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A study of the 75As(n, n'γ)75As reaction
Nuclear Physics A305 (1978) 117-143 ; : © North-Sopatd Ptrbllshlng Co ., Amsterdam Not to be reproduced by photoprint or microfilm without written permiuion from the publisher
A STUDY OF THE 'sAs(o, n'y)'sAs REACTION UGO ABBONDANNO, FERRUCCIO DEMANINS and MARIA ROSA MALISAN Istituto di Fisica dell'Universitd, Trieste and Istituto Nazionale di Fisica Nucleare, Sezione di Trieste and GIANCARLO NARDELLI Istituto di Fisica del/'Università, Padova and Istituto Nazionale di Fisica Nucleare, Sezione di Padova Received 28 December 1977 (Revised 26 April 1978) Abstract : The de-excitation gammas following the inelastic scattering of neutrons from'SAs have been studied for incident neutron energies from 1300 to 2800 keV in steps of 100 keV. The energy levels and the branching ratios of their decays have been deduced from excitation function measurements : 58 energy levels have been found in the excitation energy region below 2 .8 MeV, twelve of which are reported for the first time in this work . The experimental excitation functions and angular distributions have been compared with the theoretical predictions based on the statistical theory of the compound nucleus . Spin and parity assignments for the levels and multipolarities for the decays are proposed for excitation energies of levels up to 2300 keV. E
NUCLEAR REACTION 'SAs(n, n'y), E = 1300-2800 keV ; measured Er , a(E, E~, ~. '°As deduced levels, J, n, y-branching, 8 . Natural target .
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Information on the energy levels of °As, as reported in Nuclear Data Sheets '), is mainly based on the study of the 'SAs(y, y') [ref. Z)], 'SAs(n, n'y) [refs. 3'4)], '4Ge(3He, d) [ref. 5)] and '4Ge(p, y) [ref. 6)] reactions. Recently, new data were obtained by Schrader et al. ') who investigated the level scheme of 'SAs via the (3He, d) reaction. However, the experimentally determined structure of this nucleus still had some regions with incomplete information due to the limited bombarding energies used in the (n, n'y) reactions and the selection rules the (y, y') and (3He, d) reactions are subject to. Therefore we decided to study'SAs by means of the (n, n'y) reaction, observing the de-excitation y-rays, in order to obtain additional information on the energies, spins and decay modes of the levels of this nucleus . Neutron inelastic scattering, as a method of spectroscopic investigation, has, in 117
11 8
U . ABBONDANNO et al .
fact, some remarkable advantages, namely the absence of the Coulomb barrier (which allows the excitation of the low-lying states of the target nucleus with lowenergy incident neutrons, limiting in this way the number of competitive reactions that may complicate the y-spectra) and the absence of selection rules. For these reasons, (n, n'y) reactions have been studied intensively in recent years to gain information on the properties of nuclear energy levels. More recently, new interest in them has arisen through the advent of high-resolution and large volume truecoaxial Ge(Li) detectors. The experimental cross sections for the y-rays following inelastic scattering can be compared with the theoretical predictions of the statistical compound nucleus (Cl~ model of Hauser and Feshbach e), extended by Satchler ~ and later revised by Sheldon and Van Patter 1°). By this comparison, the spins and parities for the excited states can be determined with reasonable confidence . In the present work, the energy levels and decay modes of'SAs have been deterihined up to an excitation of 2609 keV from y-spectra obtained at incident neutron energies between 1300 and 2800 keV. Measurements of angular distributions for the decay gammas from' SAs were made at neutron energies of 1 .5, 2.0, 2.5 and 2.8 MeV. A preliminary report of the results, concerning the level scheme and the branching ratios, has been published elsewhere 12). 2. Experimental details and data analysis
The measurements were performed using the 7 MV Van de Graaff accelerator at the Laboratori Nazionali di Legnaro (LNL, Padua) . The incident neutrons were obtained with the 3H(p, n)3He reaction in a tritium-titanium target (0.84 Ci/cms) bombarded by a 3 ns pulsed beam of protons, and were incident on a cylindrical sample of powdered 'sAs metal in a thin-walled aluminium container, 4.5 cm in diaméter and 5 cm high, placed 10 cm in front of the neutron source . The energy spread of the neutrons was about 30 keV over the energy range used in the measurements and it was chiéfly due to the angular spread of the sample . The de-excitation y-rays were observed by means of a true-coaxial Ge(Li) detector with a nominal volume of 52 cm3, shielded from the tritium target by a shaped lead wedge. The full-width at half-maximum