033 0.981 ±0.050 1.173 ±0.142 0.882 ±0.292
44Sc 3.97 h 1.133 ±0.144 2.483 ±0.504 1.496 ±0.308 0.507 ±0.111 0.393 ±0.104
46Sc 83.8 d 1.395 ±0.144 1.233 ±0.068 0.878 ±0.107 1.141 ±0.095 0.509 ±0.069
47Sc 3.35 d 0.862 ±0.102 0.794 ±0.079 0.581 ±0.066 0.764 ±0.080 0.642 ±0.127
48Sc 43.7 h 1.446 ±0.164 1.304 ±0.046 0.882 ±0.131 1.205 ±0.270 1.097 ±0.334
48V 16.0 d 1.641 ±0.121 1.538 ±0.136 1.222 ±0.067 1.683 ±0.123 1.903 ±0.167
48Cr 21.6 h 1.465 ±0.560 1.050 ±0.311 - 1.647 ±0.551 -
51Cr 27.7 d 1.213 ±0.149 1.184 ±0.211 0.700 ±0.008 0.861 ±0.159 0.847 ±0.015
52Mn 5.59 d 1.683 ±0.091 1.658 ±0.068 1.032 ±0.056 1.230 ±0.083 1.246 ±0.114
54Mn 312 d 1.142 ±0.134 1.000 ±0.163 0.722 ±0.061 0.924 ±0.030 0.748 ±0.138
56Mn 2.58 h 0.388 ±0.056 0.282 ±0.034 0.091 ±0.008 - -
Rw, tot± sRw, tot 1.257±0.562 1.313±0.668 0.999±0.521 1.024±0.535 0.880±0.508
5.2.1 Radiological Declassification of Building
For radiological declassification of buildings, the SSM authority has defined a requirement that for each nuclide the activity in the buildings must be lower than a certain limit with a 95 % confidence limit. This is based on the standard ID R-11-15 from the Swedish Stan- dard Institute(SIS) [22]. This means that if the results can be assumed to have a normal distribution then FLUKA’s resulting activity, AF, can be used to calculate a corrected activity, AF, corr as in Eq.11 where AF is used with the resulting average ratio, RA, from this study and the standard deviation σRA is multiplied by 1.645 to get a confidence of 95 %, see Fig.11. If the corrected activity calculated with Eq.11is smaller than the limits set by SSM for an area in the facility, that area could be cleared. If this were to be used together with core sample analysis the RA and σRA could be adapted to fit the specific area.
AF, corr = AF(1 + 1.645σRA) RA
(11)
Figure 11: A Gaussian distribution with the lower 95 % of the area filled in.
5.2.2 High Energy Gamma rays
When calculating the activity of a nuclide that emits multiple γ rays when it decays, the weighted average of the activity is used (see Eq.8). It was observed that γ rays with higher
energies (above about 2 MeV) tend to give slightly higher activities. The only nuclides detected with energies larger than 2 MeV were34Cl in spectrum 000 01 and24Na in all 13 spectra. The nuclide34Cl emits two γ rays with energies 1177 keV and 2128 keV and24Na emits two γ rays with energies 1369 keV and 2754 keV. The activities calculated by Eq.6 for these four γ rays are shown in Tab.4. For 24Na, the 2754 keV peak gives an activity that is about 50 % higher than the 1369 keV peak. For34Cl the 2128 keV peak gives an activity which is about 20 % larger than the 1177 keV. The variation of the activities for most of the other nuclides with multiple γ rays is within ± 15 % .
Table 4: Comparison of calculated activities using two γ rays from the decay of24Na and34Cl.
