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The decay of 73Br
Nuclear Physics A142 (1970) 21 --34; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
THE DECAY OF 73Br G. M U R R A Y , W. J. K. WHITE and J. C. WILLMOTT
Physics Department, University of Manchester, England and R. F. ENTWISTLE
Physics Department, University of Otago, New Zealand Received 17 October 1969 Abstract: The energies and relative intensities of the ~-rays following the radioactive decay of 3.3 rain 73Br produced by the 59Co(160, 2n) reaction have been measured using Ge(Li) detectors. No isomeric transition with half-life greater than 1 ms has been observed in 73Br. A level scheme for 73Se is proposed. The results are discussed in the light of observed correlations of the variations in the B(E2) values and the excitation energies of the first 2 + states in doubly even nuclei and the excitation energies of the first ~+ states in odd-proton rtuclei in the region Z, N = 2 8 a n d Z , N = 5 0 .
I E
RADIOACTIVITY 7aBr [from 59Co(1~O, 2n)]; measured T~, E~,, I~,. TaSe deduced levels. J, ~r. Natural target, Ge(Li) detector.
1. Introduction The low-lying positive-parity states in odd-mass nuclei in the region, Z, N = 28 to Z, N = 50 pose several interesting questions. The so-called anomalous ~2+ and ½+ states have been the subject of some theoretical investigation and several coupling schemes have been advanced to explain them 1). The $+ states may be interpreted in terms of the Nilsson formulation but such an interpretation would seem to be of doubtful validity since, for example in the odd-mass arsenic nuclei 2), the low excitation energy of these states imply the presence of large oblate deformations (0.2-0.3). Recently Scholz and Malik 3), using a Coriolis-coupling model with a residual interaction of the pairing type, have successfully accounted for several experimentally observed features of the odd-proton nuclei in the mass range 71 < A < 85 and, in particular, have accounted for the occurrence of several low-lying positiveparity states in such nuclei. Using this model it is not possible to calculate the exact position of the 9 + states relative to the negative-parity states because of the present uncertainties in the parameters of the single-particle potentials 3). However there would appear to be some evidence to suggest that there is a correlation between the excitation energies of $+ states in odd-mass nuclei and the first 2 + states in neighbouring doubly even nuclei. The available data 4) are presented in fig. 1. There is a relatively smooth variation of excitation energy with neutron number for both the 21
22
G. MURRAYet al.
9 + a n d 2 + levels. F o r the 2 + levels m a x i m a occur at the closed shells N = 28 a n d N = 50 with a w e a k e r m a x i m u m at N = 38. T h e n a t u r e o f this b e h a v i o u r has been discussed b y Kisslinger a n d Sorensen 5). D a t a on the 9+ levels exist only for the region a b o v e N = 40 a n d it is clear f r o m fig. 1 t h a t the m a x i m u m excitation energy occurs at N = 50. The only evidence available to confirm or refute the existence o f an increase in excitation energy t o w a r d s N = 38 comes f r o m the arsenic i s o t o p e s where
Ist 9/2 +'STATE
/ / ~
As
0.5
0
33
r
>"
r
t
i
i
Br
Rb
352' 37. /
i
•
39
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,
Ist 2 + STATE
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1.5
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3oZn
32
2 Fe "
38 •
.
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0.5-
28
2
4
6
38 40 42 Neutron Number
44
46
48
50
52
Fig. 1. Plot of the excitation energies of the Ist 2 + states in doubly even nuclei and the I st ~ + states in odd-mass, even-neutron nuclei as a function of neutron number.
aaAs+o is some 120 keV a b o v e t h a t in 75 the 9 + level in 73 aaAs42. I f this p o s t u l a t e d corr e l a t i o n is to be t a k e n seriously then in the nucleus 73 asBra8 the ~+ 2 state w o u l d be exp e c t e d to be some few h u n d r e d s o f kilovolts a b o v e the g r o u n d state. A l t e r n a t i v e l y if such a c o r r e l a t i o n does n o t exist then the d a t a 4) on 77Br, 79Br, 81Br a n d 83Br might be t a k e n to i m p l y that in 73Br the 3 + state should occur at low excitation energy o r indeed be the g r o u n d state.
73Br DECAY
23
No information on 73Br has hitherto been published. In the present work a search has been made for an isomeric level in 73Br and the positon decay of this nucleus to the levels of 735e has been investigated to establish the 73Br ground state spin. Previous work 4) has shown that the 73Seground state decays with a half-life of 7.1 h predominantly to a 428 keV level in 73As which decays by a cascade of 361 keV and 67 keV transitions to the 73As ground state. A 26 keV isomeric state in 735e decays with a half-life of 42 min partly (82 %) to the 73Se ground state and partly (18 %) by positon emission to the states of 73As, the two most intense transitions following the positon decay having energies of 84 keV and 254 keV [ref. 2)]. The 73Se ground state and isomeric state have been assigned spins of ~+ and ½- respectively 6).
2. Experimental procedure In the present work the 73Br activity was produced by the 59Co(160, 2n) reaction with the Manchester Hilac. In those experiments in which chemical separation of the bromine was carried out cobalt targets of thickness 25 mg/cm 2 (purity = 99.999 %) were used to obtain the maximum possible yield. In the chemical separation the irradiated cobalt targets were added to a mixture of potassium permanganate (5 cc) and a carrier solution (1 cm 3) containing small quantities ( ~ 200 pg) of the known reaction products. Concentrated nitric acid (5 cm a) was then added and, after the target had dissolved, the solution was mixed thoroughly with carbon tetrachloride (5 cm a) and decanted. Using radioactive tracers it was found that the efficiency of the bromine separation was > 90 % and that negligible quantities of the other reactions products (i.e. Se, As, Ge, Ga) came through in the bromine fraction. The time interval between the end of the irradiation and the start of the study of the bromine activity was approximately 2.5 rain, the actual instant of chemical separation being some 2 rain after the end of the irradiation. In those experiments in which the (160, 2n) reaction cross section was studied as a function of bombarding energy chemical separation was not employed and one mg/cm2 cobalt targets, prepared by electro-deposition, were used. In the first set of experiments a 25 mg/cmz cobalt target was irradiated for 45 rain by a 0.4 #A, 64 MeV, oxygen beam and the 7-radiation from the chemically-separated bromine studied using a 25 cm 3 Ge(Li) detector. A typical spectrum, recorded immediately after the separation, is shown in fig. 2. The energies of the y-rays, shown in table 1, are the average values obtained from several such experiments and are based on the known energies of transitions in 24tAm, 57C0, 758e, 54Mn, 8sYt, 22Na, 6°C0 and 228Th. Source and calibration runs were taken alternately to minimise the effects of any time-dependent gain changes in the amplifier (Tennelec 200) and by using a relatively large number of calibration lines at close intervals over the energy spectrum the effects of any possible non-linearity in the analogue to digital converter (Laben 4096-channels) were reduced. The 7-ray intensities, shown in table 1, are likewise average values and were computed by summing the number of counts
ou
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.