of a y-line obtained by the Ge(Li) detector during operating conditions, was x 2 keV at almost all the energies up to 2.6 MeV. The time-of-flight technique with a flight path of 60 cm was used to discriminate between the background from the neutrons scattered in the Ge(Li) detector and the y-rays produced in the de-excitation of 'sAs . For monitoring purposes, a neutron time-of-flight spectrometer using a NE 213 scintillation counter was placed at a distance of4 m from the neutron source at an angle of 20° with respect to the beam direction. The charge collection was also monitored by means ofa current integrator. The y-ray spectra were recorded in a 4096 channel analyser . All the measurements on the 'sAs sample were followed by similar measurements, at the same neutron
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energies, on a ' ZC sample, for identification of background peaks arising from neutron interactions in the Ge(Li) detector and surrounding materials. Additional measurements were made employing standard calibrated y-sources in order to determine the energy scale and the efficiency of the detector . The' 6Fe(n, n'y) 56Fe reaction was finally studied at a neutron energy of 2.5 MeV, in order to obtain absolute normalisation by comparing the y-yields from the sample with the known values 's) for the 90° differential production cross sections of the 846 and 1240 keV y-rays . The decay scheme of'SAs and the y-ray production cross sections have been deduced from the analysis of the y-ray de-e~ccitation spectra. For this purpose, measurements were performed at 90° for neutron energies from 1 .3 to 2.8 MeV, in steps of 0.1 MeV, in order to determine y-excitation functions. The y-angular distributions were observed at E = 1 .5, 2.0, 2.5 and 2.8 MeV, at angles between 30° and 150° to the incident beam direction. A part of one of the y-spectra obtained in the measurements is shown in fig. 1 . It was observed at E = 2.5 MeV ;the prominent y-rays attributed to inelastic scattering in' SAs and some background peaks (B) are labeled.
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The spectral data, i.e. energy, area and FWHM of each peak after subtraction of the background, were extracted by means of an automatic computer code ' 4 ), based on a modified non-iterative method ~ s), which allows the analysis of relatively large zones of the spectra. This analysis has been performed at the HP 2116 B computer of the LNL and at the HP 2100 S computer of the Istituto di Fisica dell'Università di Trieste. A typical result of this method is shown in fig. 2. In this figure, the points represent the measured spectrum, the straight line the calculated background, and the curves the Gaussian shapes fitted to the experimental points . Photopeak yields obtained in this way have been corrected for the attenuation of both neutrons and y-rays in the sample ; the effects of multiple scattering have also been taken into account, according ref. ' 6). The cross sections for the neutrons and gammas used in the evaluation of these corrections have been obtained from refs . 1' .18) . The y-yields were normalised to the monitor counts of the neutron detector, and then corrected for the relative efficiency of the Ge(Li) detector . Finally, as explained above, absolute values have been obtained from the corrected data by comparison with those, corrected and normalised in the same way, from the 56Fe(n, n'y) 56 Fe reaction . 3. Results and discussion 3 .1 . RESULTS
The results of the measurements described in the previous section are presented in this section and compared with the predictions of the statistical theory of the compound nucleus. The first result is the decay scheme of'SAs, which is shown in fig. 3 and table 1 . Each value reported in the second column of table 1 is the weighted average of the excitation energies determined for the corresponding level . As for the observed y-rays, the assignment to transitions in' SAs was simplified by the small energy step used in the excitation function measurements . Some ambiguities were resolved by comparing the experimental data with the calculated ones, as will be described in detail in subsect. 3 .3 . The second group of results represents the excitation functions of y-rays from levels lying above 1000 keV. The data, given in absolute values, are reported in figs. X17 : the errors have been estimated as a combination of the statistical error, the error in the y-ray detector efficiency determination, the error in neutron monitoring, and the uncertainty ( f 10 /) due to the absolute normalisation to the data from the 56Fe(n, n'y) 56Fe reaction . The third group of results consists in the y-ray angular distributions . In figs . X17 they are reported as angular asymmetries with respect to 8 = 90° w(9) = Q(~~Q(90°) .
