A study of the level scheme of 74As by the 74Ge(p, nγ)74As reaction

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Nuclear Physics A213 (1973) 61--81; (~) North-HollandPublishin9 Co., Amsterdam Not to be reproduced by photoprint or microfilmwithout written permission from the publisher

A S T U D Y O F T H E L E V E L S C H E M E O F 74As

BY THE 74Ge(p, nT)74As REACTION KIKUO KIMURA

Department of Physics, Faculty of Science, Kyushu University, Fukuoka, Japan Received 24 April 1973 (Revised 9 July 1973) Abstract: A level scheme of 74As lip to about 700 keV excitation energy has been determined by threshold energy measurements on the reaction 74Ge(p ' n7)7ams" For eleven levels below 465.3 keV, spins and parities were assigned by using transition multipolarities obtained from the observed internal conversion coefficients and by comparing relative cross sections for the (p, n) reactions leading to various residual levels with those predicted by Hauser-Feshbach theory. E

NUCLEAR REACTIONS 74Ge(p, nT), E = 3.3-5.3 MeV; 74Ge(p, nce), E = 5.3 MeV, measured ET, I~,, or(E; E7), Ic¢. 74As deduced levels, J, ,-t, Icc, 7-branching, 7-multipolarities.

1. Introduction The level structure o f the d o u b l y o d d nucleus 74As has recently been studied by several authors. F i n c k h et al. 1) have d e d u c e d n u m c r o u s excited levels up to a b o u t 1.4 M e V excitation energy by n e u t r o n time o f flight studies o n the 74Ge(p, n) reaction. Based on this result Christiansen et al. 2) m e a s u r e d 7-rays following the (p, n) r e a c t i o n and c o n s t r u c t e d a decay scheme o f levels below a b o u t 700 keV t h r o u g h the ,/-ray spectra a n d 7-7 coincidence mcasurements. A study o f the 75As(p ' d)74As reaction has been r e p o r t e d by F o u r n i e r el al. 3). T h e y d e t e r m i n e d transferred l n values and spectroscopic factors f r o m a D W B A analysis o f the d e u t e r o n a n g u l a r distributions. H o w e v e r , the resolution in all these m e a s u r e m e n t s was n o t high e n o u g h to reveal the c o m p l e x level structure o f this d o u b l y o d d nuclei, and thus the level scheme so far o b t a i n e d does n o t seem to be complete. The first aim o f the present investigation is to refine the level scheme o f 74As t h r o u g h the m e a s u r e m e n t o f t h r e s h o l d energies a n d excitation functions o f the i n d i v i d u a l 7rays f r o m the (p, n) reaction. A n a t t e m p t is also m a d e to d e t e r m i n e spins a n d parities by c o m p a r i n g the o b s e r v e d (p, n) cross sections o f the various levels with the ones p r e d i c t e d by H a u s e r - F e s h b a c h t h e o r y 4). U s u a l l y it is n o t easy to d e t e r m i n e b o t h spin a n d p a r i t y by only this p r o c e d u r e . The d e t e r m i n a t i o n o f t r a n s i t i o n m u l t i p o l a r i t i e s is helpful to m a k e the assignments unique, a n d in the p r e s e n t w o r k this was a c c o m p l i s h e d by m e a s u r i n g internal conversion coefficient~ 61

62

K. KIMURA 2. Excitation functions of 7-rays

2.1. APPARATUS AND MEASURING PROCEDURE A proton beam obtained from the Kyushu University Van de Graaff accelerator was focused in a target chamber at a distance of about 10 m from the accelerator. The beam was stopped in a Faraday cup made of an iron duct 3 m in length. A target about 400/~g/cm 2 thick was prepared by evaporating isotopically enriched GeOz onto a thin carbon backing. The isotopic composition of the target material * was: 74Ge (94.48 %), 7°Ge (1.71%), 72Ge (1.71%), 73Ge (0.90%) and 76Ge (0.70%). G a m m a - r a y spectra were measured by a 30 cm 3 coaxial Ge(Li) detector ( O R T E C ) and a 1 cm 3 planar Ge(Li) detector (home made), which were fixed at 120 ° and 60 °, respectively, to the beam direction. The 1 cm 3 Ge(Li) detector was used mostly to measure low energy 7-rays below about 300 keV. It faced the thin 0.2 m m thick A1 wall of the target chamber. However, it was not useful for ~-rays below about 30 keV owing to absorption in the detector chamber and because of the dead layer of the detector. The resolutions ( F W H M ) of the 30 cm 3 Ge(Li) detector were 1.9 and 2.8 keV for 122 and 1173 keV ~-rays, respectively, while that of the I cm 3 Ge(Li) detector was 1.5 keV for 122 keV ~:-rays. The 7-ray excitation functions were measured by changing the average proton energy in steps of 20 keV in the energy range 3.3 to 4.2 MeV. During each experimental run, the beam current was adjusted so that the counting rate of the 30 cm 3 Ge(Li) detector did not exceed 1000 counts/sec. In this condition, the dead time loss of the pulse height analyser (4096 channel) was less than 10 %. The relative intensities of the 7-transitions were measured at Ep = 4.0 and 4.2 MeV by the above mentioned 1 cm 3 Ge(Li) detector, and a 45 cm 3 Ge(Li) detector (home made). Their photopeak detection efficiencies were calibrated using I A E A standard sources placed at the target position. 2.2. RESULTS In fig. 1, typical y-ray spectra taken by the 30 cm 3 Ge(Li) detector at Ep = 3.3 and 4.2 MeV are shown. According to ref. 1), the Q-value of the 74Ge(p, n)74As reaction is - 3 . 3 4 6 MeV. In the spectrum at Ep = 3.3 MeV, strong peaks marked by black triangles are due to the 7~Ge(p, 7) reaction while those marked by open triangles are due to the 74Ge(p, p ' ) reaction. Contributions f r o m contaminant isotopes are very small; they are indicated by the letter C. Background radiations from the target chamber, which was made of aluminium and brass, are rather strong, and their isotopic origins are explicitly indicated in the spectrum. In the 4.2 MeV spectrum, only the peaks due to ~-rays from the 74Ge(p, n) reaction are given energy values (in keV). The energy values of these 7-rays were determined by using the 7-rays from the V4Ge(p, 7) reaction as the internal standards, t Supplied by the Isotopes Division of Oak Ridge National Laboratory.