.
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.
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NUMBER
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1500
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609.5 614 9 74_ , t 634.5( Brl
,~--"r ~
788.1
, '
",
e
Fig. 2. Gamma-ray spectrum from 73Br produced by the 59Co(160, 2n) reaction followed by chemic,"
.
0
o
i
125.5
2K
64.9
,'%.. ~" .:,~%
~ 4g
Energies
Chemical SE
$9
16 Co( 0 , 2
Br Gamma-
?3
73Br DECAY
25
in the peak, the dependence of counter efficiency on quantum energy being corrected for by using a set of eight gamma sources of known strength supplied by IAEA Vienna. TABLE 1 Energies, relative intensities and associated half-lives of the ~-rays following 73Br decay Energy (keV)
Half-life (sec)
64.9±0.1 125.5+0.1 275.14-0.2 335.74-0.2 374.34-0.2 400.64-0.2 489.64-0.2 annihilation radiation 539.64-0.2 550.1+0.2 614.94-0.2 638.64-0.3 699.54-0.2 788.14-0.3 848.74-0.2 861.74-0.3 869.84-0.3 913.6:k0.2 930.74-0.2 995.64-0.2
2004-20 2004-20 190±40 180+20 2454-40 2104-30 2004-40 2004-30 2054-40 1404-40 2004-40 2804-50 1904-30 2504-40 1954-30 1954-40 2304-40 1854-30 1804-30 1654-40
Relative intensity a) 100 234- 2 104- 1 344- 2 84- 1 204- 1 44- 1 5744-40 84- 2 3~ 1 84- 1 44- 1 404- 3 34- 1 204- 1 34- 1 54- 1 19± 2 224- 2 74- 1
a) Intensities relative to that of the 65 keV 7-ray.
Gamma-ray spectra were recorded at 100 s intervals for 120 min after the chemical separation and from these data the half-life values shown in table 1 were deduced. There was no evidence for any initial build-up in the ~-ray yields and this, together with the fact that chemical separation took place 2 rain after the end of the irradiation, can only be interpreted on the basis that the 3.3 +__0.3 min activity is associated with the decay of a bromine isotope. The 7-spectrum recorded from the bromine fraction thirty minutes after the chemical separation is shown in fig. 3. These ~-rays have been identified with known transitions in 73As following the decay of 73Se and 73roSe with the exception of the 635_+ 1 keV and 730 +__1 keV lines which, on the basis of their energies, associated half-life (38 + 4 min) and relative intensities (7.5 : 1), are assigned to 74Se in accordance with the work of Belyaev et al. 7). The intensities of the 84 keV and 254 keV lines in fig. 3 are shown plotted as a function of time in fig. 4 and their measured decay rates of 40 + 4 min are consistent with their assignment to transitions in 73As following the decay of 73roSe. The initial build-up in the decay curves is consistent with a 3 _+1 min activity feeding the 73Se 40 min level. We thus assign the 3.3 +__0.3 rain activity to 73Br.
26
G. MURRAYet al.
bi,Ocnt,9
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101
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2{;o
~o
..... .
. ,-...:. "":.. " : '.... .'"
36o
CHANNEL
460
NUMBER
Fig. 3. Gamma-ray spectrum, from the bromine fraction, recorded 30 rain after its chemical separation.
,
84keY
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9
8Z
10-
1
~
o
(counts+ IOOI
lo6o
2obo TIME
IN
3obo
4obo
5obo
SECONDS
Fig. 4. Plots, as functions of time, of the yields of the 84 keV and 254 keV ?-rays following the 7a"Se decay and the 361 keV ?-ray following the 7aSe decay recorded from the bromine fraction.
73Br DECAY
27
The build-up and decay curve in the 73As 361 keV transition is also shown in fig. 4. This curve is completely compatible with the 73Se 40 rain level feeding the 735e 7.1 h ground state which decays to the 428 keV 73As level Due to the poor statistical accuracy of the initial points on this curve however it is not possible to completely exclude the presence of a small 3 min component in the build-up arising from direct feeding of the ~3Se ground state by the 73Br ground state. An upper limit on the amount of this direct feeding was established using the known decay schemes of 7aSe and 7arose to predict the relative intensities of the 361 keV and 84 keV transitions assuming that the 73Se ground state was populated only through the decay of the 73roSe isomeric level. This predicted value of the relative intensities was found to be identical to that observed experimentally within the limit of error of 10 ~. We conclude that the intensity of the 73Br ground state decay to the 73Se ground state is less than one-tenth that to the 7aSe isomeric state. A second set of experiments were carried out to investigate the possible existence of a low-lying isomeric state in 73Br with a half-life significantly greater than the 3.3 min ground state. The same irradiation procedure was followed as in the first set of experiments but the chemical separation procedure was not carried out until twenty minutes after the irradiation. The only 7-rays from the bromine fraction were the 635 keV and 730 keV lines following the 74Br decay and, in particular, no 7-rays following the decay of 7aSe or 7aroSe were observed. We conclude that 73Br does not have a low-lying isomeric level of half-life > 3 min. 3. The (140, 2,) yield function In the preceding section evidence has been presented for the assignment of a 3.3_+0.3 min activity to 73Br. Corroborative evidence for this was obtained from a study of the dependence of the yield of the 3.3 min activity on the bombarding energy of the heavy ion in the reaction s 9Co(160, 2n)VaBr. Thin cobalt targets (1 mg/cm 2) were bombarded for a period of 45 min by a 0.02 pA 160 beam of known energy and the yields of the 7-rays, observed using a Ge(Li) detector, recorded for a period of 10 min after the irradiation. The bombardment time of 45 rain was initially chosen to enhance any possible activities with half-lives significantly longer than 5 min in bromine nuclei with A < 74 but none, in fact, were observed. No chemical separation was carried out in these experiments because the other activities present were useful in establishing the energy dependence of the cross sections for different reactions. The yields of the 65 keV and 125 keV transitions associated with the 3.3 min activity and of the 67 keV and 361 keV v-rays following the 735e 7.1 h activity are shown as a function of heavy ion bombarding energy in fig. 5. The 73Br activity can be produced only by the (160,2n) reaction whilst the 73Se activity can be produced by the (160, rip) reaction and by the 59C0(160, 2n) 73Br(fl+)TaSe process. The similarity in yield functions of the 73Br and 7ase activities shown in fig. 5 implies either that the energy dependence of the (160, np) and the
28
G. MURRAYe t
al.
