UPTEC F 17035
Examensarbete 30 hp Juni 2017
Analysis of triple gamma coincidences for studies of the level structure
of nuclei in the 100-Sn region
Ludvig Hallberg
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
Analysis of triple gamma coincidences for studies of the level structure of nuclei in the 100-Sn region
Ludvig Hallberg
In this master thesis a thorough analysis of data collected in an experiment performed at the JYFL accelerator laboratory in Finland using a reaction with a 47-Ti beam on a 58-Ni target is presented. The prompt emitted gamma rays from the
fusion-evaporation products were detected in the gamma-ray spectrometer JUROGAM II, while the recoils were detected in the GREAT spectrometer.
The main aim of this work was to use triple gamma-ray coincidences to find out if it is possible to discover new levels and transitions in proton-rich nuclides such as 103-Sn and 101-In.
A recoil triggered gamma-cube of coincident gamma rays was constructed and analyzed with the program levit8r from the Radware software package. From an intensity estimation of the data it was concluded that 102-Ag was the strongest reaction channel and emission of 3-7 particles in the fusion-evaporation reaction was favoured.
By analyzing peaks in double-gated spectra of the gamma-cube, three new levels and seven new gamma-ray transitions up to an excitation energy of 8644 keV and an angular momentum of 22 h were proposed as a continuation of rotational band B3 published in a recent article on 102-Ag. From plots of the alignment of the newly proposed transitions no band crossing was observed, just a smooth continuation of the band.
The efficiency of the charged particle detector, UoYTube, was deduced to be about 66% for proton detection, while the probability to detect one random proton was about 10%.
Ämnesgranskare: Tord Johansson Handledare: Johan Nyberg
Populärvetenskaplig sammanfattning
I detta examensarbete på masternivå inom kärnstrukturfysik har data från ett två veckor långt experiment, som genomfördes år 2014 vid acceleratoranlägg- ningen JYFL i Finland, analyserats. Det främsta målet med experimentet var att bidra till förståelsen av strukturen hos kärnor med proton- och neutrontal i närheten av den dubbelmagiska kärnan100Sn (50 protoner och 50 neutroner).
Kärnorna i detta område på nuklidkartan är av stort intresse inom aktuell kärn- strukturforskning.De befinner sig i närheten av protondropplinjen, d.v.s. vid gränsen där protonseparationsenergin blir negativ och utanför vilken inga fler protoner kan bindas till nukliden.
Mer specifikt var huvudmålet med experimentet att studera okända energi- nivåer i atomkärnan102Sn. Vid experimentet bombarderades ett tunt strålmål av58Ni med47Ti-joner med varierande energier mellan 153-167 MeV. När den högenergetiska strålen träffar strålmålet uppstår kärnreaktioner. Först fusione- rar 47Ti och 58Ni kärnorna och bildar en kompoundkärna, 105Sn, som vidare sönderfaller genom att sända ut i huvudsak protoner, neutroner och alfapar- tiklar, vilket skapar en mängd olika slutkärnor. Denna typ av reaktion kallas fusionsevaporation och sker inom ca 10−19 s efter att strålen träffar strålmålet.
Kärnorna som skapas vid fusionsevaporationen befinner sig i exciterade till- stånd och sönderfaller snabbt till grundtillståndet genom att sända ut en kaskad av diskreta gammakvanta med energier som är specifika för varje kärna. De hu- vudsakliga detektorsystemen som användes vid experimentet var JUROGAM II (gammaspektrometer placerad vid strålmålet), UoYTube (detektor för lätta lad- dade partiklar placerad mellan strålmålet och JUROGAM II), RITU (gasfylld rekylseparator) och GREAT (rekylspektrometer vid separatorns fokalplan).
I en preliminär analys av data som erhölls vid experimentet visade det sig att instrumentets känslighet för detektion av gammasönderfall från 102Sn inte var tillräckligt stor för identifiering av dess gammaövergångar. I denna rapport användes metoden att skapa en tredimensionell kub med gammaenergier längs de tre axlarna från trippelkoincidenta gammakvanta detekterade i JUROGAM II. Till att börja med uppskattades intensitetsfördelningen av de producerade slutkärnorna från datan lagrad i den tredimensionella kuben. 102Ag var den vanligast förekommande slutkärnan. Vid fortsatt analys av gammaenergierna lagrade i en tredimensionell kub upptäcktes 3 nya energinivåer och 7 nya över- gångar i102Ag. Effektiviteten för att detektera en proton i UoYTube bestämdes också. Även andra reaktionskanaler undersöktes men dessa var för svaga för att upptäcka några nya övergångar.
Contents
1 Introduction 1
1.1 Background . . . 1
1.2 Problem . . . 1
1.3 Goal . . . 2
2 Nuclear models 2 2.1 Properties of rotational bands . . . 3
3 Gamma-ray spectroscopy 4 3.1 Gamma decay. . . 5
3.2 Level schemes . . . 5
3.3 Double and triple coincidences . . . 5
4 JR126 Experimental Setup 6 4.1 JUROGAM II. . . 7
4.2 RITU and GREAT . . . 8
4.3 UoYTube . . . 8
5 Methods and Data Analysis 9 5.1 GRAIN . . . 9
5.2 Radware . . . 9
5.3 UoYTube efficiency . . . 13
6 Results and Discussion 14 6.1 Choice of trigger . . . 14
6.2 Relative intensities . . . 15
6.3 UoYTube detection efficiency . . . 16
6.4 Search for transitions in101In and103Sn . . . 19
6.5 Analysis of102Ag . . . 21
6.6 Rotational properties of band B3 in 102Ag . . . 28
7 Conclusion and Outlook 29
8 Appendix A - Calculation of UoYTube efficiency 35
1 Introduction
1.1 Background
During two weeks in the beginning of 2014 an experiment known as JR126 was performed at the Accelerator Laboratory of the University of Jyväskylä (JYFL) in Finland. The main goal of the experiment was to discover new energy levels and γ-ray transitions in the nucleus102Sn, for which just a few levels and transitions are known until now [1]. The nucleus102Sn and its neighbors on the proton-rich side of the nuclear landscape is a region of large interests, since it is located close to the doubly magic nucleus100Sn, which features specific nuclear structure characteristics [2].
In the JR126 experiment, beams and targets of various types and energies were tried. Most of the data were collected with a 47Ti beam bombarding a thin foil of58Ni to obtain the compound nucleus 105Sn via fusion reactions.