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whose energy values had been accurately determined in a study of electron capture in 7SSe [ref. 5)]. Fig. 2 shows some of the observed excitation functions for 7-rays from the (p, n) reaction. The intensities of the different y-transitions are expressed as relative intensities, in arbitrary units. The error bars on the experimental points include errors due to statistics and background subtraction only. Fig. 3 shows an excitation function for a doublet peak, 278.4 keV+279.6 keV; the former is due to the (p, n) reaction, the latter to the (p, 7) reaction. This figure is discussed in subsect. 4.1.

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74As LEVEL SCHEME

65

Threshold energies for various residual levels in the (p, n) reaction were determined in the following manner: The observed 7-ray intensity was plotted versus the c.m. proton energy on a linear scale and the observed points lying within an energy range less than about 50 keV above the threshold were linearly extrapolated, and the intercept with the proton energy axis was taken as the c.m. threshold energy. The results are given in the third column of table 1 and are denoted by Eth(c.m. ). The errors in the threshold energies do not involve errors in the proton energy, which are probably less than 5 keV. For weak transitions, whose excitation functions could not be measured with good statistics, only an upper limit to the threshold energy was determined. I0 a

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In order to obtain the ground state Q-value of the (p, n) reaction, it was assumed that the 173.1 keV and 183.0 keV y-rays, which had the two lowest threshold energies, corresponded to transitions to the ground state• As can be seen from table 1, the above two y-rays give ground state Q-values of -3347___ 3 keV and - 3 3 4 7 _ 4 keV, respectively. These two values agree well with each other and also with the ground state Q-value, -3346_+ 5 keV, given by Finckh et al. t) from their measurement of the (p, n) neutron energy, and the tabulated value, -3346.0+ 3.6 keV, of Mattauch e t al. 6). Thus the excitation energies in 74As corresponding to all other (p, nT) thresholds were calculated by the equation, E~ = Eth(c.m.)--3347 keV. The results are tabulated in the fourth column of table 1.

66

K. K I M U R A

TABLE 1 T h r e s h o l d energies (in the c.m. system) o f y-rays following t h e 74Ge(p, n)V~As reaction a n d excitation energies o f 74As at these threshold Peak no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

Ey (keV)

Et. (c.m.) (keV)

76.2 --0.2 84.4 88.6 147.57±0.10 149.2 i 0 . 6 168.4 4-0.2 173.08 :k0.10 182.984-0.10 202.11 4-0.10 206.54:k0.10 212.0 4-0.4 215.654-0.10 224.104-0.15 227.71 ± 0 . 1 5 235.6 4-0.2 240.284-0.15 249.09±0.15 252.9 4-0.6 258.80±0.10 267.40___0.10 271.724-0.10 278.44 1 0 . 1 5 307.544-0.15 327.474-0.15 332.2 i 0 . 2 345.564-0.15 350.4 4-0.5 354.74±0.15 358.6 4-0.5 368.9 ± 0 . 6 373.6 4-0.6 379.6 ± 0 . 2 385.06=k0.15 413.0 i 0 . 6 425.7 4-0.6 427.5 ± 0 . 5 443.5 ~ 0 . 4 447.9 i 0 . 3 476.6 ± 0 . 4 494.8 ± 0 . 4 506.7 ± 0 . 6 514.l ± 0 . 4 523.5 i l . 0 525.8 ± 1 . 0 528.3 ± 0 . 6 532.9 4-0.6 548.0 --0.6 551.8 ± 0 . 6 570.6 +__0.6

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74As LEVEL SCHEME

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TABLE 1 (continued) Peak no. 50 51 52 53 54 55