(160, 2n) reaction cross sections are very similar or that the yield of the former reaction is considerably smaller than the latter over the energy range under consideration. Further evidence on this point was obtained by comparing the ratio of the yields of the 73Se 125 keV transitions and the 73As 85 keV transition, observed from 59Co targets bombarded by 160 ions at energies from 38 MeV to 60 MeV, with the ratio computed from the decay schemes of the two nuclei assuming that the 73Se was not produced directly. The observed and calculated y-ray intensity ratios were found to be consistent within experimental error (typically 10 ~o) except at bombarding energies above 52 MeV where significant deviations were noted. The sign '
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50 70 HEAVY ION BOMBARDING ENERGY [MeV]
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Fig. 5. Excitation function of y-rays emitted after the bombardment of S9Co by 160 ions. of the deviations is indicative of an enhancement of the 73As 85 keV transition relative to the 73Se 125 keV transition and these deviations are thus interpreted as the result of the (160, np) reaction. The predominant reaction at and below 52 MeV is thus (160, 2n) and the results shown in fig. 5 therefore confirm the assignment of the 3.3 min activity to the decay of 73Br. The other yield functions shown in fig. 5 will be discussed in sect• 5.
4. The positon decay of 7aBr The end-point energy of the 73Br 3.3 min positon activity was measured using a plastic scintillator from which the pulses were amplified and recorded in a Laben 1024-channel analyser. The system was calibrated using 22Na, 22STh and an americium-
73Br DECAY
29
beryllium source. Cobalt targets, of thickness 1 mg/cm2, were bombarded for 5 min by a 0.5 pA, 64 MeV oxygen beam and then placed directly in front of the counter. The Fermi-Kurie plot of a spectrum taken immediately after an irradiation is shown in fig. 6 and is observed to have two components of end-point energies 4.5 ___0.3 MeV and 3.7 +0.5 MeV. To measure the decay constant of each component, spectra were recorded at 100s intervals after the irradiation and in the analyses spectra were divided into sub-groups of 8 channels each. All sub-groups up to an energy of 4.5 MeV had a component of half-life 40_+ 5 min present and thus this activity was attributed to the decay of 74Br' both the end-point energy value and the half-life value being consistent i
i
Fermi- Kuri¢ Plot 30
Reaction t60 + 59C0
Bombarding Energy--45 MeV
_, 2 . 0 -
Component A
.SO
7.0
9,0 W ( MoC 2)
Fig. 6. The Fermi-Kurie plot of the positons emitted from 73Br and 74Br produced by the s9C0(~70, Xn) reaction with chemical separation.
with the previous measurements of Butement and Boswell s). Confirmation of the presence of 74Br in the irradiated targets comes from the observation of the 635 keV and 730 keV ~-rays in 74"Se as discussed in sect. 2. Above an energy of 3.7 MeV no subgroup was observed to have a component of half-life less than 40 min in its decay curve. Below 3.7 MeV all sub-groups showed a component of half-life 3.5-t-0.5 min. We thus attribute the component with end-point energy 3.7+_0.5 MeV to the decay of 7~Br. The above value for the 73Br end-point energy implies a Q-value of 4.7 +_0.5 MeV in good agreement with the predicted value of Mattauch et al. 9). The predicted value of 1.5+_1.0 MeV in ref. 1) is a misprint and should read 4.5+_1.0 MeV [ref. lo)].
30
G. MURRAY e t aL
5. Search for an isomeric level in 73Br
As discussed in sect. 1 there is some possibility of the existence of a 9 + ground state or near ground state level in 73Br. In either situation an isomeric state and associated 9 + --r 3 - ) might be expected in an analogous way to the E3 transition (3- __> 9+ or ~4.3 min 108 keV level and the 4.8s 208 keV level in 77Br and 79Br respectively. The possibility of an isomer in 73Br of half-life greater than about one minute has virtually been excluded by the work described in the preceeding sections, in particular by the absence of unassigned 7-rays in spectra such as fig. 2, by the absence of evidence of a build-up in the yields of the y-rays following the 3.3 min 73Br activity, by the absence of an unassigned component in the decay curve of the annihilation quanta and by the absence of any activity other than 74Br from the bromine fraction chemically separated 20 min after the end of the irradiation. A search was carried out for a 73Br isomeric state with a half-life of < 1 min. A cobalt target (thickness = 1 mg/cm z) was bombarded by an oxygen beam of known energy from the Manchester Hilac which has a pulse length of 2 ms and a repetition frequency of 10 per second. Between beam pulses the g a m m a radiation emitted from the target was studied as a function of time using a Ge(Li) detector, amplifier and multi-channel analyser (Laben 1024-channels) operated in its subgroup stepping mode, typical operating parameters of the latter being four groups of 256 channels stepped at 20 ms intervals. This irradiation and counting procedure was carried out for 45 min for each of 14 heavy-ion bombarding energies between 35 MeV and 94 MeV. Just prior to the end of each 45 min irradiation the contents of the multichannel analyser were recorded on paper tape, the memory cleared, and some five seconds after the end of the irradiation a subsidiary experiment was started in which a set of spectra were taken, at 100 second intervals, of the y-rays emitted from the activities still present in the target. F r o m a comparison of the spectra in the four sub-groups recorded between beam pulses it was possible to identify 7-rays associated with half-lives in the range 1-100 ms. Two such y-rays were observed with energies of 176 keV and 198 keV and half-lives of 25__.2 ms and 21+__10 ms respectively. The intensities of these y-rays, normalised with respect to integrated beam intensity, are plotted as a function of bombarding energy in fig. 5. From a comparison of the spectra recorded between beam pulses with these recorded after the irradiation it was possible to pick out activities with half-lives greater than 100 ms. Two y-rays, at energies of 55__ 1 keV and 102+ 1 keV, were observed each to have the same intensity in the four sub-groups recorded between beam pulses but to be absent in the spectra recorded after the irradiation implying that they were associated with half-lives in the range 0.1s-2s. The dependence of their yield on the heavy ion bombarding energy is shown in fig. 5. Also shown in the same figure is the relevant data on an observed 116 keV y-ray associated with a half-life of 15 min. A comparison of the yield curves presented in fig. 5 shows that none of the five activities discussed in the preceeding paragraph are associated with 73Br and further
73Br DECAY
31
