The level structure of 71Ge

2 I'E'I : I

Nuclear Physics A 1 4 3 (1970) 53 - - 6 4 ; (~) North-Holland Publishin# Co., Amsterdam

.A.I : 3.A I

Not to be reproduced by photoprint or microfilm without written permission from the pubfisher

T H E L E V E L S T R U C T U R E OF ~lGe J. G. M A L A N , J. W. T E P E L a n d J. A. M. D E V I L L I E R S

Atomic Energy Board, Pelindaba, Transvaal, Republic of South Africa t

Received 3 November 1969 Abstract: The level structure of 71Ge has been investigated by observing both neutrons and decay gamma rays produced in the reaction 71Ga(p, n),)VlGe at incident proton energies between 1.5 and 3.1 MeV. The excitation energies of a large number of levels below 2 MeV have been accurately determined. A level scheme has been constructed for 71Ge. Branching ratios and probable spin assignments for a number of levels are presented. E

I NUCLEAR REACTIONS 71Ga(p, n), (p, n~,), E = 1.5-3.1 MeV; measured a(E; E,, E, ,0). l E~, I~.. 71Ge deduced levels, J, ~, branching ratios. Enriched and natural targets. I 1. Introduction

The widespread use of Ge(Li) detectors has stimulated interest in the low-lying level structures of the germanium isotopes. Due to the interaction of neutrons with the detector material, a background consisting mainly of de-excitation ~-rays from the stable germanium isotopes is produced in most experiments at higher energies. The level structures of these isotopes have been investigated and are reasonably well known. Considerably less information is available on the remaining isotopes, in particular the isotope 7tGe. Previous investigations of 71Ge have been limited to studies of the fl÷ decay of 7tms and the 7°Ge(d, p) reaction. Only the ground and first two excited states at 174.8 and 197.8 keV are populated in the fl+ decay t). A few levels below an excitation of 2 MeV were seen in early (d, p) work 1). A recent experiment by Goldman 2) using a multigap spectrograph with photographic plate detection revealed a large number of levels in this energy region. Excitation energies were determined with an accuracy of 10-20 keV and In and J-values were assigned to several of the stronger transitions. We decided to study the 71Ga(p, n)71Ge reaction (Q = - 1 0 1 7 keV) by means of high-resolution neutron as well as ,/-ray spectroscopy in order to obtain information on the final nucleus in a different and independent manner. Apart from a threshold determination 3) no information has been published on the 71Ga(p, n) reaction. Preliminary results from the present study and from a 7°Ge(n, ~) capture experiment simultaneously performed at Pelindaba have been presented elsewhere 4). t Postal address: Private Bag 256, Pretoria, Republic of South Africa. 53

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2. Experimental procedure Targets prepared by evaporating either natural gallium oxide or 99.6 ~ enriched 71Ga203 onto 0.25 m m tantalum backings were exposed to the proton beam of the 3 MeV pulsed Van de Graaff accelerator at Pelindaba. The beam energy was known to an absolute accuracy of approximately 5 keV. The energy spread of the beam was less than 1 keV, which increased considerably when klystron bunching was used to produce an overall time resolution of ~ 2 ns for the neutron spectra. Neutrons were detected with a 10 cm diam. × 5 cm NE 102A plastic scintillator in a conventional time-of-flight arrangement utilizing flight paths of up to 3 m. Neutron time-of-flight spectra were obtained at several different angles for proton energies between 1.5 and 3.0 MeV in roughly 100 keV steps. Consequently, the Q-values corresponding to the observed neutron groups were deduced from a large number of measurements. A typical neutron spectrum is presented in fig. 1. The energy of the corresponding 71Ge level, as determined from these measurements, is indicated for each neutron group. De-excitation y-ray spectra were observed with a 30 cm 3 Ge(Li) detector by irradiating a natural gallium oxide target of 100 keV thickness in energy intervals o f 100 keV from 1.5 to 3.0 MeV. High resolution spectra (AEr = 3 keV at Ey = 1.33 MeV) were obtained at incident proton energies of 1578, 1650, 2172, 2300, 2700, 2900 and 3100 keV using a 40 cm 3 Ge(Li) detector and a pure 71Ga203 target of 40 keV thickness. A time-of-flight circuit was used to discriminate against ;:-rays produced by neutrons in the detectors and their surroundings. All spectra were recorded on a 4096-channel analyser with a dispersion of less than 1 keV per channel. A section of a typical spectrum is shown in fig. 2. Due to the extremely low cross section for this reaction at energies below 3 MeV it was not feasible to perform coincidence measurements between neutrons and y-rays associated with selected single levels in 71Ge. At an incident proton energy of 2.8 MeV a standard fast-slow coincidence arrangement was used only to identify the y-rays associated with the neutron flux from the (p, n) reaction.