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3. Internal conversion coefficients 3.1. APPARATUS AND MEASURING PROCEDURE The p r o t o n b e a m f r o m the Van de G r a a f f accelerator was t r a n s p o r t e d to a sectorfield d o u b l e - f o c u s i n g / 3 - r a y s p e c t r o m e t e r 7), o f m e a n o r b i t radius 18 cm, a n d was focused o n a target p l a c e d in the magnetic field o f t h e / 3 - r a y spectrometer. A t a dist a n c e o f a b o u t 3 m b e h i n d the target the b e a m was s t o p p e d in a F a r a d a y cup, which was shielded by a concrete wall. Electrons emitted f r o m the target at a b o u t 180 ° to the b e a m direction were focused on the focal p l a n e o f t h e / 3 - r a y spectrometer. Since this /3-ray s p e c t r o m e t e r has a n a t u r e o f a b r o a d - r a n g e spectrometer, electrons were detected by a 20 channel G M c o u n t e r a r r a y p l a c e d on the focal plane. T h e c o u n t e r a r r a y e x t e n d e d 11 cm along the focal line. This length c o r r e s p o n d e d to a b o u t 5 ~o o f the m o m e n t u m range. The m e a s u r e m e n t o f internal c o n v e r s i o n electrons was m a d e at a p r o t o n energy o f 5.3 MeV. The w i n d o w o f t h e / 3 - r a y s p e c t r o m e t e r was set for a m o m e n t u m r e s o l u t i o n o f 1 % . F o r electrons b e l o w 100 keV, a 2 5 0 / t g / c m z GeO2 target e v a p o r a t e d o n t o c a r b o n b a c k i n g was used, while for electrons a b o v e t h a t energy a 550 # g / c m 2 t a r g e t was used. A n electron s p e c t r u m was m e a s u r e d by changing the s p e c t r o m e t e r c u r r e n t in steps o f a b o u t 5 %, being m o n i t o r e d b y the 7-ray counts which were o b t a i n e d b y the 1 cm 3 G e ( L i ) d e t e c t o r fixed at 30 °. The electron counts, n o r m a l i z e d to the m o n i t o r counts, were then corrected for the relative solid angle o f each c o u n t e r c h a n n e l a n d the s p e c t r u m plotted. G a m m a - r a y spectra o b t a i n e d by the m o n i t o r d e t e c t o r were also used to d e t e r m i n e internal c o n v e r s i o n coefficients, which are discussed in subsect. 3.2. 3.2. RESULTS Fig. 4 shows a n electron s p e c t r u m at Ep = 5.3 MeV. The h o r i z o n t a l axis o f this figure ( p o t e n t i o m e t e r setting) is p r o p o r t i o n a l to the electron m o m e n t u m . The spacing between two a d j a c e n t d a t a p o i n t s is equal to 0.26 ~ o f the m o m e n t u m increment, w h i c h is the m o m e n t u m dispersion o f one c o u n t e r channel o f the G M c o u n t e r array.

K. K I M U R A

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Under the present window setting of the fl-ray spectrometer (1% momentum resolution) the F W H M of an electron line corresponds to about 4 channels. As seen from fig. 4, K and L conversion electrons could be resolved well but L and M conversion electrons could not be resolved except for the lowest energy transition of 76.2 keV. The 76.2 keV line looks broader than others owing to a self-absorption effect in the target. The target thickness, 250 l~g/cmz, was selected so that the whole line shape of -t IOi

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Fig. 5. Comparison of the observed and theoretical internal conversion coefficientsfor K electrons, ~K, and LWM electrons, ~L+M. The theoretical values of Hager and Seltzer s) are shown by solid and dotted lines. the 76.2 K line could be covered within one potentiometer setting. Peak areas of the different lines in fig. 4 give directly the relative intensities of internal conversion electrons at 180 ° to the beam direction. The results are given in the fourth column of table 2. The intensity of the 76.2 K line is normalized to 100. For pairs of transitions whose enegies differ by the binding energy difference of K and L + M electrons in the As atom (10 keV), the K and L + M lines of the respective transitions overlap, and in such cases summed intensities are given. For electrons above 300 keV, the 1% momentum resolution was not enough to resolve complex electron lines as expected from 7-ray spectra and thus their measurement was abandoned. The relative intensities of the y-rays observed by the 1 cm 3 Ge(Li) monitor detector are given in the second column of table 2. Internal conversion coefficients (hereafter denoted by ICC) were determined by using the electron intensities at 180° and ~-intensities at 30 °. The absolute detection efficiency of the monitor detector was measured by using I A E A standard sources. The effective solid angle of the fl-ray spectrometer including the counter array, f2p,

70

K. KIMURA

was determined by m e a s u r i n g both internal conversion electrons a n d 7-rays i n the following reactions: a 54 keV (½- ~ ) - ) transition following the 65Cu(p, n ) 6 5 Z n reaction, a n 86 keV (½- ~ ) - ) t r a n s i t i o n following the 69Ga(p, n ) 6 9 G e reaction, a n d the radioactive decays of s 7Co a n d 137Cs" I n the case of the 54 keV a n d 86 keV TABLE2 Relative intensities for v-rays and internal conversion electrons from the reaction V4Ge(p, n)V4As at Ep = 5.3 MeV and the observed internal conversion coefficients Ey (keV) 76.2

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Transition

100

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} M1 M1 MI M1 ÷E2 E2

} E1 (MI)

a) Relative intensities ofT-transitions observed at 30°. b) Relative intensities of internal conversion electrons observed at 180°. c) including the intensities of the 84.4 K and 88.6 K electrons. transitions, pure E2 transitions were assumed a n d theoretical I C C values s) were used to o b t a i n £2p. F r o m these m e a s u r e m e n t s it was f o u n d that the a b s o r p i o n in the window foil (12 p m thick mylar) of the G M c o u n t e r was less t h a n 10 ~o for electrons d o w n to a b o u t 70 keV and, at most, 30 ~o for 45 keV electrons. The cut-off energy of the w i n d o w was 25 keV. Thus it was f o u n d that the window a b s o r p t i o n may be neglected for most of the lines shown in fig. 4. It m a y affect the intensity of the 76.2 K line, b u t n o t more t h a n 10 ~o. The observed I C C values are tabulated in the fifth c o l u m n o f table 2. I n fig. 5 they are compared with the theoretical values of Hager a n d Seltzer [ref. 8)]. T r a n s i t i o n multipolarities determined by this c o m p a r i s o n are given in the