experiments are in progress to identify the nuclei to which they belong. No such short-lived activities were observed in the excitation region 40-60 MeV and we conclude that there are no ~-emitting isomeric states in 73Br with half-lives greater than 1 ms. The possibility must also be considered of an isomeric activity in :aBr which deexcites with no 7-emission either by positon decay directly to the VaSe ground state or by internal conversion. If an isomeric level were to exist then the heavy-ion reaction might be expected to favour the production of the high spin state. The possibility of direct positon emission to the VaSe ground state can be excluded on the basis of the calculations discussed in sect. 2 which showed that, at excitation energies below 52 MeV, the amount of Vase activity observed is completely consistent with that expected from the known 3.3 min 73Br decay. In the case of a strongly internally converted transition from the isomeric level to the ground state it has been estimated that if it has an ~: of less than 5 the 7-radiation would have been observed in this work. If the transition has an ~: of greater than 5 then the energy must be less than 105 keV [ref. 11)] and such an E3 transition would be expected to have a half-life greater than 200 seconds, c.f. v 7Br ' 108 keV, 4.3 rain; a 1Se' 103 keV, 57 rain; SlRb, 85 keV, 32 min. The build-up in the :3Br ground state activity due to the decay of the isomeric state would have been clearly visible in this case. From the work described above we conclude that there are no isomeric states in 73Br with half-lives greater than 1 ms. 6. Discussion of results
The proposed VaSe level scheme is shown in fig. 7 and has been established from the data in table 1 together with the cascade crossover sums shown in table 2. The 374 keV, 540 keV and 862 keV transitions have not been placed in the scheme. From previous work 2) the spins of the ground state and 26 keV levels are ~+ and ½- respectively. The absence of evidence for a 3 min build-up in the 7ase ground state decay (sect. 2) implies that the VaSe 7-rays feed the 26 keV level rather than the ground state. As discussed above, the intensity of the fl-decay from the 73Br ground state to the Vase ground state is less than 10 ~o of that to the Vase isomeric level indicating that the 73Br ground state spin is not ~+. The known odd-proton nuclei between Z = 29 and Z = 37 inclusive have ground state spins of ~2- with only three exceptions all of which have spins of -~- [ref. g)]. The allowed fl-transition from the ground state of 7aBr to the ½- isomeric level of 73Se excludes a ~- assignment. We therefore assign a spin of 3 - to the 73Br ground state. The absence of an isomer in VaBr with half-life greater than 1 ms enables a lower limit on the energy of the ~+ level to be established. If there is no low-lying ~state in V3Br then the ~+ state would necessarily have to be at least 500 keV above the ground state otherwise an E3 transition of half-life greater than 1 ms would be observed. However many of the nuclei in this region have a low-lying -~- state. If
312
3 rain 17~Br I I
2 1022/~o
641/ 427/
i_
ISI
ll2" 7•2 +
~
~
1
73 5e
b.r~
~'_ 26 ~'/'42 rain ½ 7.1 h
]FIB. 7. Proposed ~aBr decay scheme TABLE 2 E n e r g y s u m s r e l e v a n t t o t h e 91 k e V a n d 151 k e V levels DirectT-ray energy measurement
=
64.9±0.1
400.64-0.2--335.7±0.2 6 1 4 . 9 4 . 0 . 2 - - 5 5 0 . 1 -t-0.2 913.6 ±0.2--848.7±0.2 995.6±0.2--930.7±0.2
= = = =
64.9:E0.3 64.84-0.3 64.94-0.3 64.9±0.3
Direct 7-ray energy measurement 400.64.0.2--275.14-0.2 614.94.0.2--489.64.0.2 913.64-0.2--788.1 4-0.3 995.64-0.2--869.84-0.3
= = = = =
125.5±0.1 125.54-0.3 125.3 4 - 0 . 3 125.54-0.4 125.8+0.4
335.7-L0.2--275.1 i0.2 550.1 ± 0 . 2 - - 4 8 9 . 6 ± 0 . 2 699.54-0.2--638.64-0.3 848.74-0.2--788.1 4-0.3 930.74-0.2--869.84-0.3
= : ~ = =
60.64-0.3 60.54-0.3 60.94-0.4 60.64-0.4 60.94-0.4
Energies in keV.
73Br DECAY
33
such a state exists in 73Br then from single-particle estimates together with observed retardation factors of M2 transitions in this region 4) a ~+ state would need to be more than 130 keV above the ~- state if its half-life is to be less than 1 ms. The implication of the present results is therefore that the ~+ state in 73Br is at a higher excitation energy than the corresponding level in 77Br (108 keV). From the present work and previously published results 4) it is observed in the odd-mass bromine and arsenic isotopes, that the ~+ levels in N = 42 nuclei are at a lower excitation energy than in N = 38 and N = 40 nuclei respectively (see fig. 1). The apparent correlation between the excitation energies of the 79 + levels and the
0.~
B ( E 2 ) VALUES OF I ~t 2 "I" STATES (uncertointies in experirnentol values ~ 10%)
¢-.,Eu0.4
Z ~ ~
-
0.2
Z 3
3'4
3'6 3'8
I
,o
Neutron Number
!
so
Fig. 8. Plot of the B(E2) values of the first 2 + states in doub|y even nuclei as a function of neutron number.
first 2 + levels may be interpreted as the result of an increase in nuclear deformation towards the middle of the shell since, with increasing deformation, the l g~ level would be expected to penetrate more into the lf-2p shell thus leading to the observed lowering of the ~+ state with respect to the negative-parity ground state 3). This interpretation is consistent with the trend observed in the experimental B(E2) values to the first 2 + states in doubly even nuclei in this region as shown in fig. 8 where the B(E2) values are lowest around N = 38 and N = 50 and have the maximum value at N = 42. References I) I-I. Ikegami and M. Sano, Phys. Lett. 21 (1966) 323 2) G. Murray, W. J. K. White, J. C. Willmott and R. F. Entwistle, Nucl° Phys. A130 (1969) 563
34 3) 4) 5) 6) 7) 8) 9) 10) 11)
G. MURRAYet al. W. Scholz and F. B. Malik, Phys. Rev. 176 (1968) 1355 and private communication Nucl. Data 1B (1966) 4, 6 L. S. Kisslinger and R. A. Sorenson, Rev. Mod. Phys. 35 (1963) 853 G. Murray, W. J. K. White, J. C. Willmott and R. F. Entwistle, Phys. Lett. 28B (1968) 35 B. N. Belyaev, B. A. Gvozdev, V. I. Gudov, A. V. Kalayamin and L. M. Krizhanskii, Yad. Fiz. 3 (1966) 609 F. D. S. Butement and G. G. J. Boswell, J. Inorg., Nucl. Chem. 16 (1960) 10 J. H. E. Mattauch, W. Thiele and A. H. Wapstra, Nucl. Phys. 67 (1965) 1 N. B. Gove, private communication R. S. Hayer and E. C. Seltzer, Nucl. Data 4 (1968) 1, 2
THE DECAY OF 73Br G. M U R R A Y , W. J. K. WHITE and J. C. WILLMOTT
Physics Department, University of Manchester, England and R. F. ENTWISTLE
Physics Department, University of Otago, New Zealand Received 17 October 1969 Abstract: The energies and relative intensities of the ~-rays following the radioactive decay of 3.3 rain 73Br produced by the 59Co(160, 2n) reaction have been measured using Ge(Li) detectors. No isomeric transition with half-life greater than 1 ms has been observed in 73Br. A level scheme for 73Se is proposed. The results are discussed in the light of observed correlations of the variations in the B(E2) values and the excitation energies of the first 2 + states in doubly even nuclei and the excitation energies of the first ~+ states in odd-proton rtuclei in the region Z, N = 2 8 a n d Z , N = 5 0 .