3. Result and discussion A summary of the gamma-ray data collected at various proton energies is given in table 1. It contains all the g a m m a transitions which were observed in at least two separate measurements at different energies. The accuracy is estimated to be better than I keV, except for very weak lines where uncertainties of up to 2 keV prevail. The relative intensities were determined at 2.9 MeV with the 40 cm a Ge(Li) detector and have been corrected for the detector efficiency. Below 250 keV the detector efficiency was unknown. The intensities have been normalised so that ly = 100 for the 831.2 keV line. Transitions found to be in coincidence with neutrons from the (p, n) reaction have been marked with the symbol n. The fairly low sensitivity of the coincidence arrangement precluded in most cases the detection of lines with relative

?I~LEVEL$

57

intensities lower than 30. Lines which only appeared in spectra from the natural target have been omitted from the list. The lowest incident proton energy Eo at which a particular line was observed is given in the table. The real threshold may be somewhat lower than Eo depending on the strength of the line and the spacing of the measurements. The assignment of de-excitation y-rays to particular levels in 71Ge was based mainly on the results from the neutron time-of-flight experiments. Transitions given in brackets in table 1 could not be uniquely assigned or were very weak lines in the g a m m a spectra. For only a small number of lines no assignments were made. At an incident proton energy of 3.1 MeV a number of mostly weak additional lines appeared having energies of 199.1,485.9, 728.0, 1084.4, 1191.5, 1278.1, 1762.7, 1937.8, 1965.4 and 1982.4 keV. The 1965.4 keV line is associated with a level at 1965 keV seen in the neutron capture experiment 4). The lines at 1191.5, 1762.7 and 1937.8 keV seem to originate from a level at 1938.0 keV. A compilation of energy levels observed in different reactions leading to 71Ge is given in table 2. Allowing for a 10 to 20 keV systematic shift in level energies, the (d, p) work of Goldman 2) compares fairly well with the present results and a one to one correspondence of levels exists in certain regions of excitation. Neither neutrons nor ),-rays were observed from levels at 0.62, 0.89 and 0.97 MeV which were clearly observed in the stripping experiment z). A neutron group corresponding to a level at 0.62 MeV in 71Ge would have overlapped in some spectra with a peak from the 37C1(p,no)37A reaction (fig. 1). No trace of a neutron group from 71Ge is visible, however, at angles where the two groups should be kinematically separated. The levels at 1291, 1547, 1633 and 1749 keV observed in the (p, n) experiment have no (d, p) counterparts. The energies of those levels populated in the (n, y) experiment compare quite well with the present (p, n)') results. The level energies deduced from the neutron time-of-flight measurements are usually slightly higher than the corresponding values from the g a m m a transitions. This discrepancy is probably due to time-slewing. Due to the very small cross sections involved at low proton energies the 175 and 198 keV levels were not separated in the neutron time-of-flight spectra. A level at 1204.1 keV has been postulated to accommodate decay y-rays of 373.1, 615.3 and 679.8 keV. This is consistent with the neutron time-of-flight work where the neutron group corresponding to a 1211 keV level is particularly strong and slightly broadened. Furthermore, the relative intensities of these g a m m a lines are practically independent of the incident proton energy. A level at 1406.5 keV with decay y-rays of 659.1, 906.9 and 1231.8 keV has been tentatively introduced because of the broadening, and in some spectra even splitting of the neutron group corresponding to a level at 1416 keV. However, the allocation of the 659.1 and 1231.8 keV g a m m a lines to such a level is questionable, since their intensity ratio at 3. I MeV differs considerably from the values at 2.9 and 2.7 MeV, while the 906.9 keV )'-ray is very weak. In some of the neutron spectra there are indications of a weak level at 1457 keV (fig. 1), but the evidence is not conclusive. A fairly weak ),-ray could be assigned to a level at

.L G. MALAN fit al.