V~As LEVEL SCHEME

71

last column of table 2. The effect of the angular distributions of y-rays and electrons on the observed ICC values at Ep = 5.3 MeV was estimated to be 10 % or less for dipole and quadrupole transitions from levels with spin values less than 4, by using the formula for the angular distribution of y-rays following a statistical compound nuclear reaction. Thus the above effect was found to be not so large as to alter the transition multipolarity assignments of table 2. The observed ICC values quoted in parentheses correspond to the composite lines previously mentioned. For these lines, only K conversion coefficients were determined by subtracting the contributions of L + M electrons, whose intensities were obtained from the intensities of the K electrons of the corresponding transitions assuming theoretical values of K / ( L + M ) ratios. As seen from fig. 5, the obtained K and L + M conversion coefficients indicate the same multipolarities to within the experimental error. For the 76.2 keV transition, the L + M conversion coefficient includes the unresolved components of the 84.4 K and 88.6 K electrons. It should, therefore, be a little too large. Thus the 76.2 keV transition is probably El, although the possibility of M1 cannot be completely excluded.

4. Analysis and discussion 4.1. ENERGY LEVELS AND DECAY SCHEME The levels of 7gAs and their decay scheme shown in fig. 8 have been constructed by using the E x values and y-ray energies listed in table 1, also taking into account the behaviour of the excitation functions and the relative intensities of the y-transitions. The first and second excited levels at 173.1 and 183.0 keV, respectively, are assigned because, as described in subsect. 2.2, the 173.1 keV and 183.0 keV y-rays give the ground state Q-values which agree well with the previous results. In the energy range above 30 keV, no y-transitions due to the 74Ge(p, n) reaction could be observed below the threshold energy of the 173.1 keV level. As seen from table 1, the observed threshold energies of the 202.1 keV and 206.5 keV y-rays agree with each other within experimental error. But they appear to belong to different levels because their relative intensity changes largely with proton energy. From the E~ values of these y-rays, both are assigned to be ground state transitions. The same situation as above exists for the 267.4 keV and 271.7 keV y-rays. They also are assigned to be ground state transitions. F r o m 267.4 keV and 271.7 keV levels, transitions of 84.4 keV and 88.6 keV which seem to feed the 183.0 keV level are weakly observed. A 278.4 keV y-ray could not be resolved from a 279.6 keV y-ray, which was a ground state transition from the 279.6 keV (-~-) state of 75As fed by the 74Ge(p, y) reaction. In fig. 3, a summed excitation function of these y-rays is shown. Since all the excitation functions of the y-rays following the (p, 7) reaction were observed to have almost the same shape, the contribution of the 279.6 keV y-ray was subtracted from the summed excitation function by using the excitation curve for the other y-ray from the (p, 7) reaction. Result of this subtraction is indicated by the crosses in fig. 3.

72

K. KIMURA

A value for the threshold energy of the 278.4 keV 7-ray was determined from this subtracted curve. It clearly shows that the above y-ray is a ground state transition. The observed threshold energy of the 76.2 keV y-ray agrees with that of the 278.4 keV y-ray and also the intensity of the 76.2 keV y-ray relative to the 278.4 keV y-ray is observed to be constant to within about 10 ~ with respect to the proton energy. Hence it appears that the 278.4 keV level is de-excited by the 76.2 keV transition to the 202.1 keV level. But the analogue resonance at Ep = 3.804 MeV is not seen in the excitation function of the 202.1 keV transition as strongly as the resonance intensity in the 76.2 keV transition (see fig. 2) and the intensity of the 76.2 keV transition exceeds that of the 202.1 keV transition above Ep = 4.4 MeV by more than a factor of two. These facts disprove the above assignments. In their y-y coincidence work, Christiansen et al. 2) have observed that a 76.3 keV ~/-ray (76.2 keV y-ray of the present work) is in coincidence with a 147.2 keV y-ray (147.6 keV), which has been assigned to feed a 273.8 keV level, and a 182.6 keV y-ray (183.0 keV) but not with y-rays around 200 keV. In order to explain this result, they assumed a 76.3 keV-182.6 keV-14.9 keV cascade from the 237.8 keV level and attributed the 182.6 keV y-ray to a level at 197.5 keV (see fig. 7). In the present experiment, a shoulder which seems to be due to a contribution from a cascading transition is observed in the excitation function of the 183.0 keV y-ray just at the threshold energy of the 76.2 keV transition. In fig. 2 it is indicated by an arrow. The resonance at Ep = 3.804 keV is clearly seen in this excitation function and the intensity of the 183.0 keV transition is consistently larger than that of the 76.2 keV transition. These facts support the conclusion that the 76.2 keV transition is followed by the 183.0 keV transition. According to the E~ value, however, it is difficult to attribute the 183.0 keV transition to a level above 190 keV, and, as is discussed later, the 147.6 keV transition must feed the 278.4 keV level. Thus we need to assume a 76.2 keV-19.2 keV-183.0 keV cascade or a 19.2 keV-76.2 keV-183.0 keV cascade from the 278.4 keV level instead of the 76.3 keV-182.6 keV-14.9 keV cascade of Christiansen et al. At present, the possibility of the 76.2 keV-19.2 keV-183.0 keV cascade is rejected by the fact that the intensity of the 76.2 keV y-ray relative to the 278.4 keV y-ray slowly increases above Ep = 5 MeV. This may be explained by the existence of a 259.2 keV ( = 76.2 k e V + 183.0 keV) level with a relatively high spin. Direct evidence for the 19.2 keV transition could not be obtained because of its low energy. F r o m the threshold energy only, it is not clear that the 147.6 keV transition feeds which of the three levels at 267.4, 271.7 and 278.4 keV. The former two possibilities are excluded because the intensity of the 147.6 keV transition exceeds the decay intensities of the 267.4 keV and 271.7 keV levels, respectively, above Ep = 4 MeV. A 258.8 keV transition is considered to feed the 206.5 keV level but not the 202.1 keV level. This is so because when the intensity of the 258.8 keV transition is subtracted from the 202.1 keV transition, the resultant intensity tends to decrease above the threshold energy of the 258.8 keV transition, whereas for the 206.5 keV transition, only a smoothly increasing curve is obtained.