24Na
Measurement γ [keV] Activity [Bq]
000 01 1369 25610±5122
2754 39532±7887
000 02 1369 10330±2066
2754 15679±3136
000 03 1369 629±126
2754 953±191
013 01 1369 8507±1702
2754 12823±2565
024 01 1369 12579±2516
2754 19037±3807
024 02 1369 474±95
2754 715±143
277 01 1369 1190±238
2754 1785±357
685 01 1369 62±12
2754 92±18
34Cl
000 01 1177 832±192
2128 1083±223
5.3 Results from Samples
5.3.1 Sample 000
This sample was the first one in the concrete core with a thickness of 13 mm and thus the sample to get directly hit by the neutron beam. This results in it having the greatest amount of activity, so much so that for the first measurement with the detector it had to be placed at a greater distance from it. Four measurements were done on Sample 000; after 20 minutes, 7 hours, 71 hours and 11 days. On the first measurement made, 000 01, the sample was placed 32.5 mm away instead of the 8 mm the rest had. This was a distance at where no efficiency calibration had been done. Nor were there any data to do such. This is why the first measurement hasn’t been accounted for when calculating the averages. The weighted mean ratio from Sample 000 is thus 1.257 with a standard deviation of 0.562. The resulting average ratios for each nuclide can be seen in the first column of Tab. 3. These ratios vary from 0.20 (3) for 42K to 1.68 (9) for 52Mn. The strongest observed specific activities are 100 (20) Bq/g for 24Na in measurement 000 02, 6.3 (9) Bq/g for 24Na in measurement 000 03, and 1.0 (2) Bq/g for 7Be in measurement 000 04 (see Tab. 9-11 in AppendixC),
From FLUKA the total specific activity in Bq/g is obtained for each sample for the times at which the measurements were made. These total activities cannot be directly compared with the corresponding total measured activities, because some of the nuclides that were produced could not be observed experimentally. Instead of using the total simulated ac- tivities, the sum of the simulated activities for the experimentally observed nuclides were calculated. The results are shown in Tab.5. The result of the first measurement looks rea- sonable at first glance, but the true values are not possible to get from the measurements made because of the difference in measurement distance.
Table 5: Simulated and measured total specific activities and their ratios for three of the measurements of sample 000. The time of the start of the measurement after the end of the irradiation, tstart, is shown in column 1. The total simulated specific activities and the sum of the simulated specific activities for the nuclides that were observed experimentally are shown in column 2 and 3, respectively. Column 4 shows the measured total specific activities and column 5 the ratio of the values in column 3 and 4.
FLUKA FLUKA Measured Ratio
tstart total observed
[Bq/g] [Bq/g]
7.1 h 64.98±0.09 51.91±0.08 115.42±16.71 0.45±0.07 3.0 d 16.64±0.02 6.43±0.01 11.03±1.77 0.58±0.09 11.1 d 11.37±0.01 2.96±0.01 2.35±0.45 1.26±0.24
The activity ratios shown in Tab.5increase with the time after the irradiation. This could be an indication that short-lived nuclides are more difficult to to simulate correctly, while the activity ratio is close to 1 for the longer lived nuclides, which are the only ones observed after many days have passed since the end of the irradiation.
5.3.2 Sample 013
This sample had a thickness of 11 mm and was placed right after sample 000, at a distance of 13 mm into the concrete core. The sample was measured twice with a start of the measurement 8.1 hours and 6.0 days after the end of the irradiation. According to Tab.
3, the average activity ratio for all nuclides is 1.31 and its standard deviation is 0.67. The activity ratios vary from 0.28 (3) for 56Mn to 2.5 (5) for 44Sc. The strongest observed specific activities are 100 (20) Bq/g for24Na in measurement 013 01 and 1.1 (2) Bq/g for
7Be in measurement 013 02 (see Tab. 12-13 in AppendixC). The decay of 38Cl and 41Ar were not observed in this sample, probably because of their short half lives compared to the time of the measurements.
5.3.3 Sample 024
This sample had a thickness of 12 mm and was placed as the third one, 24 mm into the core.