58

TABLE 1 C o m p i l a t i o n o f g a m m a rays f r o m 7 a G a + p for incident p r o t o n energies between 1.5 a n d 2.9 M e V E~, (keY) 165.2 174.8 247.2 301.3 307.1 326.4 331.3 342.4 349.8 373.1 390.9 439.6 447.6 465.0 477.0 499.7 511.1 517.6 526.6 533.4 572.2 580.4 596.5 601.5 615.3 630 632.8 639.5 646.1 659.1 667.2 679.8 696 708.0 712.2 747.2 757.8 788.6 798.6 808.1 824.2 831.2 839.2 845 851.3 855.0 867.4 882.1 887.6 894.6

Intensity ")

Eo (keY) n)

> 14 203 27 115 n 14 1 15 5 125 n 18 5 4 < l 321 n 20 25 43 n 6 56 4 9 < 1 44 n ~4 ~ 12 5 5 5 6 6 4 142 n 24 31 2 32 n 6 27 n 8 100 n 5 5 10 4 3 3 14 <1

1600 1500 1900 1500 2172 1650 2172 2700 1650 2700 1650 1700 2700 2700 1500 1578 1500 2700 2172 2172 1900 2700 2172 1650 2400 1572 2172 2300 2700 2700 1500 2700 2700 1800 2700 2172 2900 2400 2700 2100 2700 2100 2700 2700 2172 2700 2900 2900 2700 2172

Assignment XaITa(p, p'y) 174.8-0 747.1-499.7 lSlTa(p, p'y) 831.2-524.3 524.3-197.8 831.2-499.7 524.3-174.8 1204.1-831.2 588.7-197.8 2aNa(p, p'y) (i 543.0-1095.8) 1212.4-747.1 ~Li(p, p'y), ~Be(fl)TLi * 499.7-0 annihilation peak 1348.8-831.2 1026.4-499.7 708.1-174.8 747. ! -174.8 1288.3-708.1 (1095.8-499.7), 7*Ge(n, n' 7) 71Ga(p, 7)TZGe 1204.1-588.7 7tGa(p,y)V2Ge (831.2-197.8), (808.1-174.8) 1139.4-499.7) (1454.2-808. I ) (1406.5-747.1), see text ~gGa(p, y)7°Ge 1204.1-524.3 72Ge(n, n'7) 708.1-0 (1543.0-831.2), (1212.4-499.7) 747. l - 0 1779.5" 1288.3-499.7 1298.7-499.7 808.1-0 1348.8-524.3 831.2-0 27Al(n, n'y), 56Fe(p, p'y), (n, n'y) 1026.4-174.8 (1378.9-524.3) (1698.6-831.2) 1629.4-747.1 1476.5-588.7

59

71G¢ LEVELS

Table 1 continued E~, (keV)

Intsnsity ")

Eo (keV) b)

906.9 914.7 921.1 935.2 952.4 964.5 976.5 983.7 994.9 1006.8 1014.2 1017.3 1026.7 1033.7 1038.7 1044.5 1058.8 1095.6 1098 1139.5 1182.9 1204.2 1212.7 1231.8 1239.7 1247.5 1258 1267 1273.5 1298.9 1331.7 1349.2 1378.7 1414.5 1424.1 1542.9 1599.0 1605.6 1617.4 1629.6 1633.6 1743.7 1779.5