74As

LEVEL SCHEME

73

The observed intensities of the 7-rays from levels above 506.7 keV were mostly very weak and only upper limits for their threshold energies could be determined. However, because the energy of the first excited state is as large as 173.1 keV, it is only easy to observe ground state transitions from the upper limit of the threshold energy. Level positions above 506.7 keV were determined from the ground state transitions and then the remaining cascading transitions were set in the level scheme by considering the similarity of the excitation functions with the ground state transitions. As a result of this, most of the cascading transitions could be fixed in the level scheme, except some weak transitions. 4.2. SPIN-PARITY ASSIGNMENTS The ground state spin of 74As has been uniquely assigned as 2 - from the l o g f t values of the positon and negaton decays 9), the fl-7 angular correlation work of ref. 10)] and the fl-spectrum shape analysis of ref. 11). Hence it is possible to assign TABLE 3

Optical potential parameters Incident particle

Proton a)

V (MeV) W (MeV) Vs.o. (MeV) ro (fm) rl (fm) rc (fro) ao (fm) al (fm) b (fro) imaginary potential form

59.9-0.55 E 12.6 7.5 1.25 1.25 1.25 0.65 0.47 differential Woods-Saxon

Neutron b) 46.0 14.0 7.0 1.30 1.42 0.62 0.50 G aussian

a) Ref. 12). b) Ref. 13).

parities and also to set limits on the spin values of the excited levels of 74As below 465.3 keV, by using the transition multipolarities discussed in subsect. 3.2, except for the 332.2 keV, 385.1 keV and 447.9 keV levels for which internal conversion electron measurements were not possible. In this section, in order to make the spin assignments unique, the observed cross section ratios (of an excited level to the 173.1 keV level) of the (p, n) reactions are compared with those calculated using Hauser-Feshbach theory 4) for various spin values of the residual levels. The comparison of the observed cross section ratios with the calculated ones is made only in the threshold region of each residual level, up to 100 keV above the threshold, and the spin assignment is made stepwise to higher levels.

f

I

I

3 !6 5

2~ I-

G

3 85

313o

422.2 keY

i

3!60

2021 keV

! 38b

3165

H

~ 390

I-

I

CENTER

3 8o

(

÷

I

I

3190

+ 2*

2÷

H

C

MASS

÷

3165

OF

385

426.0 keY

310

+--".

+ ~

206 8 keY

3ao

i

i

_~ 3e5

3!65

PROTON

/'

[

267.4 keY

3170

i

ENERGY

~.tgo

i+ + 2*

I

3 15

l i _~ 3Bo

- - -

I 385 (MeV)

,

,

f

3170

--

,4479 key

3165

/

2 7 1 7 keY

,

.-

~ 390

+

4 ÷

K

3.175

4-

2-

E

,

I

~

278 4 keY

,

-

3~ 5

{'~/z~ ~ / -

319o

3.70

.u~.

468.3 keY

I 3.65

tG-----/

'

F

" 2*

3195

L

31,,5

,

Fig. 6. Comparison of the observed and theoretical cross sections for various residual levels in the 74Ge(p, n)74As reaction. Relative cross sections to the 173.1 keV level are compared with Hauser-Feshbach calculations. The theoretical curves are averaged over the energy loss of the protons in the target (30 keV). See the text for an explanation of the solid and dotted lines.

5.75

4*

43-

~ J

_

A

-- 2/2-

313-

213-

f t ~ - - -

3BO

385. I key

i

$160

....