I E
RADIOACTIVITY 7aBr [from 59Co(1~O, 2n)]; measured T~, E~,, I~,. TaSe deduced levels. J, ~r. Natural target, Ge(Li) detector.
1. Introduction The low-lying positive-parity states in odd-mass nuclei in the region, Z, N = 28 to Z, N = 50 pose several interesting questions. The so-called anomalous ~2+ and ½+ states have been the subject of some theoretical investigation and several coupling schemes have been advanced to explain them 1). The $+ states may be interpreted in terms of the Nilsson formulation but such an interpretation would seem to be of doubtful validity since, for example in the odd-mass arsenic nuclei 2), the low excitation energy of these states imply the presence of large oblate deformations (0.2-0.3). Recently Scholz and Malik 3), using a Coriolis-coupling model with a residual interaction of the pairing type, have successfully accounted for several experimentally observed features of the odd-proton nuclei in the mass range 71 < A < 85 and, in particular, have accounted for the occurrence of several low-lying positiveparity states in such nuclei. Using this model it is not possible to calculate the exact position of the 9 + states relative to the negative-parity states because of the present uncertainties in the parameters of the single-particle potentials 3). However there would appear to be some evidence to suggest that there is a correlation between the excitation energies of $+ states in odd-mass nuclei and the first 2 + states in neighbouring doubly even nuclei. The available data 4) are presented in fig. 1. There is a relatively smooth variation of excitation energy with neutron number for both the 21
22
G. MURRAYet al.
9 + a n d 2 + levels. F o r the 2 + levels m a x i m a occur at the closed shells N = 28 a n d N = 50 with a w e a k e r m a x i m u m at N = 38. T h e n a t u r e o f this b e h a v i o u r has been discussed b y Kisslinger a n d Sorensen 5). D a t a on the 9+ levels exist only for the region a b o v e N = 40 a n d it is clear f r o m fig. 1 t h a t the m a x i m u m excitation energy occurs at N = 50. The only evidence available to confirm or refute the existence o f an increase in excitation energy t o w a r d s N = 38 comes f r o m the arsenic i s o t o p e s where
Ist 9/2 +'STATE
/ / ~
As
0.5
0
33
r
>"
r
t
i
i
Br
Rb
352' 37. /
i
•
39
l
t
,
Ist 2 + STATE
>t9 re
1.5
~< I.O bJ
3oZn
32
2 Fe "
38 •
.
Se
0.5-
28
2
4
6
38 40 42 Neutron Number
44
46
48
50
52
Fig. 1. Plot of the excitation energies of the Ist 2 + states in doubly even nuclei and the I st ~ + states in odd-mass, even-neutron nuclei as a function of neutron number.
aaAs+o is some 120 keV a b o v e t h a t in 75 the 9 + level in 73 aaAs42. I f this p o s t u l a t e d corr e l a t i o n is to be t a k e n seriously then in the nucleus 73 asBra8 the ~+ 2 state w o u l d be exp e c t e d to be some few h u n d r e d s o f kilovolts a b o v e the g r o u n d state. A l t e r n a t i v e l y if such a c o r r e l a t i o n does n o t exist then the d a t a 4) on 77Br, 79Br, 81Br a n d 83Br might be t a k e n to i m p l y that in 73Br the 3 + state should occur at low excitation energy o r indeed be the g r o u n d state.
73Br DECAY
23
No information on 73Br has hitherto been published. In the present work a search has been made for an isomeric level in 73Br and the positon decay of this nucleus to the levels of 735e has been investigated to establish the 73Br ground state spin. Previous work 4) has shown that the 73Seground state decays with a half-life of 7.1 h predominantly to a 428 keV level in 73As which decays by a cascade of 361 keV and 67 keV transitions to the 73As ground state. A 26 keV isomeric state in 735e decays with a half-life of 42 min partly (82 %) to the 73Se ground state and partly (18 %) by positon emission to the states of 73As, the two most intense transitions following the positon decay having energies of 84 keV and 254 keV [ref. 2)]. The 73Se ground state and isomeric state have been assigned spins of ~+ and ½- respectively 6).
2. Experimental procedure In the present work the 73Br activity was produced by the 59Co(160, 2n) reaction with the Manchester Hilac. In those experiments in which chemical separation of the bromine was carried out cobalt targets of thickness 25 mg/cm 2 (purity = 99.999 %) were used to obtain the maximum possible yield. In the chemical separation the irradiated cobalt targets were added to a mixture of potassium permanganate (5 cc) and a carrier solution (1 cm 3) containing small quantities ( ~ 200 pg) of the known reaction products. Concentrated nitric acid (5 cm a) was then added and, after the target had dissolved, the solution was mixed thoroughly with carbon tetrachloride (5 cm a) and decanted. Using radioactive tracers it was found that the efficiency of the bromine separation was > 90 % and that negligible quantities of the other reactions products (i.e. Se, As, Ge, Ga) came through in the bromine fraction. The time interval between the end of the irradiation and the start of the study of the bromine activity was approximately 2.5 rain, the actual instant of chemical separation being some 2 rain after the end of the irradiation. In those experiments in which the (160, 2n) reaction cross section was studied as a function of bombarding energy chemical separation was not employed and one mg/cm2 cobalt targets, prepared by electro-deposition, were used. In the first set of experiments a 25 mg/cmz cobalt target was irradiated for 45 rain by a 0.4 #A, 64 MeV, oxygen beam and the 7-radiation from the chemically-separated bromine studied using a 25 cm 3 Ge(Li) detector. A typical spectrum, recorded immediately after the separation, is shown in fig. 2. The energies of the y-rays, shown in table 1, are the average values obtained from several such experiments and are based on the known energies of transitions in 24tAm, 57C0, 758e, 54Mn, 8sYt, 22Na, 6°C0 and 228Th. Source and calibration runs were taken alternately to minimise the effects of any time-dependent gain changes in the amplifier (Tennelec 200) and by using a relatively large number of calibration lines at close intervals over the energy spectrum the effects of any possible non-linearity in the analogue to digital converter (Laben 4096-channels) were reduced. The 7-ray intensities, shown in table 1, are likewise average values and were computed by summing the number of counts
ou
.
.
.
.
.
.
.
.
500
.
.
~
..
~ 374.3
'
:"; ":'~,:.~ :''~ . . . . .
400.6
275.1|
'
335.7
.
.
.
CHANNEL
.
.
I000
.
.
.