The compound nucleus decays instantaneously via emission of protons, neutrons and α particles, and creates a variety of different final product nuclei. Since
105Sn is located far out on the proton-rich side of the valley of stable nuclei, the cross section for 105Sn to decay through proton emission dominates over α particle and especially over neutron emission. The final product nuclei, also known as recoil nuclei or evaporation residues, are created in excited states and decay promptly to the ground state by emitting cascades of γ-ray quanta with specific discrete energies. These γ rays were detected by the γ-ray spectrometer JUROGAM II, which was located around the target. The recoil nuclei were, after transportation through the gas-filled recoil separator RITU [3], detected in a focal plane detector. The time between the detection in JUROGAM II and the detection of recoil nuclei in the focal plane for an event of γ rays in coincidence was in average 0.43 µs.
To observe γ rays from102Sn, which has a known isomer at state 6+ with a half life of 0.8 (3) µs [1], three γ-ray detectors of the GREAT spectrometer were placed around the focal plane, to detect delayed γ rays in coincidence with prompt γ rays in JUROGAM II. Unfortunately it was not possible to see any γ rays from102Sn, because of too much random γ rays and too low cross section for the reaction channel.
Although the data from the experiment was not good enough to study102Sn, new nuclear structure data could potentially be observed in some of the nuclei produced in the stronger reaction channels e.g. 103Sn, 101In, 102Cd, 101,102Ag and96Pd.
1.2 Problem
In γ-ray spectroscopy experiments quite commonly the resulting energy spectra are complex with many overlapping peaks. A common method to clean up the γ-ray spectra is to detect at least two γ rays in coincidence and create a two dimensional matrix with the γ-ray energies on the two axes. A narrow window, overlapping with the energy of the γ-ray peak of interest, can be set on one of the axes and a spectrum with counts in this window can be projected on to the other axis. This is called gating or slicing and is an efficient method to sort out undesired and background counts and to build level schemes of the nuclei.
Usually, many more events in the dataset are needed to analyze a matrix than
a singles spectrum based on detection of only one γ ray.
In the data from JR126 a large number of nuclei were created with many characteristic γ-ray energies. The γ-ray spectrum consisted of peaks almost everywhere. Even with a γ-matrix it was hard to distinguish different peaks, since quite frequently different nuclei had peaks with the same energies. In order to improve the sensitivity, a γ-cube can be created, by selecting events where at least three γ rays were detected in coincidence. With a γ-cube a double-gate can be set at two energies on two of the axes with a spectrum projected down on the third axis. This procedure cleans up the spectra to a large extent. However, considerably more events are needed to analyze a γ-cube than a γ-matrix.
In the JR126 dataset, some reaction channels were very strong, in particular the ones associated with emission of 2-4 protons. To reduce the contribution of these channels in the γ-ray spectra, the UoYTube detector was used as a charged particle veto detector. However, a high efficiency of UoYTube is required for this to be useful. If the data can be cleaned in this way, it may be possible to obtain new information about nuclides in the100Sn region produced in this experiment.
1.3 Goal
The main purpose of this thesis is to contribute to the understanding of the structure of nuclei in the region of the doubly magic nucleus 100Sn with 50 protons and 50 neutrons. The nuclei in this region are of great interest for current nuclear structure research [2]. The mentioned technique of constructing a γ-cube from triple coincidences will be used in the analysis. By taking double- gates on known transitions in the nuclei produced in the reactions, these nuclei can be investigated for new energy levels and transitions. Nuclei of particular interest are103Sn, where only five excited levels are known until now, as well as
101In,102Cd,101,102Ag and96Pd.
2 Nuclear models
It was early understood, from results showing that the nuclear binding energy is increased for even-even nuclei and that some proton and neutron numbers are magic, that the nucleus has an inner structure. Many different nuclear models exist that aim to describe this and other features of the nucleus. The shell model is, for nuclei with more than a few tens of nucleons, used to describe the inner structure of the nucleus. One of them is the shell model, which in its no-core approach has been very successful to describe the structure of light nuclei with up to a few tens of nucleons. For heavier nuclei, a core nucleus, usually with a magic number for the proton or neutron particle, is chosen in the shell model and the calculations are performed for a small number of valence particles or holes with respect to the core. In the shell model both protons and neutrons are considered to be placed in separate shells or orbits with specific angular momentum and parity. In each orbit only a certain number, depending on angular momentum and spin, of nucleons can fit according to Pauli exclusion principle.
Between two shells, which are energy levels, there is a gap in energy. The ground state of a nucleus is obtained when the protons and neutrons are in the
lowest energy shells. If a nucleon is moved up to a higher shell an excited state is formed with an excitation energy equal the energy difference between the two shells. When the nucleon returns from the higher to the lower shell a γ ray may be emitted with an energy corresponding to the difference in energy between the excited and ground state.
Single-particle excitation was the first considered way of exciting a nucleus.
However, results showing that some low and close lying excited states of nuclei (in the order of 0.1 MeV) were too small in energy to originate from single- particle excitations (in the order of MeV). This phenomenon was explained by states occurring from collective motions of the nucleons in nuclei with spherical or deformed shapes. Such states, originating from collective motion, build up vibrational and rotational bands of energy levels. [4] [5]
2.1 Properties of rotational bands
The nuclei in the region close to 100Sn are well described by the shell model.
With increasing number of particles or holes from the closed shell Z = N = 50, collective structures appear. An example of this is 102Ag (Z = 47, N = 55) in which rotational structures have been observed [6]. Since rotation of a spherical symmetric nucleus cannot be observed, a deformed nucleus is required to give rise to collective rotational excitations. Deformed stable nuclei are mostly found for nuclei with unfilled shells between the magic numbers [7].
Figure 1: Angular momentum vectors and axis notations for a deformed nucleus with a prolate shape.
Consider a prolate, rugby-ball shaped, nucleus with the coordinate axes as in Fig.1. Since this shape is symmetric around the z axis, only rotations around the x axis (considered here) and the y axis can give rise to states due to collective motion.
In Fig. 1, ~I is the total nuclear angular momentum, ~Ix the rotational angular momentum, ~K the projection of ~I (or ~J ) on the z axis and ~J the angular momentum not coming from the rotation. J can for example originate from~ vibrations, odd nucleons outside a closed shell or pair breaking of two nucleons with angular momenta ~j1and ~j2. The summed component ~J in the x direction is called the alignment ix. If ~J consists of n nucleons then ~J =P
n~jn and ix=X
n
~jn· x (1)
is the alignment. The total nuclear alignment ~Ixis the projection of ~I onto the rotation axis. The vectors ~I, ~Ixand ~J have to fulfill the relation
I = ~~ Ix+ ~J (2)
The energy of a rotating nucleus is given by [5,8]
E(I) = ~2
2J(~I − ~J )2 (3)
where J is the moment of inertia for rotations around the x axis. According to Eq.3, different energy levels arise for different I. Such energy levels build up a rotational band. Eq.3 leads to the Routhian [8]
Eω(I) = E(I) − ωIx (4)
which is the energy in the rotational frame. Also the kinetic moment of inertia is a quantity of interest for rotational bands. The kinetic moment of inertia is given as
J = ~I
ω (5)
here assuming I ≈ Ix [9].