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The absolute values are reported in table2, as integrated cross sections Q(E, Ey) = 4aa° obtained from a Legendre-polynomial fit to the experimental values . It should be.noticed that the errors reported in table 2 and the errors of the angular distributions reported in figs . 4-17 do not take into account the uncertainty due to the absolute normalisation . 3.2. COMPOUND NUCLEUS CALCULATIONS
Since the predictions of the statistical theory of the Chi on differential cross sections have been described in detail in ref. 1 °), the presentation of the calculations and results obtained in the present work will refer to the symbols and nomenclature adopted there. All the calculations have been performed by means of the code MANDY ` 1) at the CDC 6200 computer of the Centro di Calcolo delfUniversità di Trieste. As we pointed out in the introduction, the comparison between measured and calculated cross sections can provide information on the level spins and parities and the radiation multipolarities . Our study began then by testing our calculations (in a special way, the transmission coefficient set) on a particular level (Ei = 1349 keV,fig. l1), for which the possible Jx values have been limited by previous studies. In fact, this level had been observed also in the'4Ge(3 He, d)'SAs reaction S" '), and the deduced value of lp = 1 allowed J` to be ~= or ~- . By observing the experimental angular distributions of the 1349 and 948 keV y-rays, the value J = ~ can be excluded since it would imply isotropic distributions. However calculations of the excitation function have been performed for J* _ ~-, ~+, ~- using the transmission coefficients calculated with an optical model code ' ~ adopting the parameters of Becchetti and Greenlees 2 ~ : in this framework, the experimental values have been best fitted when J~ _ ~- . Calculations performed with another set of transmission coefficients z') give, for this level and for JR = ~-, results which are less than the experimental data of about 20 ~. Calculated cross sections have shown enough sensitivity to parity to allow, in some cases, parity assignments. In our calculations, a partial wave cutoff at 1~ = 4 has been used [see ref. 1 ~]. As for theextra CN exit channels considered in the r-term, wehave taken into account all'sAs levels that could be excited at the neutron energy considered (their number varied from 22 at E = 1 .3 MeV to 59 at E = 2 .8 MeV) . The extra exit channels have been confined solely to the inelastic and elastic channels because the other possible decay channels ['sAs(n, p)' S Ge (Q = - 0.416 MeV) and'sAs(n, a)'2Ga (Q = 1 .198 MeV)] have negligible transmission coefficients at E -<- 2.8 MeV. As explained in ref. 1 ~, the measured cross sections of the observed y-rays may be affected by contributions of cascades from higher levels, which modify not only the absolute values but also the shapes of the angular distributions. In our calculations, we took into account this fact only when the cross sections of cascades were greater
'sAs(n, n'y)~ sAs
127
than about 10 ~ of the observed y-ray cross section, being 10 % of the error in the absolute value of the measured data. In what concerns the analysis of the angular distributions, which has been performed together with the study of the excitation functions, their shapes are sensitive also to the mixing ratio of the multipolarities of the y-rays . In order to extract this parameter, every experimental angular distribution has been developed in a Legendrepolynomial least-square fit. The experimental aZ and a4 values have been reported on the ellipse yielded by the programme MANDY, and in this way the value of S has been determined. If this comparison between the a* led to a double-valued solution for S, the lower value of S has been systematically adopted. If possible, excitation functions and angular distributions have been jointly studied in order to determine J, n and S. In the absence of angular distribution data, only J (and in some cases, tentatively, n) can be proposed . 3 .3 . DISCUSSION
In the excitation energy range Ex _<_ 886 keV we obtained twelve levels and confirmed all levels reported in Nuclear Data Sheets 1) except the uncertain 585 .7 keV level observed in the (3He, d) reaction S). The level reported at 304 keV was not observed in our time-gated measurements because of its 17.53 ms half-life. Since our measurements started from E = 1 .3 MeV whereas the y-ray excitation functions are sensitive to the spins and parities involved in the observed transitions especially near the threshold, it follows that the excitation functions of the levels below 1 MeV are not significant. On the other hand, neither can the angular distributions of their decays yield useful information because they are disguised by the contributions of the many cascades from the higher levels. Therefore, for the levels with E_ <__ 886 keV we give only the experimental integrated cross sections of table 2. In our CN calculations, we adopted the spin-parity values reported in column 9 of table 3, which are derived from previous works and are compatible with the cross sections of the observed transitions to these levels . For the higher levels, we report in table 3 the previous spin-parity assignments compared with those suggested from our study of excitation functions and angular distributions. The spin and parity determined for the levels involved, and, in some cases, the multipolarities and mixing ratio of the radiation, are reported in the figure concerning the observed transition . In the following, a discussion of the results obtained for each level is presented in some detail . The level at Ex = 1042 keV. In the present measurements, we observed the excitation functions of the two y-decays of this level, EY = 740 keV and EY = 643 keV, and the angular distributions for the latter. In ref. a) J = ~ is proposed whereas in ref. ') an !p = 3 or 4 transition is observed to a level at 1048 f 5 keV : from our excitation function calculations, in which the contribution of a pure cascade from
U . ABBONDANNO et al.