< 1 3 45 6 5 9 9 < 1 5 3 ~ 17 ~ 17 15 18 22 2 2 89 n

2700 2700 2172 2900 2700 2300 2700 2900 2900 2700 2000 2700 2172 2700 1500 2900 2900 2172 2100 2300 2400 2700 2300 2700 2700 2700 2700 2700 2700 2400 2700 2700 2700 2700 2700 2900 2700 2900 2900 2900 2700 2900 2700

64 n 15 7 15 n 4 6 4 4 3 4 52 n 7 5 7 14 4 4 4 < 1 2 ~ 8 ~12 6 15

Assignment (1406.5-499.7), see text 1414.5--499.7 1095.8-174.8 1743.5-808.1 1476.5-524.3 1139.4-174.8 (1565.5-588.7) (1792.2-808. I ) 64Zn(P, P'7), (n, n'7) (1506.5-499.7), (1204.1-197.8) (1212.4-197.8), 27Al(n, n' 7) (1543.0-524.3), see text 1026.4-0 (1558.5-524.3) (1212.4-174.8), 69Ga(p,y), ~gGa(p, ~ty) (1792.2-747.1) (1558.5-499.7) 1095.8-0 1139.4--0 1378.9-174.8 1212.4---0 (1406.5-174.8), see text 1414.5-174.8

1298.7-0 1506.5-174.8 1348.8-0 1378.9-0 1414.5-0 1599.0-174.8 1543.0-0 1599.0-0 1792.2-174.8 1629.4--0 19F(p,y)2°Ne 1743.5-0 27Al(p,y), zsSi(p, p'y)

") Intensities were measured at/~p = 2.88 MeV with a VlGazOa target. Lines marked n were found to be in coincidence with neutrons. b) Threshold (see text). 1454.2 keX.T A d d i t i o n a l s u p p o r t f o r a p o s t u l a t e d e x c i t e d s t a t e a t t h i s e n e r g y is f u r n i s h e d b y t h e (d, p ) w o r k , w h i c h s h o w s d e f i n i t e e v i d e n c e o f a level a t 1.45 M e V . T h e level a t

60

J. G. MALAN et al. TABLE 2 Level energies in 71Ge 71Ga(p ' n7 ) a) (keV)

71Ga(p, n) (keV)

0 174.8 197.8 499.7 524.3 588.7

504i 5 527+ 5 594+ 5

0 176 198 501 525 590

708.1 747.1 808.1 831.2

713752, 812+ 835,

5 5 5 5

709 748 809 832

1030-t- 5 1099+ 5 1140+_ 8

1097 1140

1026.4 1095.8 1139.4 1204.1 1212.4 1288.3 ! 298.7 1348.8 1378.9 (1406.5) 1414.5 (1454.2) 1476.5 1506.5 1543.0 (1558.5) (1565.5) 1599.0 1629.4 (1698.6) 1743.5 1792.2

4+ 5 183- 5

7°Ge( n, 7) b) (keV)

1211, 1291, 1302+ 1352, 1383__+_ 1416+ (1457+ 1481 :1511± 1547-:1565 +

8 8 8 8 8

1300 1379

8 8) 8 8 8 8

1602___ 8 1633± 8 1701,10 1749+10 1797 _-kI 0

7OGe(d ' p) c) (MeV)

Spin a)

0 0.16 0.19 0.48 0.51 0.57 0.62 0.70 0.73 0.79 0.81 0.89 0.97 1.03 1.09 1.12 1.16

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1.28 1.34 1.38

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1.41 1.45 1.47 1.50

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1.55 1599

1.59

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1.69 (1740)

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1938.0 1965.4

1965

1.94 1.96

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") Excitation energies are known to an accuracy o f ! 0 . 5 keV or less for levels below 1.2 MeV. The error for the remaining states is less than I keV. Brackets imply questionable gamma-ray allocations. b) Ref. 4). Gamma-ray energies are known to 0.7 keV. c) Ref. z). The precision is + 10 keV, except for the 0.16 and 0.19 MeV levels, where an error o f i 2 0 keV is given. d) These values were derived from all available information. Spins given in brackets are either less favoured values, or have been strongly suggested by (d, p) experiments without being supported by the present work.