±

- -

t

3+

ro-3

IO

I

183.0 keY

~

i

IO

P

3.15 5

f

/ //~

I

I (3-

~0

74"As LEVEL SCHEME

75

Relative cross sections of the (p, n) reactions to the various levels of 74As were determined from the intensities of the (p, nT) transitions observed at 120 ° after correcting for contributions from cascading transitions and internal conversion electron decays. Barrier transmission coefficients were calculated using the program ELIESE-2 [ref. 1,) ], employing Perey's potential i 2) for protons on 7,Ge and Moldauer's potential 13) for neutrons on 74As. The potential parameters used are listed in table 3. The notation used is standard. The comparison of the observed cross section ratios with the calculated ones is shown in fig. 6. Corrections for the width fluctuation is) were neglected in the calculation. The solid curves in fig. 6 were calculated by neglecting the levels lying above the level under consideration and the dotted curves were calculated including all neutron channels. All of these curves are averaged over the energy loss of the protons in the target (30 keV). Elastic and inelastic channels were included in the calculations but (p, 7) channels were neglected unless otherwise stated. The extreme left of each abscissa in fig. 6 corresponds to the threshold position of each 74As level. 4.2.1. The 173.1 k e V and 183.0 k e V levels. Both the 173.1 keV and 183.0 keV levels decay by M1 transitions to the ground state (2-). Therefore, their spins are 1-, 2 - or 3-. The cross section of the 183.0 keV level relative to the 173.1 keV level was calculated for various possible spin combinations of these levels. In fig. 6a, spin combinations assumed are indicated at the right of all curves. The upper values correspond to the assumed spins of the 183.0 keV level. The calculated curves assuming a spin of 1- for the 183.0 keV level are omitted because they largely deviate upwards from the observed points. The best fit with experiment is seen for the case where the 173.1 keV and 183.0 keV levels have spins, 1- and 3- respectively. A discrepancy around the threshold is due to an enhancement of the cross section of the 173.1 keV level by the analogue resonance at Ep = 3.580 MeV, which is 10 keV above the threshold of the 173.1 keV level and just at the threshold of the 183.0 keV level. This resonance is the ground state analogue of 75Ge [ref. t6)] and thus its spin is 1 - . Since the 173.1 keV level is fed almost by s-wave neutrons around this resonance, the existence of resonance enhancement indicates that the spin of the 173.1 keV level is 0 - or 1-. This supports the above assignment of 1-. A dotted curve was calculated including neutron channels to the 202.1 and 206.5 keV levels whose spins were assumed to be those assigned in the following discussion. The effect of these channels is only about 20 ~o at 100 keV above the threshold of the 183.0 keV level. In addition, a dash-dotted curve was obtained including radiative capture channels. They were included in the calculation by merely adding transmission coefficients of the form T,~ = 2~z(F~)/D s, where J denotes the spin of the compound state, to the sum for outgoing channels. The mean radiative width ( F ~ ) was obtained from the systematic trend of the experimental radiative capture width for neutrons 17) with mass number and it was assumed to be independent of J. The mean level spacing D a was calculated by Newton's semiempirical level density formula as). An improved fit with the observed points is seen for the dash-dotted curve.

76

K. KIMURA

4.2.2. The 202.1, 206.5, 267.4 and 271.7 k e V levels.

The parities of these levels are given by the transition multipolarities of the respective ground state transitions. From figs. 6B-E the spins of the above levels are uniquely given, namely the 202.1 keV level is 2 - , the 206.5 keV level is 1 +, the 267.4 keV level is 3 - and the 271.7 keV

74~S ~n _

750 740 727

_

-

-

-

-

"

-

-

-

-

-

"

-

o~

I~'3 {

-

~r

•

646.0 628.4

625

-

726

" 711.1

0 d,.o

647 -

779 _

727.5

.3

7 1 0

696 683

_

I ~'3 -

-

626

o

-

-

-

-

-

-

5

1

-

-

-

-

-

-

-

-

541

I

-

-

442

4

-

-

337

2+4 - -

265

513.0

~o~

463 446

384

-

276 266

-

204

-

578

]

4+2

Lt~ e.~

1

535

-

551.0

477.2 445.3

421

-

-

,*0 0

550 529

-

-

584

-

-

-

421.0

;r

' 384.7

1 I

[

I

N

I

l]

1

~[

E.FINCKH et. al.

-

]

-

173

273.8

~ o¢~O~ ~

. . . .

~ -

" 267.8

~ ~ 206.0 ~ ~ 200.2 --

197.5 172,9

14.9 0

0 (p,n)

332.2

o ~

( p,n I ' ) J.CHRISTIANSEN et. af.

l

-

-

. . . . .

198 (t60)

2

0 ( p,d )

R. FOURNIER e l . e l .

Fig. 7. Energy levels of V4As obtained in previous work. Only the levels in the energy range of the present work are shown.

level is 4 - . The excitation functions of the 267.4 keV and 271.7 keV 7-rays exhibit resonance enhancement at Ep = 3.804 MeV, like that of the 183.0 keV 7-ray. This suggests that the 267.4 keV and 271.7 keV levels have spins similar to that of the 183.0 keV level. Therefore, the 3 - assignment to the 267.4 keV level and the 4 - assignment to the 271.7 keV level are reasonable. 4.2.3. The 259.2 and 278.4 k e V levels. Although no direct evidence for the 259.2 keV level could be obtained, the spin of this level is tentatively assigned as 4 + or 4 - . This is so because, as discussed in subsect. 4.1, the neutron feeding of this level ap-

74As L E V E L S C H E M E

77

~ w ~ O ~

743.4 716.~

701

-w

...... .

.

.

.

.

.

.

.

.

.

.

.

.

.

5

. ,

;',I

I!llll] ~1 : ~ e

- ~h~

,

-;==-~T I I ~

.