NUMBER
.
F $39.6 ~ ~ ~,550.1
480.6
~'~7-"~-"T,
1500
~7-'*'i"
609.5 614 9 74_ , t 634.5( Brl
,~--"r ~
788.1
, '
",
e
Fig. 2. Gamma-ray spectrum from 73Br produced by the 59Co(160, 2n) reaction followed by chemic,"
.
0
o
i
125.5
2K
64.9
,'%.. ~" .:,~%
~ 4g
Energies
Chemical SE
$9
16 Co( 0 , 2
Br Gamma-
?3
73Br DECAY
25
in the peak, the dependence of counter efficiency on quantum energy being corrected for by using a set of eight gamma sources of known strength supplied by IAEA Vienna. TABLE 1 Energies, relative intensities and associated half-lives of the ~-rays following 73Br decay Energy (keV)
Half-life (sec)
64.9±0.1 125.5+0.1 275.14-0.2 335.74-0.2 374.34-0.2 400.64-0.2 489.64-0.2 annihilation radiation 539.64-0.2 550.1+0.2 614.94-0.2 638.64-0.3 699.54-0.2 788.14-0.3 848.74-0.2 861.74-0.3 869.84-0.3 913.6:k0.2 930.74-0.2 995.64-0.2
2004-20 2004-20 190±40 180+20 2454-40 2104-30 2004-40 2004-30 2054-40 1404-40 2004-40 2804-50 1904-30 2504-40 1954-30 1954-40 2304-40 1854-30 1804-30 1654-40
Relative intensity a) 100 234- 2 104- 1 344- 2 84- 1 204- 1 44- 1 5744-40 84- 2 3~ 1 84- 1 44- 1 404- 3 34- 1 204- 1 34- 1 54- 1 19± 2 224- 2 74- 1
a) Intensities relative to that of the 65 keV 7-ray.
Gamma-ray spectra were recorded at 100 s intervals for 120 min after the chemical separation and from these data the half-life values shown in table 1 were deduced. There was no evidence for any initial build-up in the ~-ray yields and this, together with the fact that chemical separation took place 2 rain after the end of the irradiation, can only be interpreted on the basis that the 3.3 +__0.3 min activity is associated with the decay of a bromine isotope. The 7-spectrum recorded from the bromine fraction thirty minutes after the chemical separation is shown in fig. 3. These ~-rays have been identified with known transitions in 73As following the decay of 73Se and 73roSe with the exception of the 635_+ 1 keV and 730 +__1 keV lines which, on the basis of their energies, associated half-life (38 + 4 min) and relative intensities (7.5 : 1), are assigned to 74Se in accordance with the work of Belyaev et al. 7). The intensities of the 84 keV and 254 keV lines in fig. 3 are shown plotted as a function of time in fig. 4 and their measured decay rates of 40 + 4 min are consistent with their assignment to transitions in 73As following the decay of 73roSe. The initial build-up in the decay curves is consistent with a 3 _+1 min activity feeding the 73Se 40 min level. We thus assign the 3.3 +__0.3 rain activity to 73Br.
26
G. MURRAYet al.
bi,Ocnt,9
tD00
1000
" U.I Z
"~c.
Z
<
t~t/)
~t/)
t'~
~
~
t~
o
t.P
r,-
!a ~, , . . . ~ ; ,
-..
100
,. ".:,, .:'-.
:...;.. 8
101
" .
2{;o
~o
..... .
. ,-...:. "":.. " : '.... .'"
36o
CHANNEL
460
NUMBER
Fig. 3. Gamma-ray spectrum, from the bromine fraction, recorded 30 rain after its chemical separation.
,
84keY
w tn o
9
8Z
10-
1
~
o
(counts+ IOOI
lo6o
2obo TIME
IN
3obo
4obo
5obo
SECONDS
Fig. 4. Plots, as functions of time, of the yields of the 84 keV and 254 keV ?-rays following the 7a"Se decay and the 361 keV ?-ray following the 7aSe decay recorded from the bromine fraction.
73Br DECAY
27
The build-up and decay curve in the 73As 361 keV transition is also shown in fig. 4. This curve is completely compatible with the 73Se 40 rain level feeding the 735e 7.1 h ground state which decays to the 428 keV 73As level Due to the poor statistical accuracy of the initial points on this curve however it is not possible to completely exclude the presence of a small 3 min component in the build-up arising from direct feeding of the ~3Se ground state by the 73Br ground state. An upper limit on the amount of this direct feeding was established using the known decay schemes of 7aSe and 7arose to predict the relative intensities of the 361 keV and 84 keV transitions assuming that the 73Se ground state was populated only through the decay of the 73roSe isomeric level. This predicted value of the relative intensities was found to be identical to that observed experimentally within the limit of error of 10 ~. We conclude that the intensity of the 73Br ground state decay to the 73Se ground state is less than one-tenth that to the 7aSe isomeric state. A second set of experiments were carried out to investigate the possible existence of a low-lying isomeric state in 73Br with a half-life significantly greater than the 3.3 min ground state. The same irradiation procedure was followed as in the first set of experiments but the chemical separation procedure was not carried out until twenty minutes after the irradiation. The only 7-rays from the bromine fraction were the 635 keV and 730 keV lines following the 74Br decay and, in particular, no 7-rays following the decay of 7aSe or 7aroSe were observed. We conclude that 73Br does not have a low-lying isomeric level of half-life > 3 min. 3. The (140, 2,) yield function In the preceding section evidence has been presented for the assignment of a 3.3_+0.3 min activity to 73Br. Corroborative evidence for this was obtained from a study of the dependence of the yield of the 3.3 min activity on the bombarding energy of the heavy ion in the reaction s 9Co(160, 2n)VaBr. Thin cobalt targets (1 mg/cm 2) were bombarded for a period of 45 min by a 0.02 pA 160 beam of known energy and the yields of the 7-rays, observed using a Ge(Li) detector, recorded for a period of 10 min after the irradiation. The bombardment time of 45 rain was initially chosen to enhance any possible activities with half-lives significantly longer than 5 min in bromine nuclei with A < 74 but none, in fact, were observed. No chemical separation was carried out in these experiments because the other activities present were useful in establishing the energy dependence of the cross sections for different reactions. The yields of the 65 keV and 125 keV transitions associated with the 3.3 min activity and of the 67 keV and 361 keV v-rays following the 735e 7.1 h activity are shown as a function of heavy ion bombarding energy in fig. 5. The 73Br activity can be produced only by the (160,2n) reaction whilst the 73Se activity can be produced by the (160, rip) reaction and by the 59C0(160, 2n) 73Br(fl+)TaSe process. The similarity in yield functions of the 73Br and 7ase activities shown in fig. 5 implies either that the energy dependence of the (160, np) and the
28
G. MURRAYe t
al.