3 Gamma-ray spectroscopy
Gamma-ray spectroscopy is a technique to measure energy spectra of γ rays. It is a widely used method to deduce the structure of nuclei. The most common type of γ-ray detector used is the high-purity germanium (HPGe) detector which has an excellent energy resolution.
Three energy dependent processes occur for γ rays incident on a γ-ray de- tector. At energies less than a few hundred keV photoelectric absorption dom- inates in germanium. For energies between a few hundred keV up to several MeV, Compton scattering is dominant. At higher energies pair production is the dominant process [10]. Photoelectric absorption is the preferred process since all the energy transfer from the γ ray to an electron, which deposits all its energy in the detector. However, in the energy range of the γ rays in this experiment, Compton scattering followed by photoelectric absorption was the dominant process in germanium.
The JUROGAM II spectrometer consists of a large number of HPGe de- tectors. However, in some experiments the timing is more important than the energy resolution. In such cases various fast scintillators are used. In other words, the choice of detector depends on the type of experiment.
3.1 Gamma decay
According to the already mentioned nuclear shell model, the nucleus is built up by orbits of nucleons. The nucleus always strives to be in the lowest energy state, the ground state. Hence, a nucleus in an excited state can decay by emitting a γ ray with an energy equal to the energy difference between the initial and final state, if the small recoil energy is neglected. For a state at high excitation energy the γ-ray decay proceeds usually via several intermediate states, which leads to an emission of cascades of γ rays.
Most often the γ rays are emitted promptly (in less than 10−12 seconds), however some γ rays can be delayed. These delayed γ rays are due to isomeric states, which are long-lived excited states, with half lives of about 10−9 sec- onds or longer [11]. The probability for a γ-ray transition to occur depends on the quantum numbers of the initial and final state. Gamma radiation is elec- tromagnetic radiation and originates either from variations of an electric or a magnetic field. From momentum and parity conservation of the electromagnetic interaction the multipolarity of the transition can be deduced.
The probabilities are given from the selection rules of γ transitions [12]. For an emitted γ ray with angular momentum L = 1 the transition is of dipole type, L = 2 of quadrupole type and so on, where L = 0 is forbidden. The conservation of parity defines if it is of electric or magnetic multipolarity type since they transform differently under parity.
3.2 Level schemes
From discovered transitions and energy levels, a level scheme of the nucleus can be constructed. Energy levels are marked as horizontal lines and arrows between the levels denote the γ-ray transitions. In the level scheme of a nucleus all paths lead down to the ground state. From a level scheme one can easily see which transitions are in coincidence with each other. The lowest energy state for a specific spin is called the yrast state. In a decay of a high-spin state the yrast and near-yrast states are the ones usually observed in the experiments.
3.3 Double and triple coincidences
A nucleus created in a fusion-evaporation reaction is formed at a high-spin state.
The nucleus deexcites to the ground state by emitting a cascade of γ rays. If multiple γ-ray detectors are used in an experiment, then several of the γ rays in a cascade can be detected in coincidence, i.e. simultaneously within a narrow time window. If two γ rays are detected simultaneously then a γ-matrix can be constructed with the γ-ray energies along the axes. A gate can be obtained in the two dimensional matrix by selecting a narrow energy interval on one axis and within this interval the values on the other axis are projected down to the second axis. By setting a gate, a coincidence spectrum for specific energies is obtained. This can be done for higher dimensions in the same way. In this thesis a three dimensional γ-cube is constructed from three coincident γ-ray transitions.
In the JR126 experiment, 39 different γ-ray detectors were used and hence theoretically 39 γ rays could be detected simultaneously. In practice it happens that more than three γ rays are detected simultaneously, but in this experiment
not more than 12 were detected at the same time. However, an event with four γ rays cannot be put into the cube. Instead there are four ways to select three γ rays out of four. It is here called the increment and more generally it is given by the binomial formula
N =n d
(6) where N is the number of increments, n the number of γ rays detected in an event and d the dimension of the object to be constructed (3 for a cube and 2 for a matrix).
4 JR126 Experimental Setup
Figure 2: Overview of the JR126 experiment, with the target chamber, the quadrupole and dipole magnets and detector chamber of RITU. Not shown in the figure are the JUROGAM II γ-ray spectrometer and the UoYTube charged particle detector, which are located around the target chamber, and the GREAT spectrometer with various recoil, γ-ray and electron detectors that are located around the detector chamber. [13]
A58Ni target of two different thicknesses was bombarded by a47Ti beam with velocities and energies shown in Table 1. The target was located in a target chamber in the center of JUROGAM II. Inside the target chamber the CsI- detector UoYTube was located. The recoil nuclei flew into RITU and were stopped in the focal plane surrounded by the GREAT spectrometer.
Runs Elab [MeV] β Target thickness Size of raw data
[mg/cm2] [GB]
36-43 157 0.0344 0.5 124
44-46 167 0.0325 0.5 112
64-74 153 0.0346 0.5 329
75-88 153 0.0322 0.75 91
89-112 157 0.0335 0.75 509
Table 1: Selected data sete (also known as runs) used in the analysis of the present work. β is the veloocity of the incoming beam in units of the speed of light, c.
4.1 JUROGAM II
JUROGAM II was the γ-ray spectrometer located around the target in the ex- periment. It consisted of 39 high purity germanium (HPGe) detectors, divided into 15 tapered detectors (one HPGe crystal) and 24 clover detectors (composite detector with 4 HPGe crystals) provided by the Gamma Pool of EUROBALL equipment. All detectors were surrounded by bismuth germenate (BGO) Comp- ton suppression shields.
In Fig. 3 a schematic view of the setup around the target is shown. The detector angles were calculated with the incident beam pointing in the direction 0◦. The JUROGAM II detectors were uniformly distributed in rings at four angles around the beam axis as illustrated schematically in Fig. 3. Five tapered detectors (T01 to T05) were located at 157.6◦, nine tapered detectors (T06 to T15) at 133.57◦, 12 clover detectors (Q01 to Q12) at 104.5◦ and 12 clover detectors (Q13 to Q24) 75.5◦.