J m

o.l~+j

• • .t~ I •

"

~:,~

i I I I l i1111111

I

;

I

I

I

I

"

IIIIIrll

i ill

; ;

Ill

L I

I

2*

,~

IIIII IIIII IIIIItllllilllillll

+ ~+++1!1

650.0 626

•2

617.1 586.0 552.

~

. , , , , , . . , , , , .I ,I I 1 ' IIIIi

J

.

II,

,, !!

I

506.

7

465.

3 9

447 • 446.6 426.0 422.2

385,

l IIIl l[ll[

~3 - - "

~, L - , . ~ -. J j j.j j j j j. j i j

i

.

.

.

II.

111111] l Ft+ .

.

J

.

.

J J.

.

Ji.

.

ii

.

[/

I

I

332,2

278,

+'+.

F

514.1

206.5 202. J83.0 J73.1

'

Fig. 8. Proposed level scheme of V4As. Widths of the arrows indicate relative intensities of 7transitions at Ep = 4.2 MeV. Branching ratios are given in parentheses.

pears very small and the 259.2 keV level appears to decay by the 76.2 keV transition (El or M1) to the 183.0 keV level (3-). The intensity of the (p, n) reaction leading to the 278.4 keV level was taken as the sum of the intensities of the 76.2 keV and 278.4 keV transitions. Since the 278.4 keV transition was not given uniquely as E1 or M1 from the observed ICC, the HauserFeshbach cross sections for the 278.4 keV level were calculated for both positive and negative parity cases. From fig. 6F, the 278.4 keV level is expected to be 2 + or 3 + and thus the 278.4 keV transition is probably El. The excitation functions of the 76.2 keV and 278.4 keV 7-rays show resonance enhancement at Ep = 3.804 MeV as strongly as those of the 183.0 keV, 267.4 keV and 271.7 keV 7-rays, which de-excite 3- and 4 levels. On the other hand, the excitation function of the 202.1 keV 7-ray, which deexcites a 2 - level, does not show this resonance. Thus spin 3 + seems to be preferable for the 278.4 keV level than 2 +. The dash-twice-dotted curve is the sum of the calculated

4

J

78

K. KIMURA

cross sections of the 259.2 keV and 278.4 keV levels assuming spins 4 + and 3 +, respectively, and the dash-thrice-dotted curve is that of the 259.2 keV and 278.4 keV levels assuming spins 4 + and 3 - , respectively. The former one better reproduces the observed cross section. 4.2.4. The 332.2 k e V level. The observed intensities of the transitions from the 332.2 keV level were very weak. From the upper limit of these intensities, the spin of this level is given as 1 => 4. It is further restricted to I = 4 or 5 because levels with I > 6 are almost impossible to excite with observable intensity by protons in the energy range 3 to 4 MeV. 4.2.5. The 385.1 k e V level. F r o m the 385.1 keV level, a ground state transition and a faint cascading transition feeding the 173.1 keV level were observed. If this level decays also to the 202.1 keV level, as Christiansen et al. have proposed, the energy of this cascading transition should be 183.0 keV. Thus it will be entirely masked by the transition from the 183.0 keV level. A 4 - assignment predicted from fig. 8G involves an ambiguity due to this transition, if it exists. 4.2.6. The 422.2, 426.0, 446.8 and 465.3 k e V levels. All of these levels decay by M1 transitions to the low-lying positive parity levels and thus their parities are all positive. The observed cross section of the 426.0 keV level is well fitted by the calculated curve for a spin of 2 +. The 168.4 keV (M1 + E 2 ) and 240.3 keV (M1) transitions, feeding the 278.4 keV (3 +) and 206.5 keV (1 +) levels, respectively, confine the spin of the 446.8 keV level to only 2 +. The calculated cross sections of this level also gives a good fit to the observed values for a spin of 2 +. The observed cross sections of the 422.2 keV level are not well fitted by the calculated ones for all the spins 0 +, 1 + and 2 +. A spin of 1 + is predicted as being the most probable. As for the 465.3 keV level, the calculated cross sections for spins of 0 + and 1 + are equally close to the observed values and thus a spin of 0 + or 1 + is predicted for this level. In the excitation region above 400 keV, the neglected neutron channels in the Hauser-Feshbach calculation have less effect on the calculated cross section than in the lower excitation region because the decay width of the compound state is dominated by neutron decays to the levels below 278.4 keV. Some ambiguities in the above spin assignments to the levels above 400 keV are caused by the following reasons: The spin dependence of the calculated cross sections becomes less with increasing excitation energy. In particular, with the proton potential given in table 3, the barrier transmission coefficients for s- and p-wave protons are very close in value to each other above Ep = 4 MeV and thus the calculated (p, n) cross sections for residual levels with spins of 0 -+, 1+ and 2 + are also close in value above an excitation energy of 400 keV, as shown in figs. 6H and L. Furthermore, as we go higher in excitation energy, the number of branching transitions from a particular level becomes progressively large and thus it becomes difficult to obtain the exact intensities of (p, n) reactions to various levels from the observed intensities of (p, nv) transitions.