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TABLE 3
Comparison between spin and parity values reported in the literature and obtained in the present work Level no.
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E - 27 M~V
LO
Ei 6I7 Y~V
Ob
E - 2 " AbV l0
E (NbV)
us "
Fig. 4. Excitation functions and angular distributions of both y~ecays from the 1042 keV level. The solid lines are the calculations for J` _ ~- . the dashed lines those for J` _ ~+ .
13 0
U. ABBONDANNO et at. l06]
~~
47q
7!4 k~V
]/s_
MhE2
C~-a67
106]
_
E~ " 106] k~V
p
M1+E2
~/2 C " -L2s ]/
a2
u
Ls E " LSMN
2 .0
2.s
E(M~V)
0 .1 LS E " 2SM~V
LO
u
20
2.3
E (M~V)
LO
E " 20M "V
QS
u
! " 23 MrV
! - 2.~Mdv
Lo
Lo E ~ 2! M~V
a5
L1 av 1
0
-1
E~~ ~92 k~V 1 t
106]
1
I
o
372
MI+E2 1
C " a"1
~/2 -
QhI -1
ro. s ~e . s Fig. 5. Excitation functions and angular distributions of three y-decays from the 1063 keV level. The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes; the dashed line is the excitation function calculated assuming the opposite parity for the decaying level.
the 1301 keV level has been taken into account, good agreement is found (fig . 4) for J = ~. The level at Ex = 1063 keV. The excitation function and the angular distributions for the prominent decay from this level (EY = 784 keV) have been studied. The experimental points for this y-ray compare better with the calculations for Jx = ~-, confirming thespin assignment given in ref. 4). Calculations have been performed also for other decays (E7 = 1063 keV and 492 keV) : in the limit of statistical errors, they fit the experimental data (fig . 5). The level at Ez = 1075 keV In the present work, two gammas are found which can be assigned to a level of this energy . The excitation function and the angular distributions for the decay to the ground state have been studied and confirm the assignment J = ~ proposed in refs. z " 4 " ') (fig . 6). The levels at E_ = 1080 keV and Ei = 1095 keV. The existence of levels at these energies was first proposed by McMurray et al. °) to account for the presence of a 816 keV y-line, observed at E < 1130 keV. This gamma was attributed to either a transition from a 1080 keV level to the 265 keV level (J~ _ ~-) or a transition from a 1095 keV level to the 279 keV level (J` _ ~-). We tested both hypotheses in our
'sAs(a, a'y)'sAs a7s
E~ 107s 4~Y
s t
-0
MNIY
ye'a
f -Wf
;f!-
~~51-5~i+ E - tSM~V
u
~
~
E
m
3
Lo
u LO
u u
ns k v
~i~
w as
~~
LO
u
E~-
131 _________
T f f ~
tow
"
E - L! M.V
xes
20
__ T Et+w
~.s
t
y`t' f-tm
3h-
E (M~l
LO 2
E - 20 M~V
T
' '
E - 7A MrV
1 T
t
E - ?3 M .V
T
E - 2f M1r
E - 23 M.V
T
u
r
to
Fig. 6. Exçitatioa functions and angular distributions of y-decays from the 1075 and 1080 keV levels. The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes; the dashed lines arb the excitation functions calculated assuming the opposite parity for the decaying level.