Figure 3: Schematic view of the setup around the target chamber. The angles of the JUROGAM II detectors (T = tapered, Q = clover) with respect to the beam direction are shown as well as the UoYTube detector, which is located inside the taget chamber. Red wavy lines indicate γ rays. Green arrows indicate protons and α particles. The drawing is not to scale.
4.2 RITU and GREAT
The recoil ion transport separator (RITU) [3] transported the recoil products from the target chamber to the gamma recoil electron alpha tagging (GREAT) spectrometer [14], placed around the detector chamber in Fig. 2. RITU, consisted of three quadrupole magnets and one dipole magnet in the order Q1DQ2Q3 (Fig. 2) and was constructed for heavy-ion separation with a sepa- ration efficiency of around 25% for 210,211Ac [3]. Since thin targets were used in the experiment a majority of the beam ions passed through the target with- out any collision. The dipole magnet had an interior bending angle of 25◦ to separate fusion evaporation products (recoil ions) from beam ions. The recoil ions were then transported through two quadrupole magnets to focus the recoils before they entered GREAT.
The GREAT spectrometer included a variety of detectors: multi wire pro- portional counter (MWPC) and double-sided silicon strip detectors (DSSSD) for recoil detection, HPGe planar and clover detectors for X- and γ-ray detection and silicon PIN diodes for detection of conversion electrons. The main aim of the detectors for γ rays and electrons was to identify the isomeric transitions in
102Sn. In the present work, GREAT was only used as an event trigger for fusion- evaporation events by saving a time stamp (MWPC) and the energy deposition (DSSSD) of the recoil ions.
4.3 UoYTube
The University of York Tube (UoYTube) is a six sided detector for charged particles consisting of 96 CsI-detectors [15]. As seen in Fig. 3, UoYTube was
placed inside the target chamber. UoYTube detects light charged particles, mainly protons and α particles. In the version of the detector used in the present experiment, it was not possible to distinguish between detection of different types of charged particles. UoYTube could therefore be used either as a charged particle veto detector or as a detector to gate on a specific number of detected particles.
In this experiment UoYTube was used as a rough reaction channel selec- tor. For example in order to observe 101In (1p3n channel), only events when 1 particle was detected in UoYTube were considered. The detection efficiency of UoYTube is an important parameter for this selection process. If a UoYTube detector is triggered by noise or random events then more charged particles can be registered than were emitted by the compound nucleus. The probability of this was here called the leakage and was also investigated in this report.
5 Methods and Data Analysis
5.1 GRAIN
The data set (runs) used in this analysis are listed in Table 1. The collected raw data was sorted into events to be analyzed. The sorting was done by the Java based program GRAIN [16]. A time window and the detectors to trigger on were selected with a configuration file. In this analysis the recoil detection in the DSSSD detectors of GREAT were used as the event trigger. Since the recoils were detected in GREAT after the γ rays of the corresponding nuclei were detected in JUROGAM II, the time window stretches from 620 ns before the recoil was detected until the time of the recoil detection. In the sorting, a user written file with Java code was used [17]. The code adjusted the gains of all HPGe and UoYTube detectors, performed addback of Compton scattering between the four crystals of each clover detector and corrected for the Doppler shifted γ rays. The user code was compiled to run with GRAIN and it produced for the different detectors various energy and time histograms, stored in a file with extension aida (used by GRAIN). The user code was modified to suit the aim of the analysis done in this work, namely to select events with at least three coincident γ rays detected in JUROGAM II. Such events were saved to a list mode file (event-by-event file) with the energies calibrated to 0.5 keV/ADC- channel.
In GRAIN different detectors can be used as triggers. A test by using only JUROGAM II as an event trigger (no requirement of detecting a recoil in the DSSSD) was also performed in this work.
5.2 Radware
From the list file produced by GRAIN a γ-cube was created with incub8r, which is a program in the Radware software package [18]. For events with more than three γ rays, incub8r uses Eq. 6 during the γ-cube construction to obtain all combinations (increments) with three γ rays. After the cube was created with incub8r, the further analysis was done with the Radware level scheme builder program levit8r [18]. To perform an analysis in levit8r a cube (incub8r), parameters of an efficiency calibration, 1D background subtraction spectrum, a
level scheme file and parameters of FWHM (Full Width Half Maximum). An efficiency calibration of JUROGAM II was obtained in the following way. Ex- perimental values of the peak efficiencies were obtained from calibration runs with 133Ba and 152Eu sources placed in the target position and with UoY- Tube mounted inside the target chamber [19]. This efficiency calibration was performed for an other experiment than JR126 [19], but it was judged to be good enough for the present work. The following function was used for a fit to the measured efficiency data points for all JUROGAM II detectors summed together:
ln() = ((A + Bx + Cx2)−G+ (D + Ey + F y2)−G)−1/G (7) Here, is the relative peak efficiency of JUROGAM II, x = ln (Eγ/100), y = ln (Eγ/1000) and Eγ is the γ-ray energy in keV. Six of the seven parameters in the function were free during the fit (A, B, D, E, F , G), while C was kept fixed with a value of 0. The fitting was performed in MATLAB by using this function
ln() = A + Bx (8)
or Eγ values smaller than 240 keV and this function
ln() = D + Ey + F y2 (9)
for Eγ values larger than or equal to 240 keV. Numerical values of A-F obtained from Eq. 8 and 9 were inserted into Eq. 7 to fit G. The numerical values of the fitted parameters were A = 0.115, B = 2.62, D = 1.38, E = −0.482, F = 0.0161 and G = 6.00, which were used as input values for levit8r. A plot of the measured efficiency data points and of the function using these parameters is shown in Fig. 7.
0 500 1000 1500
Relative peak efficiency 0
1 2 3 4 5 6 7 8
Gamma-ray energy (keV)
Data points Fitted curve
Figure 4: Relative peak efficiency versus γ-ray energy for JUROGAM II.
The levit8r cube is created, to optimize the size, with a non-linear energy calibration with fixed FWHM. Hence a FWHM-calibration has to be performed to obtain linear energy calibration in the cube. The parameters f , g and h are given by
F W HM =p
f2+ g2x + h2x2 (10) where x = ch/1000 is the channel number divided by 1000 [18]. The width was obtained from fitted peaks in gf3 and then fitted to f , g and h in MATLAB.
0 0.5 1 1.5 2 2.5
x (ch./1000) 4
5 6 7 8 9 10 11 12 13 14
FWHM
Fitted curve Data points
Figure 5: FWHM versus x for JUROGAM II.