Three measurements were made: start of measurement 1.5 hours, 80 hours and 18.2 days after the end of the irradiation. Compared to sample 000 and 013 almost all nuclides have a slightly lower activity ratio, see Tab.3.This is also seen as a smaller average activity ratio of 0.99, although with the standard deviation of 0.52 it does not differ significantly from the values for samples 000 and 013. As for sample 013, the smallest and largest activity ratios were obtained for 56Mn, 0.091 (8), and and 44Sc, 1.5 (3). The strongest observed specific activities are 130 (20) Bq/g for24Na in measurement 024 01, 5.0 (7) Bq/g for24Na in measurement 024 02, and 1.3 (3) Bq/g for7Be in measurement 024 03. Tab.6show the result of the first measurement done on sample 024 (see Tab. 14-16 in Appendix Cfor all results from sample 024),
The nuclide 48Cr could not be observed in the measurements of sample 024 although it was expected be seen. It has a half-life of 21.6 hours and two strong γ rays at 112.3 keV and 308.2 keV, which were observed in several of the measurements of the other samples.
Table 6: Measured values vs Simulation from FLUKA, sample 024 01.
FLUKA Measured Ratio
Nuclide Activity [Bq/g]±Unc Activity [Bq/g]±Unc Ratio± Unc
24Na 51.166±0.051 132.031±18.673 0.388±0.055
56Mn 3.174±0.006 34.990±3.121 0.091±0.008
7Be 1.781±0.002 1.070±0.227 1.665±0.355
42K 1.631±0.007 4.812±0.966 0.339±0.069
43K 1.182±0.006 1.266±0.119 0.934±0.092
47Sc 0.824±0.003 1.367±0.274 0.602±0.123
52Mn 0.750±0.002 0.633±0.073 1.185±0.141
44Sc 0.635±0.004 0.407±0.083 1.561±0.330
51Cr 0.591±0.001 0.738±0.169 0.801±0.185
28Mg 0.347±0.003 0.832±0.097 0.418±0.053
28Al 0.347±0.003 1.014±0.204 0.343±0.072
48Sc 0.306±0.002 0.312±0.053 0.983±0.176
41Ar 0.230±0.002 0.380±0.078 0.605±0.129
48V 0.184±0.001 0.374±0.054 0.493±0.073
47Ca 0.165±0.001 0.147±0.033 1.123±0.263
22Na 0.162±<0.0005 0.105±0.023 1.544±0.347
54Mn 0.141±<0.0005 0.207±0.043 0.680±0.141
38Cl 0.095±0.001 0.269±0.050 0.354±0.068
46Sc 0.046±<0.0005 0.053±0.014 0.865±0.234
57Co <0.0005±<0.0005 0.038±0.010 <0.013±<0.0005
5.3.4 Sample 277
Sample 277 had a thickness of 9 mm and was placed a bit further down in the concrete core, at a distance of 277 mm from the front side. The activity of this sample was lower than of samples 000, 013 and 024, which probably is due to its position in the core. Sample 277 was measured two times: start of the measurements at 24.2 hours and 10.2 days after the end of the irradiation. The average activity ratio for all nuclides is 1.02 with a standard deviation of 0.54 (see Tab. 3). The strongest observed specific activities are 17 (2) Bq/g for 24Na in measurement 277 01 and 0.34 (7) Bq/g for 7Be in measurement 277 02 (see Tab. 17-18 in Appendix C). 56Mn was not observed in this sample due to the relatively short half life.
5.3.5 Sample 685
The second last sample in the concrete core was sample 685. It was placed at 685 mm from the front side of the core and had a thickness of 9.5 mm. Two measurements were made: start at 59.8 hours and 16.9 days after the end of the irradiation. Since the sample was placed so far from the front of the core it received significantly less irradiation by the neutrons (see section Profile). The induced activities were so low that many of the nuclides barely could be identified in the γ-ray spectra. 56Mn and48Cr was not observed in this sample due to the relatively short half lives. The average activity ratio for all nuclides is 0.88 with a standard deviation of 0.51 (see Tab. 3). The strongest observed specific activities are 0.7 (1) Bq/g for24Na in measurement 685 01 and 0.044 (9) Bq/g for7Be in measurement 685 02 (see Tab. 19-20 in AppendixC).
5.4 Profile
The results of a simulation of the total activity as a function of the depth in the concrete core is shown in Fig. 12. The decrease of the activity is exponential except in the first 10 mm to 20 mm where the decrease is slower.