CN calculations : the assignment J = ~ is excluded, for both levels, by the trend of the experimental angular distributions, which exhibit a remarkable anisotropy . Assuming J = ~ for the 1080 keV level, and a branching ratio of 55 ~ for the $15 keV decay, the calculated angular distributions have too weak an anisotropy to fit the experimental data, whereas the calculated excitation functions are in reasonable agreement with the experimental yield: Therefore, we performed the calculations with J = ~ for this level and good agreement was found between experimental data and theoretical curves (positive parity, see fig. 6). With the hypothesis that the 815 keV y-ray is a decay from a 1095 keV level with a branching of 100 ~, the assignment J = ~ for such a level cannot account for the experimental W(~ considered, since a ~ ->' ~ spin sequence implies distributions with a peak at 90° or with too slight a concavity upwards . A J = ~ assignment to this level describes well the angular distributions for the 815 keV y-decay, but is in total disagreement when the absolute values of the excitation function are considered (see fig. 7). The level at Ex~ = 1101 keV. In ref. a) a y-ray with E7 = 1102 keV (first observed at E = 1720 keV) was assigned to the 1503 keV level. In the present work, a 1101
13 2
U. ABBONDANNO et al. lon
E,- els t.v
~s-
sn __ i
u
_m 3
~
s.o
Ls E-LSMa
as
E (M .v)
a: E-2sM .v
T
~ E - 20 A4V
no
s.s
E (M.v)
Ls i
Q3
1
Z
n~
Ls
La 4
~
~+
1 1 i ~ i i ~ 1 ~ i
E,- 1101 4~V
E-23Mw .
I
I
I
I
I
Ls LO I
0 cw i
-1
1
0
-1
co~ Y
Fig. 7. On the left-hand side, the excitation function and angular distributions of the 815 keV y-ray if considered as a decay from a 1095 keV level. The solid lines are the calculations for !` _ } -, the dashed one is the excitation function calculated for J` _ }+ . On the right-hand side, the excitation function and angular distributions of the 1101 keV y-ray: solid lines are the calculations for J` _ }+, the dashed one is the excitation function calculated for J` _ }- .
keV y-ray was observed at E = 1 .3 MeV together with one at 796 keV : assuming that both decays come from the same level, we propose a level at 1101 keV. The 1102 keV y-decay observed by McMurray et al. a) corresponds rather to a 1105 keV y-ray that we first identified at E = 1 .8 MeV. We studied the excitation function and the angular distributions of the 1101 keV decay, because the other y-line, although stronger, overlaps the 799 keV y-line too much to give good data. From the flat trend of the angular distributions and from the values of the excitation function, for which we took into account the E,, = 403 keV cascade from the EX = 1503 keV level, J = ~} is proposed (see fig. 7). The level at Ex = 1128 keV. The previous assignments are J = ~, given in ref. °), and J~ _ ~-, given in refs. z . s" '). The assignment J = ~ is confirmed (fig . 8) by the flat structure ofthe angular distributions. The excitation function suggests a positive parity for this level . The level at E_ = 1172 keV. This level decays to the 279 keV level with the 893 keV y-ray and to the 572 keV level with the 600 keV y-ray. In ref. a) the suggested spin values are ~, ~ and The ~ assignment being excluded by the anisotropy of the
u.
"As(n, n'y)'SAs n]e
!~~ I1]~t~V
El
nn
I~i+
_____ ° _
LO
133
L0
/ 4]
sn
Yri
E E ~ i9] bV
4S
L]
1 .3 ! ~ 1.] M~V
7 .0
_s 3
"
"
u
].o
Ls
E - ] .0 M~V
E tM.v)
E ~ 23 M~V E ~ 2! AwV
LO OS
Ol
LZ
E (M~V) 1.0 .S 0
4!
LZ
] .S
M] +EI i~-0 .P
-"- 5-~
~3 ~
/
n/ta
M3+EI i~-QA]
E ~ 23 M~V
"
"
"
"
" "
! ~ 7.OM~V
Od
!~~ 600 4~V ! ~7 .tM~V
E ~ 2.eM~V 1 .3 L0
cep B
"
cep 0
Fig. 8. Excitation functions and angular distributions of y-decays from the 1128 and 1172 keV levels . The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes ; the dashed curves are calculated assuming the oppositie parity for the decaying level.