The numerical values of the parameters obtained was f = 4.469, g = 3.011 and h = 4.896.
The background spectrum, required as input to levit8r, was created by using the command bg in the Radware program gf3. The background was drawn under the one-dimensional γ-ray spectrum obtained as the total projection of the γ-cube.
Figure 6: Background subtraction of projection spectrum. All counts below the red line are subtracted.
A level scheme file is needed to run levit8r. One can either start by creating a new empty file or by using an existing one, downloaded for example from the Radware website [20].
The analysis of the γ-cube is performed in levit8r by setting single or double gates on peaks and by checking for coincident peaks in the gated spectra.
When found, new levels and γ-ray transitions can be added to the level scheme.
A fit of the coincidence intensities in the γ-cube can then be done by using the level scheme as a model. In the spectrum display of levit8r, measured and fitted γ-ray spectra and their residuals are shown. One can choose to display either the total projection spectrum, individual or sums of single or double gated spectrum. This is very useful for building of the level scheme.
Figure 7: Screenshot of a run in levit8r.
The result of the fitting procedure had to be taken with caution since some- times negative (unphysical) intensities were obtained. Negative intensities may occur if two or more coincident transitions are located close in energy. In this case, one of the transitions may obtain an intensity that is too high which is compensated by a negative intensity of one of the other transitions. The back- ground subtraction and the efficiency calibration, which never are perfect, may also effect the fitted intensities. Gamma-rays that do not feed other energy levels and are the only transitions from the initial level can not be fitted, since they have no coincidences that depend on their intensity [18].
5.3 UoYTube efficiency
The probability PkK() to detect k of K emitted particles using a detector with an efficiency to detect one particle is described by the well known binomial distribution
PkK() =K k
k(1 − )K−k (11)
For UoYTube, is different for protons and α particles.
In the data from JR126 experiment, peaks from 102Ag (3p channel) could be observed in γ-ray spectra gated by detection of 4 particles in UoYTube.
Hence, in addition to the 3 protons emitted in the evaporation reaction one random particle was detected, in this report denoted as the leakage x. Here it is assumed that the probability of detecting random particles was low enough for only taking into account terms of first order in x, which later would be seen to be a valid assumption. Eq. 11 is only valid without any leakage and has to be improved, as explained in the next paragraph.
By using a reaction channel that only can be reached by proton emission, e.g.
102Ag (3p) or102Cd (2p1n), the probability to detect 0 particles in UoYTube is a product of the probability to detect none of the K emitted protons, which according to Eq. 11 is P0K() = (1 − )K, and the probability to detect zero
of one random particle, which by using the binomial distribution formula is (1 − x). This gives the following equation for the detection of zero particles of K emitted, with the effect of detection of random particles included:
P0K(, x) = (1 − )K(1 − x) (12) The probability for UoYTube to register 1 particle of K emitted is the sum of the probability to detect 0 actual protons together with one random particle, (1 − )Kx, and the probability to detect one actual proton together with zero random particles, K1(1 − )K−1(1 − x):
P1K(, x) = (1 − )Kx +K 1
(1 − )K−1(1 − x) (13) By combining Eq. 12and13the leakage x and efficiency can be obtained from
P1K = (1 − )K− P0K+ K P0K
(1 − ) (14)
x = 1 − P0K
(1 − )K (15)
Eq. 14 can be solved numerically for and this value can then be used in Eq.
15to obtain x.
6 Results and Discussion
6.1 Choice of trigger
A comparison of using an event trigger, which required only the detection of 3 γ rays in JUROGAM II, with a trigger which in addition to the 3 γ rays required a detection of a recoil in the focal plane was performed. In Fig. 8aa spectrum produced with the recoil triggered triple γ-ray events is compared to a spectrum produced by triggering only on 3 γ rays in Fig. 8b. The number of events is 8.3 times larger and the number of increments 6.o times larger in the spectrum that did not require a recoil detection. It is clearly seen in Fig. 8 that the energy resolution is much worse in Fig. 8b.
The reason for the broader peaks in the JUROGAM-only triggered spectrum can be explained by the acceptance angle of RITU. When the events required a detection of recoils by GREAT, reactions in which the recoils entered RITU at a larger angle than the acceptance angle, were rejected. When the JUROGAM- only trigger was used then all γ rays were accepted even if the recoil angle was larger than the acceptance angle. If the recoil angle was larger than the acceptance angle of RITU, the transversal momentum spread of the recoils was increased. This increase led to a larger Doppler broadening of the γ rays because the Doppler correction was done by assuming that all recoils moved along the beam axis at 0◦.
200 600 1000 1400
E (keV)
5 15 25 35
Counts ( 10
6)
0.5 1.5 2.5 3.5 4.5
141 157
196 241 249 275304 349 399 466 484 593 636 776 861 909 1291 1368
504
x
b) Trigger:
a) Trigger: DSSSD
Figure 8: Comparison of spectra projected from the γ-cubes using two different event triggers. The spectra shown are without any background subtraction.
Thus, it was concluded that only recoil triggered events were useful for the proceeding analysis done in this work.
6.2 Relative intensities
The relative intensities of all reaction channels observed in the experiment were estimated by using the intensities of transitions fitted with levit8r for the different reaction channels. Since the fitting procedure in levit8r can have intensities of large uncertainties, as a complementation a comparison of the counts in peak areas of some dominant peaks in the background subtracted projection spectrum of the γ-cube was also performed. All intensities in Table2 were normalized to 100 for102Ag. Since the intensities in Table2were deduced from estimates obtained from both the fitting of level schemes in levit8r and comparison of area counts of some peaks there is no distinct method of obtaining the uncertainties of the intensities.
Channel Relative intensity
102Ag (3p) 100
101Ag (3p1n) 68
102Cd (2p1n) 55
99Pd (1α2p/4p2n) 39
98Pd (4p3n/1α2p1n) 23
101Pd (4p) 12
100Ag (1α1p/3p2n) 11
101Cd (1α/2p2n) 9
103Cd (2p) 5
100Pd (4p1n) 4
99Ag (1α1p2n/3p4n) 3
103In (1p1n) 1
102In (1p2n) 0.3
Table 2: Relative intensities of all observed reaction channels in the experiment.
102Ag was normalized to 100.
It was observed that the strongest channels were the ones with emission of several protons. This was reasonable since 105Sn is located far out on the proton-rich side of the stability valley. Emission of 3-7 particles was favoured.