74As LEVEL SCHEME

79

5. Conclusions More levels are assigned in the present work compared to the previous work summarized in fig. 7. In particular, from the results of the measurement of threshold energies and excitation functions for the (p, nT)transitions, pairs of levels are assigned around 270 keV, 422 keV and 447 keV excitation energy, and some transitions are assigned differently from the decay scheme in fig. 7. The 183.0 keV transition, which was attributed to a level at 197.5 keV by Christiansen et aL, had to be assigned as a ground state transition. Instead, in order to explain the 76.2 keV-183.0 keV cascade, the existence of a level at 259.2 keV was assumed. I f this level is fed mainly by the 19.2 keV transition through the 278.4 keV level and decays by the 76.2 keV-183.0 keV cascade only, the 278.4 kgV ( E l ) transition must be hindered so as to compete with the 19.2 keV transition. A single particle half-life (in Weisskopf units) of 278.4 keV transition is 3 × 10 -14 sec for E1 and that of 19.2 keV transition is 10 -11 sec for E1 and 5 × 10 -1° sec for M1. Hence, the 278.4 keV transition must be 103 to 105 times more hindered than the 19.2 keV transition. This is not extraordinary because hindrance factors for E1 usually scatter in the range from 103 to 107. Christiansen et al. [ref. 2)] have obtained a half-life T~ = 26.8+0.5 ns through the 147.6 keV-76.2 keV cascade and attributed it to a level at 273.8 keV. According to the decay scheme of the present work, this lifetime should be attributed to the 278.4 keV level or the 259.2 keV level. If we attribute it to the 278.4 keV level, the 278.4 keV (El) transition is hindered by a factor of 107 and the 19.2 keV transition is hindered by a factor of 5 × 103 for E1 and by a factor of 10 a for M1. While if we attribute it to the 259.2 keV level, the 76.2 keV transition is hindered by a factor of 3 x 104 for E1 and by a factor of 5 × 10 a for M1. All of these hindrance factors are well expected to occur for medium weight nuclei and thus there is uncertainty about an isomeric state having the above lifetime. The positive parity levels around 440 keV excitation energy decay mainly by M I transitions and have no appreciable transitions to negative parity levels. Conversely, negative parity levels mostly decay to negative parity levels. These indicate hindered nature of E1 transitions between the low-lying levels of 7 CAs. This can be interpreted by a simple discussion within the framework of the shell model. The main configurations of the low-lying positive parity states of 74As are considered to be [(nlf:, 2p~, 2p~)', (vlf~, 2p~, 2p~_)-n]j+ and those of negative parity states are considered to be [(nlf~, 2p~, 2p~)m,(vlg~)n]j-. Thus E1 transitions between these configurations are strictly forbidden because neutrons have to change their angular momenta by more than two units. Mixed components of the 2d~ or lf~ orbits allow E1 transitions. But these orbits are outside of the majoi shell (N, Z = 28-50) and their mixing into the low-lying levels of 74As may be small. Fournier et al. 3) determined transferred In values for the 75As(p, d)74As reaction. A part of their results are shown in fig. 7. The 198 keV (In = 1) state appears to correspond to the 206.5 keV (1 +) level. Around 200 keV excitation energy three more

80

Ko KIMURA

levels are assigned in the present work, namely the 173.1 keV ( 1 - ) , 183,0 keV ( 3 - ) and 202.1 keV ( 2 - ) levels. A m o n g them the 1- and 2 - levels may be weakly excited in the (p, d) reaction because they can be excited only by l, = 0 or 2 transfers, whereas the 3s, and 2d~ orbits are almost unfilled by neutrons in the g r o u n d state o f 75As. As for the 183.0 keV ( 3 - ) level it can be excited by I. = 2 or 4 transfers. The absence o f a 1, = 4 distribution m a y be due to the fact that the 183.0 keV level has a small amplitude o f the configuration [75Asg.s.(vlgk)-l]3- or that a l, = 4 transfer is masked by the strong 1, = 1 transfer. The 265 keV (l, = 2 + 4) state corresponds to one o f the 267,4 keV ( 3 - ) and 271.7 keV ( 4 - ) levels or both. The 337 keV (1, = 4) state is clearly the 332.2 keV (I = 4 or 5) level. F r o m the assignment o f l, = 4, the parity o f the 332.2 keV level assigned to be negative in fig. 8. The 442 keV (I, = 1) state, which is reported as possibly being a multiplet, corresponds to the positive parity levels a r o u n d 440 keV. In general, it seems that there are no contradictions between the parities assigned in the present w o r k and those given by the 1. = values o f the (p, d) work. I n the course o f preparation o f this paper, a study on the 73Ge(aI-Ie, d)7~As reaction has been reported by Rosner et al. 19). It is rather difficult to correlate their deuteron spectra with the present level scheme because o f the p o o r resolution. But it seems that their lp values obtained do not contradict the present spin-parity assignments. The author would like to express his thanks to Prof. A. I s o y a for his interest and advice t h r o u g h o u t this work. He wishes to express thanks to Prof. T. K u r o y a n a g i for his advice, and also to the laboratory staffof the Van de Graaff accelerator at K y u s h u University for their support. He also wishes to thank Prof. I. K u m a b e for use o f the 30 cm 3 Ge(Li) detector. The a u t h o r would like to acknowledge the application o f the p r o g r a m E L I E S E - 2 by Dr. S. Matsuki. He is greatly indebted to Mr. M. T a n a k a for use o f the 1 cm 3 Ge(Li) detector.

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74As LEVEL SCHEME 12) 13) 14) 15) 16)

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