71Ge LEVELS

61

1565 keV is quite strongly excited in the (p, n) work (fig. 1) and probably consists of a doublet at 1558.5 and 1565.5 keV. Gamma rays of 1033.7 and 1058.8 keV may be associated with a level at 1558.5 keV. Unfortunately it was not possible to compute reliable branching ratios at different energies as the 1033.7 keV line is not well resolved from the strong 1026.7 and 1038.7 keV transitions. The existence of a level at 1565.5 keV is supported only by a single gamma transition of 976.5 keV and is therefore uncertain. The identification of a level at 1698.6 keV with the 1701 keV neutron level is also doubtful, since the former value is again only supported by a single gamma transition of 887.4 keV. Additional unresolved doublets in the neutron spectra may well account for some of the gamma lines which could not be placed in the present level scheme, or which have been tentatively allocated to existing resolved levels. There is however, no definite evidence for this from the present work. In the last column of table 2 spins and parities are presented for most of the excited states. These values were derived by taking into account the decay modes and relative (p, n) cross sections from the present work, as well as the results of the (n, ~,) and (d, p) measurements 4,2). Unfortunately the spins of only the ground and first two excited states are known with certainty 1), so that it is often difficult to draw definite conclusions from the branching ratios. A few remarks on the assignments made to levels below 1.2 MeV are however, appropriate. The levels at 499.7, 708.1 and 1095.8 keV are directly fed from the ½+(n, V) capture state, allowing spins of ½, ~, or ~+. The latter value is excluded by the strong transitions (100 7o, 96 7o and 60 7o respectively; see table 3) to the ½- ground state of 71Ge. In the (d, p) experiments 1,2) all of these levels were found to have ln = 1 angular distributions and were assigned 2) spins of ½- on the basis of the Lee-Schiffer effect. In the (p, n) reaction, however, these states are strongly populated from the ~71Ga ground state. Hauser-Feshbach calculations show that this implies spins of ½, ~, or ½ for these levels, rather than ½. The results therefore seem to prefer a spin of ~for the relevant states. For the 524.3 keV level an l, = 2 stripping pattern was found, implying a spin of ~+ or i +. The observed decay of this level to the 197.8 keV ~+ state excludes the former possibility. A spin of ~+, however, would lead to an El transition strength of 12 7o to the ~- level at 174.8 keV, compared to an E2 strength of 88 700 to the 197.8 keV level. This branching ratio would rather suggest a spin of -~ or ~ for the 524.3 keV level. The state at 588.7 keV decays only to the 197.8 keV level, suggesting a spin o f ~ or ~ -~. Since strong (p, n) excitation occurs, spins above are improbable. The 747.1 and 1026.4 keV levels are also strongly excited in the (p, n) reaction; considering their decay modes spin values of ~ or ~ -are indicated. In contrast the 808.1 keV level is only weakly excited and decays strongly to the ground state, implying spin values of ½ or ½÷. The strongly populated 831.2 and 1139.4 keV levels decay mainly to the ground state and are both directly fed from the (n, ~) capture state. Spin values of ~+ or 3 - are suggested for these levels. Similar arguments have been used to limit spin values for a number of higher excited states. The complete level and decay scheme of 71Ge resulting from this work is given in

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fig. 3. Transitions given in brackets in table 1 appear as dashed lines in this figure. Branching ratios for only those levels with unambiguously assigned decays (solid lines) are summarized in table 3; transitions indicated by dashed lines have been ignored for this purpose. These values represent averages over all measurements with the 40 cm 3 Ge(Li) detector. The assignment o f decay y-rays to specific levels is based u p o n the existence o f a neutron group o f the correct energy, the correct threshold behaviour o f the relevant y-rays, the non-dependence o f the branching ratios on the TAnI~ 3 Branching ratios in 7tGe Level 524.3 708.1 747.1 831.2 1026.4 1095.8 1139.4 1204.1 1212.4 1288.3 1298.7 1348.8 1378.9 1414.5 1476.5 1599.0 1629.4 1743.5 1938.0