Only two reaction channels with 2 particles emitted (103Cd and 103In) were observed, with low intensities. The number of particles emitted depends mainly on the incoming beam energy. With decreased beam energy, channels with 1-2 emitted particles might be favored. On the other hand the total reaction cross section decreases quickly at lower energies.
6.3 UoYTube detection efficiency
In Fig. 9, a time spectrum showing the JUROGAM II time stamp minus the UoYTube time stamp is displayed. The narrow peak in the spectrum, with a maximum close to 108 counts/10 ns, was due to true coincidences between charged particles detected in UoYTube and γ rays detected in JUROGAM II, originating from the same reaction event. The horizontal background was due to random detection of γ rays and charged particles. It covers the whole time window displayed with a roughly constant value of 106 counts/10 ns. The random events, x in Eq. 15, give rise to an undesired increase of the number of detected charged particles per event. This means that a γ ray belonging to the 2p reaction channel could be detected in an event in which 3 particles were detected, due to the detection of a random particle in UoYTube.
Figure 9: Time spectrum obtained with GRAIN, showing the JUROGAM II time stamp minus the UoYTube time stamp on the x axis and the number of counts per 10 ns on the y axis.
In order to select the true coincidence events, and as few random events as possible, a gate on the energy-time spectrum as shown in Fig. 10was used. The two gates marked in Fig. 10were tested separately with the resulting efficiencies and leakages displayed in Table3. Both the efficiency (solved numerically from Eq. 14) and leakage (Eq. 15) are important parameters when using UoYTube either as a veto detector or as a channel selection detector. The rather low proton detection efficiency gave only a probability of 29.7% to detect 3 out of 3 emitted protons with gate 2 for example. Thus the 3p- and 4p-channels were dominantly found in lower p-channels. Also the 0p- and 1p-channels were affected by the low efficiency since higher channels were mixing with these channels. The proton detection efficiency prevented the UoYTube detector to be used as an efficient veto detector to search for traces of for example103Sn and101In.
Figure 10: Energy-Time spectrum of UoYTube detector from GRAIN, with the two event selection gates. The colormap is in logarithmic scale. UoYTube time is referring to the time of γ-ray detection in JUROGAM II time minus time of charged particle detection in UoYTube.
The counting of random events, leakage, was also a problem with UoYTube.
For gate 1 in Fig. 10 it is seen in Table 3 that the leakage was about 14%.
By decreasing the gate (gate 2), the leakage was reduced to 9.6% for 102Ag.
However, by constraining the gates the efficiency decreased. In Table 3 the intensities of the following three strong peaks were extracted to deduce the proton efficiency and leakage of UoYTube: 275, 304 and 466 keV for102Ag and 368, 592 and 776 keV 102Cd. Since, in this experiment, 102Ag was a stronger reaction channel than 102Cd, the statistics was better and the uncertainties lower for 102Ag than for 102Cd. The calculations leading to the results shown in Table3are given in Appendix A.
An attempt, to determine α particle detection efficiency α of UoYTube using the reaction channels 101Cd (1α) and 99Pd (1α2p), was also made. The strongest peaks in101Cd and99Pd were, however, too weak to give any reliable results of α.
Nr. of charged particles Gate 1 Gate 1 Gate 2 Gate 2 detected in UoYTube 102Ag 102Cd 102Ag 102Cd
k = 0 0.0304(7) 0.0838(16) 0.0346(8) 0.0939(18)
k = 1 0.192(4) 0.372(7) 0.208(4) 0.396(8)
k = 2 0.404(6) 0.436(9) 0.419(7) 0.435(9)
k = 3 0.309(4) 0.0863(34) 0.297(4) 0.0636(30) k = 4 0.0554(8) 0.0225(8) 0.0367(6) 0.0125(5) k = 5 0.0156(2) 0.00700(25) 0.00804(13) 0.00370(14)
P 0.672(8) 0.677(7) 0.663(7) 0.670(8)
x 0.139(64) 0.197(47) 0.096(61) 0.138(50)
Table 3: Proton detection efficiencies, pand leakages, x, for UoYTube obtained in the present work. Values on row 3-8 and column 2-5 are the experimental determined values of PkK(, x) for 102Ag and 102Cd computed from summed counts in peak areas in levit8r (See Appendix A). Only k = 0, 1 are used to compute P and x.
6.4 Search for transitions in
101In and
103Sn
It is clearly seen in Fig. 11aand11bthat in the γ-ray spectra obtained by setting double gates in the recoil triggered γ-cube on a pair of known transitions in103Sn and 101In, no evidences for reaction channels leading to 103Sn and 101In could be observed. In Fig. 11c, a γ-ray spectrum with double-gates on two known transitions on102In is shown. For this nucleus, which was the weakest reaction channel observed in this work, several of the other known transitions in 102In are clearly visible.
For the weakly populated reaction channels like103Sn the number of counts in the γ-cube was quite small. In order to increase the statistics a test that use a recoil triggered γ-matrix instead to search for transitions in 103Sn was performed. The γ-matrix that was used for this had a factor of about 3 times more events than the γ-cube.
-5 5 15
-5 5 15
100 300 500 700 900 1100
E (keV)
0 10 20 30 40
Counts
289289 298
578
187 359 387 1136
466
l l l l l l l l l l l l l l
l l l l l l
l l l l l l l l l l l l l l
l l l l l l
442
382376
316
250190 222 246
a) 103Sn
b) 101In
c) 102In
861592
517
Figure 11: Double gates set on the recoil triggered γ-cube. The dashed red lines indicate where the expected coincident transitions should occur in 103Sn and
101,102In. a) Gate on 168 keV and 1029 keV in103Sn [21], b) gate on 1310 keV and 342 keV in 101In [22] and c) gate on 145 keV and 1137 keV in 102In [23].
The expected energies according to102In level scheme are marked with red in c), while other visible peaks are marked in black: 187 (102Ag), 246 (101Ag), 466 keV (102Ag), 516 keV (101Ag), 593 keV (102Ag/102Cd) and 863 keV (101Ag/102Cd).
In Fig. 12, two spectra are shown that were obtained by setting single gates in the γ-matrix on known transitions in103Sn [21]. The red lines indicate
expected γ-ray transitions for 103Sn. As seen the expected γ rays of 103Sn were not observed in the gated spectra. Neither in the background subtracted spectra, not included in this report, any γ rays from103Sn were observed. The weakness of using a γ-matrix was that only a single-gate could be used. In this experiment there were numerous reaction channels which have peaks at the same energies and a single-gated spectrum becomes complex when the reaction channels mix. The 241 keV, 269 keV and 861 keV peaks, which are the strongest ones in Fig. 12, are due to transitions in102Cd and101Ag, respectively. Hence the γ-matrix was not usable for this type of analysis.