The neutron induced activity profile in Fig.12 can be compared to the results obtained by O. Tegmyr [11] and shown in Fig.13. In this work a similar concrete core was irradiated with 170 MeV protons. The activity profile has a sharp drop around 110 mm which corresponds to the range of 170 MeV protons in concrete. The activity in the core after this depth is induced by secondary neutrons that are produced along the core. The two activity profiles are naturally very different up to the depth corresponding to the range of the protons but have a similar exponential shape after that depth.
Figure 12: A FLUKA simulation of the specific activity of the concrete core two hours after the end of the irradiation with neutrons having an ANITA energy distribution.
0 10 20 30 40 50 60 70
Depth in core [cm] 100
101 102 103 104 105 106 107
Activiy [Bq/cm3] A1 A3 A8 A44 A71
Simulated activity in core immedately after irradiation stop
Figure 13: A FLUKA simulation of the activity per unit volume of a concrete core irradiated with a 170 MeV proton beam. The Fig. is from ref. [11].
As an example, the simulated and experimental specific activities for 24Na and 7Be are shown in Fig.14 at the time when the neutron irradiation was stopped. The activities are shown as a function of the depth in the concrete core. The experimental data points in the figure were obtained by using Eq. 3 to calculate A0 with At taken from the tables in AppendixCfor24Na and 7Be.
Figure 14: Simulated (blue) and experimental (red) specific activities at time 0 (end of irradiation) as a function of the depth in the concrete core sample for24Na (top) and7Be (bottom).
6 Discussion
The results of the present study show that the simulated and measured specific activities, averaged over all measurements and all observed nuclides, are similar: the ratio of simulated (FLUKA) to measured specific activities of 1.07 was obtained. The distribution of the ratios for the different nuclides is quite wide with a standard deviation of 0.55 (51 %). About 30 % of the ratios in table 3, deviate from 1.0 by more than 2 times their uncertainties.
Due to this large variation of the ratios for the different observed nuclides, the ratio averaged over all nuclides is not very useful. The ratios for each nuclide could, however, be used as a complement for measured values. For instance, when performing declassification work of the premises in a facility, one may need to take up to 20 drill samples per square meter (depending on the confidence of the measurements). By using the results of FLUKA simulations and Eq.11, the number of drill samples could be reduced significantly.
The nuclide 24Na (t1/2 = 15 h) has the largest specific activity for the samples that mea- sured within 3 to 4 days after the end of the irradiation. The ratios of simulated to measured specific activities for24Na were significantly smaller than 1.0: the values ranged from 0.10 to 0.40. As mentioned in section 5.2.2, the activities determined for the 2754 keV γ ray is significantly larger than for the 1369 keV γ ray. The reason for this is that the extrapolated full-energy efficiencies of the HPGe detector above 1.5 MeV are too small. By excluding the 2754 keV γ ray from the calculation of the measured activities, the ratios for
24Na will become about 25 % larger, but they will still be significantly smaller than 1.0.
A total of 37 measurements of 16 concrete core samples were made. In this work, there was only time available to analyze 13 measurements of 5 of the samples. Thus, a large portion of the available data is not included in the report. Analyzing the whole data set and including the calculated ratios may possibly lower the standard deviation of the overall ratios. However, the main results obtained in this work from the analyses of 5 samples will most likely not be altered.
In this project, the experimental uncertainties are rather large. The reason for this is that within the time frame of the present work it was not possible to correct the full-energy efficiencies for effects due to true coincidence summing. To include the uncertainties due to the TCS, a systematic uncertainty of the measured activities of ±20 % was assumed. This uncertainty is in most cases much larger than the statistical uncertainties. This problem can be solved by using a computer program (analytical or Monte Carlo simulation) to correct the efficiencies for the TCS effect. Another possibility would have been to measure all the samples at a greater distance from the detector, at which the TCS is negligible.
This would, however, require much longer measurement times to get enough statistics of the peaks in the γ-ray spectra. Even with this rather large systematic uncertainty of