angular distributions, the calculations of excitation functions were performed for the ~+, ~-, i+ and i- assignments. The best agreement (see fig. 8) was found for J i. The level at Ex = 1203 keV. Between the previous assignments, J = ~ [refs. a ~ S)] and J = ~ [refs. the angular distributions of the 1203 keV y-ray favour J = ~. function suggests (fig . 9) negative parity for this level. The excitation The level at E_ = 1301 keV. A level at this energy was deduced by McMurray et al. to attribute the 1020 keV y-ray and was assigned J = ~. Betts etal. s) observed a level at 1297 keV which could correspond to this level, and for it they proposed ~ - ,~+~ ~+ = 2) . We have studied the excitation function and the angular distri(lv butions of the 1020 keV decay and calculations have been performed for J~ _ ~+, ~-, ~+ ; as for the excitation function, good agreement has been obtained both for Jx = ~ - and J~ _ ~+ (see fig. 9), but the experimental angular distributions favour the latter assignment . -The level at E_ = 1309 keY. McMurray et al. °) suggested the existence of. two levels at this energy, each decaying with a pair of y-rays, which we too have observed . 2
2,
4)
s)],
~
13 4
U. ABBONDANNO et al. E,-,2o2
~.v
,w,
-~--(ai27 II+MY i - -L73 279 ~ ~,,Z_
E r- 1020 t.V T 120~ 0
e E - LS M.V L2
2A
1
a"
M1+E2
3,/2 C-QS!
2 ,
13 ~ E (M.V)
~ ~~~
E - 20 M .V
20
2a
E (Mrlr) '
L2
L2 .-. _s
4!
3
E-13M.V
E-10A4V
L2
E-2AM.V
0 .!
L2
u
! -2" M.V L2
1f
a"
E-1" M.V
p
~ .. " ~.. " Fig. 9. Excitation functions and angular distributions of y-decays from the 1203 and 1301 keV levels . The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes or near each curve; the dashed curve is calculated for parity opposite to that indicated.
To the one decaying with the 1309 and 1044 keV y-rays they assigned J = ~, ~, ~, to the other, decaying with the 1006 and 908 keV y-rays, they tentatively assigned J = ~, -~. In our work, we have attributed all the observed decays to a unique level the experimental angular distributions of the 1309, 1044 and 1006 keV y-rays are well fitted by the calculations with J = ~. The 1044 keV y-ray excitation function is in sufficient agreement with this assignment (fig . 10). The level at Ex = 1349 keV. This level has already been discussed in subsect. 3.2 ; in fig. 11 the excitation functions and the angular distributions for its decays are reported . The level at Ex = 1371 keV. Three decays (with EY = 242, 549 and 1371 keV) are assigned to this level . The excitation function and the angular distributions of the transition to the ground state have been considered and the calculations favour J` _ ~- (fig . 12) as also McMurray et al. °) suggested. The level at Ex = 1420 keV. The previous assignments for this level were J = ~, ~ [ref. a)]. We studied the excitation function and the angular distributions of the 1420 and 1142 keV decays and the best agreement was obtained with an assignment J` _ ~- (fig . 12) .
"As(n, n'yf SAs
135 1309
E
5 b
E~ IOM keV
307
LO 17ov
~,
463
LS
0
-_
20
E ~ LS /AeV
Ml+E4
y`t" " ~-0 .37 ~/4
4,S
E (MV)
LS LO
M4+E3 i~-0 .7 M1+E4
i ~ 0.76
ftf1 + ~1~1-
E ~ 20h~V
Ey 17091uV E ~ 23 A~V
43 E ~ 4 .0 NoV
LS
E ~ 2SMoV IA
LO
03
u
E~2" MeV IA
E~2" MV
'
+
.3 0 cec "
1
LO
E~4. " MV
Er ~ 1006 YeV
coy "
Fig. 10 . Excitation function and angular distributions of y-decays from the 1309 keV level. The solid lines are the calculations for the spin, parity and b-values shown in the decay schemes; the dashed curve is calculated for parity opposite to that indicated.