200 600 1000 1400
2 6 10 14 18
200 600 1000 1400
E (keV)
2 6 10 14
Counts ( 10
3)
168 289289 298298 578 13181029
578l
l l
l l l l
l l l l l
l l l
l l l
l l l l
l l l
l l
-matrix, gate on 168 keV
-matrix, gate on 1029 keV Expected 103Sn :s
---
a)
b)
241 861861
269241
Figure 12: Gamma-ray spectra obtained from the γ-matrix by setting single gates on the known transitions a) 168 keV and b) 1029 keV in 103Sn. Dashed red lines indicate expected coincident peaks of the other known transitions in
103Sn [21]. The spectra shown are not background subtracted.
6.5 Analysis of
102Ag
The nucleus 102Ag ,produced via emission of 3 protons, was the strongest re- action channel in the experiment (see Table 2). By analyzing the γ-cube some new energy levels and transitions in 102Ag could be identified, see Fig. 13 and Table 4. The proposed new levels and transitions found in this work were all based on the recently published paper on102Ag [6] in which the negative-parity rotational band B3 was observed up to E = 6457 keV and Iπ= 19−.
655
1292
637
483 986 503
466 770 304
488 955 467
350 748 398 157 (1360)
(1120)
969
792
817
555 (705)
(1427) (722) (760)
(1482)
18 5802
17 5165
16 4682
19 6457
(20 ) (7162)
15 4179
14 3713
13 3409
12 2921
11 2454
10 2104
9 1706
8 1549
(21 ) (7884)
(22 ) (8644)
102
Ag Band B3
Figure 13: Proposed level scheme of band B3 in102Ag. The energies of the new levels and transitions are given in parenthesis.
E(level) (keV) Iπ
7162 20−
7884 21−
8644 22−
Eγ (keV) Ei (keV) Spin assignment (Iiπ→ Ifπ)
705.4(0.4) 7162 20− → 19−
722.0(0.6) 7884 21− → 20−
760.3(0.4) 8644 22− → 21−
1120.3(0.9) 5802 18− → 16−
1359.7(0.6) 7162 20− → 18−
1427.0(0.6) 7884 21− → 19−
1482.2(0.6) 8643 22− → 20−
Table 4: The proposed new energy levels and transitions in 102Ag as obtained in the present work. Spins and parities are tentative. Uncertainties of Eγ were obtained from levit8r.
The proposed new levels were assumed to increase in spin units of 1~ and the γ-ray transitions were assumed to be of type M1 (∆I = 1) or E2 (∆I = 2).
The uncertainties of the relative transition intensities obtained from levit8r were too large and therefore not shown in Table4. Here follows a discussion of the assignment of the newly found levels and transitions in102Ag.
A peak at 1120 keV is seen in Fig. 14b,16aand16b, while it is not observed in Fig. 15aand 15b. In the 1120-503 keV double-gated spectrum in Fig. 14a, all transitions belonging to band B3 in102Ag [6] are observed as strong peaks, except 483 keV, 637 keV and 1292 keV. Hence, the 1120 keV γ ray is proposed to be an E2 transition between the known energy levels at (E = 5802 keVIπ= 18− and E = 4682keV Iπ= 16−).
200 400 600 800 1000 1200 1400 E (keV)
0.1 0.3 0.5 0.7 3Counts (10) 0.9
1050 1150 1250 1350 1450 0
40 80 120
1050 1150 1250 1350 1450 0
20 40
0.1 0.3 0.5
141141 157157 236236251260260 275275 304304 350350 466+467466+467 483+488488 517517 637 705 839839 860 955955 1120 13681368 14271427
665665655
a) Double-gate 1120 and 503 keV
b) Double-gate 503 and 655 keV
398398
Figure 14: a) Double-gated spectrum (1120 keV, 503 keV), b) double-gated spectrum (503 keV, 655 keV).
In Fig. 14b and 15a a peak at 705 keV is observed. From [6] there exists both a 705.1 keV transition (Iiπ = 8+ → Ifπ = 6−, Ei = 846 keV) and a 702.9 keV transition (Iiπ= 8− → Ifπ = 8+, Ei= 1549 keV) further down in the level scheme in coincidence with the B3 band. However, if these two peaks give rise to the 705 Kev peak seen in Fig. 14band15a, there should be a strong 705 keV peak also in Fig. 16b, which clearly is not the case. Also in Fig. 16a the 705 keV peak is observed to be in coincidence with 1368 keV. Also in Fig. 17b the 705 keV peak is clearly observed to be in coincidence with 1368 keV. Since none of the two already known transitions (702.9 keV, 705.1 keV) are in coincidence with 1368 keV, this strongly suggests a new 705 keV transition higher up in the level scheme. This led to the proposal of a new level at (E = 7162 keV, Iπ= 20−).
With this new energy level, a search for a possible 1360 keV E2 transition from this level to the level at (E = 5803 keV , Iπ = 18−). Signs of a 1360 keV peak can be hard to observe since there is a strong 1368 keV transition just below the B3 band. In Fig. 15b, which was gated on 483 keV and 637 keV, the 1360 keV and 1368 keV transitions are observed in a strong mixed peak, as well as all the transitions in the B3 band. Since 1360 keV occurs strongly in the 637 keV and 483 keV double-gated spectrum (Fig. 15b), hence the 1360 keV transition was proposed to be in band B3 between the levels at (E = 7162 keV,Iπ = 20− and E = 5802 keV,Iπ= 18−).
0.1 0.3 0.5
1050 1150 1250 1350 1450 0
20 40 60
1050 1150 1250 1350 1450 60
140 220
200 400 600 800 1000 1200 1400
E (keV) 0.2
1.0
3Counts (10) 1.8 141141157157 236+239236260260275275 304304 350350 398398 466+467466+467 488488503503 517517 655 705
705 722
839839 861861 955955 1078 13681360+1368 14271427
a) Double-gate 483 and 1292 keV
b) Double gate 483 and 637 keV
969969
Figure 15: a) Double-gated spectrum (483 keV, 1292 keV), b) double-gated spectrum (483 keV, 637 keV).
In Fig. 16aand15ba 722 keV peak is observed. According to [6] there is a 723.1 keV transition from level E = 3177 keV in band B4. However, the 723.1 keV transition is not very strong and it is not in coincidence with 517 keV, as in Fig 17a. By gating on 1360 keV and 722 keV in Fig. 17aall the dominant transitions in band B3 are observed except the 655 keV transition and a reduced 705 keV peak. With a new 722 keV γ-ray transition, a new energy level was proposed at Iπ = 21− and E = 7884 keV. As for the previous levels it was suspected that an E2 transition with an energy of about 1427 keV should exist.