(160, 2n) reaction cross sections are very similar or that the yield of the former reaction is considerably smaller than the latter over the energy range under consideration. Further evidence on this point was obtained by comparing the ratio of the yields of the 73Se 125 keV transitions and the 73As 85 keV transition, observed from 59Co targets bombarded by 160 ions at energies from 38 MeV to 60 MeV, with the ratio computed from the decay schemes of the two nuclei assuming that the 73Se was not produced directly. The observed and calculated y-ray intensity ratios were found to be consistent within experimental error (typically 10 ~o) except at bombarding energies above 52 MeV where significant deviations were noted. The sign '
~
,
,
1.0
198 kzV
>. <
7
n. f13 rv
eV
3S¢ 361 key
102 kzV z Lfl
j
3S¢ 67k
z 0.5
;.L ,'n
73Br 65ke
/ SSk,V
\
.J
<
o
73Br 12S ' / / /
k,v.~/
30
X
50 70 HEAVY ION BOMBARDING ENERGY [MeV]
90
Fig. 5. Excitation function of y-rays emitted after the bombardment of S9Co by 160 ions. of the deviations is indicative of an enhancement of the 73As 85 keV transition relative to the 73Se 125 keV transition and these deviations are thus interpreted as the result of the (160, np) reaction. The predominant reaction at and below 52 MeV is thus (160, 2n) and the results shown in fig. 5 therefore confirm the assignment of the 3.3 min activity to the decay of 73Br. The other yield functions shown in fig. 5 will be discussed in sect• 5.
4. The positon decay of 7aBr The end-point energy of the 73Br 3.3 min positon activity was measured using a plastic scintillator from which the pulses were amplified and recorded in a Laben 1024-channel analyser. The system was calibrated using 22Na, 22STh and an americium-
73Br DECAY
29
beryllium source. Cobalt targets, of thickness 1 mg/cm2, were bombarded for 5 min by a 0.5 pA, 64 MeV oxygen beam and then placed directly in front of the counter. The Fermi-Kurie plot of a spectrum taken immediately after an irradiation is shown in fig. 6 and is observed to have two components of end-point energies 4.5 ___0.3 MeV and 3.7 +0.5 MeV. To measure the decay constant of each component, spectra were recorded at 100s intervals after the irradiation and in the analyses spectra were divided into sub-groups of 8 channels each. All sub-groups up to an energy of 4.5 MeV had a component of half-life 40_+ 5 min present and thus this activity was attributed to the decay of 74Br' both the end-point energy value and the half-life value being consistent i
i
Fermi- Kuri¢ Plot 30
Reaction t60 + 59C0
Bombarding Energy--45 MeV
_, 2 . 0 -
Component A
.SO
7.0
9,0 W ( MoC 2)
Fig. 6. The Fermi-Kurie plot of the positons emitted from 73Br and 74Br produced by the s9C0(~70, Xn) reaction with chemical separation.
with the previous measurements of Butement and Boswell s). Confirmation of the presence of 74Br in the irradiated targets comes from the observation of the 635 keV and 730 keV ~-rays in 74"Se as discussed in sect. 2. Above an energy of 3.7 MeV no subgroup was observed to have a component of half-life less than 40 min in its decay curve. Below 3.7 MeV all sub-groups showed a component of half-life 3.5-t-0.5 min. We thus attribute the component with end-point energy 3.7+_0.5 MeV to the decay of 7~Br. The above value for the 73Br end-point energy implies a Q-value of 4.7 +_0.5 MeV in good agreement with the predicted value of Mattauch et al. 9). The predicted value of 1.5+_1.0 MeV in ref. 1) is a misprint and should read 4.5+_1.0 MeV [ref. lo)].
30
G. MURRAY e t aL
5. Search for an isomeric level in 73Br
As discussed in sect. 1 there is some possibility of the existence of a 9 + ground state or near ground state level in 73Br. In either situation an isomeric state and associated 9 + --r 3 - ) might be expected in an analogous way to the E3 transition (3- __> 9+ or ~4.3 min 108 keV level and the 4.8s 208 keV level in 77Br and 79Br respectively. The possibility of an isomer in 73Br of half-life greater than about one minute has virtually been excluded by the work described in the preceeding sections, in particular by the absence of unassigned 7-rays in spectra such as fig. 2, by the absence of evidence of a build-up in the yields of the y-rays following the 3.3 min 73Br activity, by the absence of an unassigned component in the decay curve of the annihilation quanta and by the absence of any activity other than 74Br from the bromine fraction chemically separated 20 min after the end of the irradiation. A search was carried out for a 73Br isomeric state with a half-life of < 1 min. A cobalt target (thickness = 1 mg/cm z) was bombarded by an oxygen beam of known energy from the Manchester Hilac which has a pulse length of 2 ms and a repetition frequency of 10 per second. Between beam pulses the g a m m a radiation emitted from the target was studied as a function of time using a Ge(Li) detector, amplifier and multi-channel analyser (Laben 1024-channels) operated in its subgroup stepping mode, typical operating parameters of the latter being four groups of 256 channels stepped at 20 ms intervals. This irradiation and counting procedure was carried out for 45 min for each of 14 heavy-ion bombarding energies between 35 MeV and 94 MeV. Just prior to the end of each 45 min irradiation the contents of the multichannel analyser were recorded on paper tape, the memory cleared, and some five seconds after the end of the irradiation a subsidiary experiment was started in which a set of spectra were taken, at 100 second intervals, of the y-rays emitted from the activities still present in the target. F r o m a comparison of the spectra in the four sub-groups recorded between beam pulses it was possible to identify 7-rays associated with half-lives in the range 1-100 ms. Two such y-rays were observed with energies of 176 keV and 198 keV and half-lives of 25__.2 ms and 21+__10 ms respectively. The intensities of these y-rays, normalised with respect to integrated beam intensity, are plotted as a function of bombarding energy in fig. 5. From a comparison of the spectra recorded between beam pulses with these recorded after the irradiation it was possible to pick out activities with half-lives greater than 100 ms. Two y-rays, at energies of 55__ 1 keV and 102+ 1 keV, were observed each to have the same intensity in the four sub-groups recorded between beam pulses but to be absent in the spectra recorded after the irradiation implying that they were associated with half-lives in the range 0.1s-2s. The dependence of their yield on the heavy ion bombarding energy is shown in fig. 5. Also shown in the same figure is the relevant data on an observed 116 keV y-ray associated with a half-life of 15 min. A comparison of the yield curves presented in fig. 5 shows that none of the five activities discussed in the preceeding paragraph are associated with 73Br and further