The level at Ex = 1430 keV. The spin values previously suggested 2' a, s) for this level were } and } ; the angular distributions of two y-rays attributed to this level exclude a ~ assignment since they are not isotropic. The choice of J = ~ is in better agreement with the experimental data and a positive parity assignment seems slightly favoured (fig . 13). The level at Ex = 1503 keV. In refs . s,a) J = ~, ~ are suggested for this level . We have studied the excitation function of the 1303 keV y-decay and the theoretical curves calculated with JR = ~t are in reasonable agreement with the experimental data . Parity assignment is not unequivocal, but near the threshold better results are obtained with J` _ ~+ . The angular distribution of the 403 keV y-ray at E = 2.8 MeV confirms this assignment, even if the associated errors are large (fig . 13). The level at Ex = 1580 keV. A y-ray with E,, = 1300 keV was first observed at E = 1720 keV by McMurray et al. a) who considered it a decay from either a 1565 or 1580 keV level. They suggested a spin value of ~ for the decaying state. In the present work a weak y-ray with EY = 1580 keV was also observed (from E = 2.1 MeV) which could be the transition to the ground state from a level with the same energy. So we based our calculations for the 1300 keV y-ray on the assumption that
l36
U . ABBONDANNO et al.
EY 1749keV
lu9
Er 91 "keV
r68
IA
r00
!~..~.__ ._
.___-~_ 1319~__ MI+E4 C~0 .i0 0
4A
E-LSA4V
25
as
MI+E4
C- L73
~h
1
F
-
_ 1/2
C-L2S
~
,~r
~
1
e
E (MeVI
-~ii T~
u
M1+E4
LO
E - 2.SMeV
20
23
E (MeV)
as
E - 20MeV L4
_m
E - 2"MeV ds
3
Lo
E-uM .v L2
ns
t
I
T
as E - 2"MeV ü
E - Ze MeV F
!1- ""1keV
ce~ "
cec
Fig . 11 . Excitation functions and angular distributions of y-decays from the 1349 keV level . T'he solid lines are the calculations for the spin, parity and b-values shown in the decay schemes ; the dashed curve and the dotted one are the excitation functions calculated, respectively, for Je = }+ and J` _ }- .
it is a decay from a 1580 keV level and the theoretical excitation function calculated with Jx = ~- is in good agreement with the experimental data (fig . 13). The level at Ex = 1606 keV. A level at this energy was observed by Moreh and Shahal Z), McMurray et al. 4) and Betts et al. s) who suggested, respectively, J = (~, ~), J = ~ and J* _ (~-, ~-). In the present work, four y-decays are assigned to this level, of which the cross sections of two have been studied. The anisotropy of the observed angular distributions exclude the J = ~ assignment, whereas the excitation function of the 1606 keV y-ray is well fitted by the calculations with JR = ~- (fig . 14). The level at Ei = 1654 keV. A level at this energy was observed in almost all the previous works, but information on it has been contradictory. In ref. ~), a 1655 keV y-ray was observed, but not assigned . McMurray et al. 4) assigned 1656 and 1083 keV y-rays to the decay from this level and their CN calculations gave a best fit to the excitation cross section for J = ~. A level near this energy (at 1669 ± 7 keV) was deduced by Betts et al. s ), to which they assigned J~ _ ~-, ~- ( lp = 3). Finally, Schrader et al. ') found a level at 1660 f 10 keV, to which there is a corresponding mixed lP value of (1 +4). In our work, we attribute three y-decays to a level at 1654 keV. The excitation function calculations for J = ~ give values too low, whereas we
1371 E~ 177l Y~V
v
0
"~
OS
E1 ~1470t~V
1470
73
E (M~VI
MI+E7
0
2A
E ~ 73 M~I
0 1 .0
MI+E7
779
03
--~__~_-~T` 20
E ~ 20 M~V
;/i i ~ 44~_
M1+E7
i~-47~-
i ~ -Q37
2d
E (MN)
E ~ i" M~V
QS ET~ 1147 E~V "
E ~ 23 /MV 3
0 l-
1
E ~ 73 M~V
4
7A
"
1 7A
E (MJl)
1
E~2" MN E~2 " MN LO
T 43 a~ "
a~ "
Fig. 12. Excitation functions and angular distributions of y-decays from the 1371 and 1420 keV levels . The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes; the dashed curves are calculated for parity opposite to that indicated.