In Fig. 14a and 14b a 1427 keV peak is observed. By gating on 1427 keV in Fig. 16ball dominant peaks of the B3 band are seen except 705 keV (reduced) and 722 keV. Especially 637 keV occurs strongly in Fig. 16b and a 1427 keV transition was proposed to be in close connection with this transition. Thus, the 1427 keV transition is placed between the new level at (E = 7884 keV, Iπ= 21−) and the level at (E = 6457 keV, Iπ = 19−).
0.5 1.5 2.5 3.5 4.5 5.5
1050 1150 1250 1350 1450 0
40 80
1050 1150 1250 1350 1450 0
20 40 60
200 400 600 800 1000 1200 1400
E (keV)
0.0 0.4 0.8 1.2 1.6 2.0 2.4
Counts (102) 141141 157157 236236 260260 275275 350350304304 466+467466+467 483+488483+488503503 517+521517 637637 666666 705705 760 839839 861861 876876 969969955955 986 11201120 13681368
a) Double-gate 655 and 705 keV
b) Double-gate 655 and 1427 keV
722 1482
398398
Figure 16: a) Double-gated spectrum (655 keV, 705 keV), b) double-gated spec- trum (655 keV, 1427 keV).
0.2 0.6 1.0 1.4 1.8 2.2 2.6
1050 1150 1250 1350 1450 0
10 20 30 40
1050 1150 1250 1350 1450 0
10 20 30 40
200 400 600 800 1000 1200 1400
E (keV)
0.5 1.5
2Counts (10) 2.5 141141 157157 236 260260275275 304304 350350 466+467466+467 483+488483+488 517517 555555 637637 705705 748 760 839839 955 969
11201120 1292 1368
503503 760
429429
a) Double-gate 1360 and 722 keV
b) Double-gate
1368 and 655 keV 1427
398398
Figure 17: a) Double-gated spectrum (1360 keV, 722 keV), b) double-gated spectrum (1368 keV, 655 keV).
In Fig. 17aa 760 keV peak is observed. In Fig. 18athe 760 keV transition is observed to be in coincidence with all peaks in band B3 below 1427 keV. Hence a new level Iπ = 22− at E = 8644 keV was proposed. As for the other B3 band levels, an E2 transition (22− → 20−) was expected to be observed with an energy of 1482 keV. In Fig. 16aa small 1482 keV peak is observed. In Fig.
18b all the dominant transitions of the B3 band are observed below the 19− level. The fact that both 760 keV and 1482 keV are observed and adds up to the energy of the 22− level, reinforces the proposal of this level.
0.2 0.6 1.0 1.4
1050 1150 1250 1350 1450 0
10 20
1050 1150 1250 1350 1450 0
10 20
200 400 600 800 1000 1200 1400
E (keV) 0.0
0.2 0.4 0.6 0.8
2Counts (10) 1.0 141141157157 260260275275 304304 350350 466+467466+467 483+488 483+488503503 517517 557 619637637 705 746 818 969 1120 12921292 1368
839 1368
a) Double-gate 1427 and 760 keV
b) Double-gate 705 and 1482 keV
1482
655655+665 722
398398
Figure 18: a) Double-gated spectrum (1427 keV, 760 keV), b) double-gated spectrum (705 keV, 1482 keV).
Rastikerdar [24] observed some transitions that were proposed to belong to
102Ag, but could not be placed in the level scheme. Some of those were the 1427 keV, 758 keV, 722 keV, 1484 keV and 1357 keV, which are similar in energy to five of the six new γ-transitions proposed in this work. All new levels and γ-transitions assigned to band B3 in 102Ag in this work are shown in Fig. 13 and Table4.
In ref. [6] a likely cascade of transitions with energies 428 keV, 553 keVm 484 keV and 488 keV feeding the 11−level in band B3 was mentioned. This cascade was also observed in the present work, but it could not be placed in the102Ag level scheme. In addition, there is a γ-ray transition with the energy 663 keV that definitely belongs to 102Ag. It is in strong coincidence with both 428 keV and 553 keV, but could not be placed in the level scheme.
No other new levels or transitions could be found in102Ag nor in any of the observed reaction channels.
6.6 Rotational properties of band B3 in 102Ag
In Fig. 19, five plots show the rotational properties according to Eq. 1-5 for the rotational band B3. From the present work the new transitions are the three last ones and the fifth from the end in Fig. 19. A smoothly increasing continuation of the alignments, kinetic moment of inertia (MOI), Routhian and excitation energy for the proposed new transitions is observed. Hence, band B3 just continues without any band crossings with the new proposed levels and transitions included.
At a rotational frequency of about 350 keV the characteristic S-shape curve can be seen in Fig. 19a and 19c. The phenomenon that gives rise to this is the so called backbending, which originates from the breaking of neutron or proton pairs in time reversed orbits. In ref. [25] and [26], the negative parity band B3 was assigned to be based on the πg9/2⊗ νh11/2 configuration. The backbending observed in this band in ref. [6] was assigned to be due to an alignment of a pair of g7/2 neutrons. The Routhian steadily decreases with increasing frequency and the kinetic MOI increases smoothly for the new data points in Fig. 19. A comparison with theoretical models, such as the tilted axis cranking model [27,28] as used in [6], could not be performed within the limits of the present work.
100 300 500 700
(keV) 1
3 5 7 9 11
Alignment, ix () -3.0 -2.0 -1.0 0.0 1.0
Routhian (MeV)
2 6 10 14 18 22
Ix ()
10 14 18 22 26
Spin ( )
16 20 24 28 32 36 40
Kinetic MOI (2/MeV)
1 3 5 7 9
Excitation energy (MeV)
a)
b)
c)
d)
e)
Figure 19: Plots of rotational properties (Eq. 1-5) of the negative-parity band B3 in 102Ag. Filled and unfilled squares denote transitions origin from levels with eve and odd spins, respectively.
7 Conclusion and Outlook
In this report a thorough analysis of data from an experiment performed at the nuclear research center JYFL in Finland has been presented. The compound nucleus 105Sn was created from fusion of a 47Ti beam and a 58Ni target. The compound nucleus decays through instantaneous evaporation of protons, neu- trons and α particles leading to a number of different residual nuclei. Gamma rays emitted promptly, when the fusion evaporation products decay from excited states to the ground state, were detected by the γ-ray spectrometer JUROGAM