Gmmna lines (intensities) 349.8 (12) 708.0 (96) 747.2(~ 30) 831.2 (72) 1026.7 (23) 1095.6(~ 60) 1139.5 (82) 679.8 (12) 1212.7 (78) 788.6 (87) 1298.9 (91) 1349.2 (15) 1378.7 (50) 1414.5 (61) 952.4 (24) 1599.0 (56) 1629.6(m 50) 1743.7 (51) 1937.8 (51)

326.4 (88) 533.4 ( 4 ) 572.2(~ 55) 331.3 (It) 851.3 (14) 921.1 (~ 40) 964.5 (11) 615.3 (79) 465.0 (22) 580.4 (13) 798.6 ( 9 ) 824.2 (20) 1204.2 (50) 1239.7 (25) 887.6 (76) 1424.1 (44) 882.1 (m 50) 935.2 (49) 1762.7 (28)

247.2(~ 15) 307.1 (17) 526.6 (63) 639.5 ( 7 ) 373.1 ( 9 )

517.6 (65) 914.7 (14)

1191.5 (21)

incident p r o t o n energy, and in the case o f the stronger transitions, the enhancement o f y-rays associated with the neutron flux in the coincidence experiments. G a m m a transitions have been dashed in fig. 3 for various reasons. The y-rays at 596.5, 1014.2 and 1038.7 keV coincide with well-known b a c k g r o u n d lines, the contributions o f which are difficult to estimate. The y-rays at 632.8, 712.2 and 1006.8 keV have each been assigned to two different levels. The intensity ratios for the 632.8 keV line at different proton energies show that it is probably associated with the 831.2 keV level. However, in view o f the suggested spin values for this level a 632.8 keV transition to the 197.8 keV level is unlikely. Unfortunately this y-ray could not be observed near threshold, so that neither o f the possible transitions from the 808.1 or 831.2 keV levels could be excluded on experimental evidence. The 712.2 keV y-ray was barely resolved from a very strong 708.1 keV transition (fig. 2), so that it was impossible to determine its intensity reliably in order to apply the constant branching ratio test. The allocation o f the 1006.8 keV line to the 1506.5 keV level is not satisfactory when

64

J.O. MALANet

aL

the branching ratios from different measurements are considered. The other possible assignment of this ),-ray is to the 1204.1 keV level (table 1). Within the proposed level scheme the 447.6 keV line can only originate from the 1543.0 keV level. Its intensity suggests a different origin, however. Another fairly strong line which may originate from the level at 1543.0 keV is a 1017.3 keV transition to the 524.3 keV level (table 1). However, the energy of this ),-ray is too low by 1.4 keV. This may be due to the fact that it is not well separated from the strong 1014.2 keV line. The energy of the 855.0 keV transition from the 1378.9 keV level varies considerably between different measurements, so that interference from a fluctuating background contribution is suspected. 4. Conclusion A level and decay scheme has been constructed for 71Ge from the results of high resolution (p, n) and (p, n)') experiments on 7 ~Ga. The results attained give evidence of the benefits to be gained by observing both neutrons and decay v-rays from the same (p, n) reaction. The proposed level scheme is not only in fair agreement with previously available information, but also contains a considerable amount of new data contributing towards a better understanding of the level structure of 71Ge. We wish to acknowledge the contribution of Dr C. H. Bornman to the initial stages of the experiment. We would also like to thank Dr D. Reitmann and our colleagues for the interest shown in this work. References 1) Nuclear Data Sheets, National Academy of Sciences-National Research Council, Washington, D.C., Nuclear Data B1-6-13 (1966) 2) L. I-L Goldman, Phys. Rev. 165 (1968) 1203 3) C. I-LJohnson, C. C. Trail and A. Galonsky, Phys. Rev. 136 (1964) B1719 4) C. Hofmeyr, B. C. Winkler, J. A. M. de Villiers, J. G. Malan and J. W. Tepel, Int. Symp. on neutron capture gamma-ray spectroscopy, Studsvik (Aug. 1969)