73Br DECAY
31
experiments are in progress to identify the nuclei to which they belong. No such short-lived activities were observed in the excitation region 40-60 MeV and we conclude that there are no ~-emitting isomeric states in 73Br with half-lives greater than 1 ms. The possibility must also be considered of an isomeric activity in :aBr which deexcites with no 7-emission either by positon decay directly to the VaSe ground state or by internal conversion. If an isomeric level were to exist then the heavy-ion reaction might be expected to favour the production of the high spin state. The possibility of direct positon emission to the VaSe ground state can be excluded on the basis of the calculations discussed in sect. 2 which showed that, at excitation energies below 52 MeV, the amount of Vase activity observed is completely consistent with that expected from the known 3.3 min 73Br decay. In the case of a strongly internally converted transition from the isomeric level to the ground state it has been estimated that if it has an ~: of less than 5 the 7-radiation would have been observed in this work. If the transition has an ~: of greater than 5 then the energy must be less than 105 keV [ref. 11)] and such an E3 transition would be expected to have a half-life greater than 200 seconds, c.f. v 7Br ' 108 keV, 4.3 rain; a 1Se' 103 keV, 57 rain; SlRb, 85 keV, 32 min. The build-up in the :3Br ground state activity due to the decay of the isomeric state would have been clearly visible in this case. From the work described above we conclude that there are no isomeric states in 73Br with half-lives greater than 1 ms. 6. Discussion of results
The proposed VaSe level scheme is shown in fig. 7 and has been established from the data in table 1 together with the cascade crossover sums shown in table 2. The 374 keV, 540 keV and 862 keV transitions have not been placed in the scheme. From previous work 2) the spins of the ground state and 26 keV levels are ~+ and ½- respectively. The absence of evidence for a 3 min build-up in the 7ase ground state decay (sect. 2) implies that the VaSe 7-rays feed the 26 keV level rather than the ground state. As discussed above, the intensity of the fl-decay from the 73Br ground state to the Vase ground state is less than 10 ~o of that to the Vase isomeric level indicating that the 73Br ground state spin is not ~+. The known odd-proton nuclei between Z = 29 and Z = 37 inclusive have ground state spins of ~2- with only three exceptions all of which have spins of -~- [ref. g)]. The allowed fl-transition from the ground state of 7aBr to the ½- isomeric level of 73Se excludes a ~- assignment. We therefore assign a spin of 3 - to the 73Br ground state. The absence of an isomer in VaBr with half-life greater than 1 ms enables a lower limit on the energy of the ~+ level to be established. If there is no low-lying ~state in V3Br then the ~+ state would necessarily have to be at least 500 keV above the ground state otherwise an E3 transition of half-life greater than 1 ms would be observed. However many of the nuclei in this region have a low-lying -~- state. If
312
3 rain 17~Br I I
2 1022/~o
641/ 427/
i_
ISI
ll2" 7•2 +
~
~
1
73 5e
b.r~
~'_ 26 ~'/'42 rain ½ 7.1 h
]FIB. 7. Proposed ~aBr decay scheme TABLE 2 E n e r g y s u m s r e l e v a n t t o t h e 91 k e V a n d 151 k e V levels DirectT-ray energy measurement
=
64.9±0.1
400.64-0.2--335.7±0.2 6 1 4 . 9 4 . 0 . 2 - - 5 5 0 . 1 -t-0.2 913.6 ±0.2--848.7±0.2 995.6±0.2--930.7±0.2
= = = =
64.9:E0.3 64.84-0.3 64.94-0.3 64.9±0.3
Direct 7-ray energy measurement 400.64.0.2--275.14-0.2 614.94.0.2--489.64.0.2 913.64-0.2--788.1 4-0.3 995.64-0.2--869.84-0.3
= = = = =
125.5±0.1 125.54-0.3 125.3 4 - 0 . 3 125.54-0.4 125.8+0.4
335.7-L0.2--275.1 i0.2 550.1 ± 0 . 2 - - 4 8 9 . 6 ± 0 . 2 699.54-0.2--638.64-0.3 848.74-0.2--788.1 4-0.3 930.74-0.2--869.84-0.3
= : ~ = =
60.64-0.3 60.54-0.3 60.94-0.4 60.64-0.4 60.94-0.4
Energies in keV.
73Br DECAY
33
such a state exists in 73Br then from single-particle estimates together with observed retardation factors of M2 transitions in this region 4) a ~+ state would need to be more than 130 keV above the ~- state if its half-life is to be less than 1 ms. The implication of the present results is therefore that the ~+ state in 73Br is at a higher excitation energy than the corresponding level in 77Br (108 keV). From the present work and previously published results 4) it is observed in the odd-mass bromine and arsenic isotopes, that the ~+ levels in N = 42 nuclei are at a lower excitation energy than in N = 38 and N = 40 nuclei respectively (see fig. 1). The apparent correlation between the excitation energies of the 79 + levels and the
0.~
B ( E 2 ) VALUES OF I ~t 2 "I" STATES (uncertointies in experirnentol values ~ 10%)
¢-.,Eu0.4
Z ~ ~
-
0.2
Z 3
3'4
3'6 3'8
I
,o
Neutron Number
!
so
Fig. 8. Plot of the B(E2) values of the first 2 + states in doub|y even nuclei as a function of neutron number.
first 2 + levels may be interpreted as the result of an increase in nuclear deformation towards the middle of the shell since, with increasing deformation, the l g~ level would be expected to penetrate more into the lf-2p shell thus leading to the observed lowering of the ~+ state with respect to the negative-parity ground state 3). This interpretation is consistent with the trend observed in the experimental B(E2) values to the first 2 + states in doubly even nuclei in this region as shown in fig. 8 where the B(E2) values are lowest around N = 38 and N = 50 and have the maximum value at N = 42. References I) I-I. Ikegami and M. Sano, Phys. Lett. 21 (1966) 323 2) G. Murray, W. J. K. White, J. C. Willmott and R. F. Entwistle, Nucl° Phys. A130 (1969) 563
34 3) 4) 5) 6) 7) 8) 9) 10) 11)
G. MURRAYet al. W. Scholz and F. B. Malik, Phys. Rev. 176 (1968) 1355 and private communication Nucl. Data 1B (1966) 4, 6 L. S. Kisslinger and R. A. Sorenson, Rev. Mod. Phys. 35 (1963) 853 G. Murray, W. J. K. White, J. C. Willmott and R. F. Entwistle, Phys. Lett. 28B (1968) 35 B. N. Belyaev, B. A. Gvozdev, V. I. Gudov, A. V. Kalayamin and L. M. Krizhanskii, Yad. Fiz. 3 (1966) 609 F. D. S. Butement and G. G. J. Boswell, J. Inorg., Nucl. Chem. 16 (1960) 10 J. H. E. Mattauch, W. Thiele and A. H. Wapstra, Nucl. Phys. 67 (1965) 1 N. B. Gove, private communication R. S. Hayer and E. C. Seltzer, Nucl. Data 4 (1968) 1, 2