obtain a more satisfactory agreement for J = ~ . The parity assignment is uncertain, but the cross section of the most intense decay (E7 = 1083 keV) favours the choice of J` _ ~+ (fig . 14). The level at Ex = 1685 keV. A 1686 keV y-ray was observed, but not assigned in ref. 3). The existence of a level at this energy was first proposed in ref. a) but no indication was given as to its spin. In the present work, we also observed a y-ray with this energy at E = 1 .8 MeV, together with five other y-rays, which can be considered as decays from this level. The excitation functions of the 1685 and 1066 keV y-rays have been studied and J` _ ~- can be tentatively proposed (fig . 15). The level at E_ = 1688 keV. This level must correspond to the 1690 keV level observed in ref. 4). We attribute to it three possible decays, of which we studied the excitation function of two (EY = 1688, 1423 keV) . Both the experimental excitation functions are well fitted by the calculations for J = ~ (fig . 15). The level at E_ = 1873 keV. A level at this energy was observed in refs. a. a . e) and assigned J = (~, ~) in ref. Z). In the present work, it has been attributed five decays. The spin assignment has been deduced from the study of the excitation function of the 1873, 1675 and 1594 keV decays and a spin value of ~ can be proposed, which is
13 8
U. ABBONDANNO et al. E! ~ 1170 kcV
_~
1303
__~ ~~-
Ey~ 17W kW
1101
l1+M3
e~-QS~ i
19e /
b v
/
/
0
E ~ 7.3 MeV
Yz.H
1470
!
nse 0 20
+
Ml+E7 e ~-0 .3
EI+M2
1h
e~-133 ~_ ZS
E (MeV) LS E ~ 2e MeV
LO
2A ,
?}
E(M~VI
LO
OS
LO
~
E~
2e MeV
!~
2e MeV
E~ 107 keV
!!- 303 k~
OS 1
0 ce~ 9
-1
LS
7A
if'
E (M~V~
Fig. 13 . Excitation functions and angular distributions of y-decays from the 1430, 1503 and 1580 keV levels. The solid lines are the calculations for the spin, parity and b-values shown in the decay schemes ; the dashed curves are calculated for parity opposite to that indicated.
consistent with the concavity of the 1873 keV y-angular distribution at E = 2.8 MeV (fig . 15). The level at E_ = 1899 keV. A y-raywith this energy was observed, but not assigned by Wilenzick et al. 3). In the presentwork, it isproposed as the transition to the ground state from a level at the same energy, which may decay through two other y-rays . The excitation function and the angular distribution of the 1899 keV decay have been studied : calculations for J-values of ~ and ~ have been performed and all seem to be compatible with the experimental data (fig . 16). The level at E_ = 1909 keV. This level must correspond to the 1908 ± 7 keV level observed in ref. s) and to the 1903 ± 5 keV level observed in ref. '). In both these works, the level was assigned -~+ . We attribute two decays (the 1645 and 1629 keV y-rays) to a level at this energy, which can be considered to confirm the suggested spin value. The agreement between the experimental data and the calculations for J = ~ is not good, because of the very small values of the excitation functions, as can be seen in fig. 16 where, besides parity, two values of the branching ratio have also been considered for the 1645 keV y-ray (obviously a branching ratio of 100
"As(n, n'y)"As 1606 ~ 0
â
EI+M2 MI+E2
i ~-QS
i
43E
3,/'1 _ _ I~
1634 617 37Y
~7
L0
44 as E~~ 16061aV YA
E ~ 20 M~V 1.3
1
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I
i.S
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____
i
T
,
.
~"
a4 .4 0
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C " _a75
E i -1634 4~V
.4 0
E, ~ 1037 hV LS E ~ 7 .e AMV
E ~ 2! M~V
as
0
E,~10l7 4~V E ~ 2! M~V
1 .3
3/2~ ~ EI+M7
.4 0
.3 0
3
139
~ 20
T
' ~ - -23
E (AbV)
1 .7 ae
E~ ~ 1003 NV
.o . o ~~. e Fig. 14 . Excitation functions and angular distributions of y-decays from the 1606 and 1654 keV levels. The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes; the dashed curves are calculated for parity opposite to that indicated.
for this y-ray leaves the 1629 keV y-ray unassigned). Consequently very little can be said definitely about this level, but a spin value of ~ can be adopted, because other spin values would yield cross sections too large for these decays . The leoel at Ex = 1988 keV. A level at this energy is proposed for the first time in the present work . The excitation functions of the 1988 and 1518 keV y-decays have been studied and the calculations with J = ~ give curves which are in sufficient agreement with the experimental data (fig. 16). The level at E_ = 2000 keV. A y-ray with this energy was first observed at E = 2.3 MeV and was considered as the transition to the ground state of a new level . Its angular distribution at E = 2.8 MeV has an evident concavity downwards which can be reproduced well by assigning J = ~ to the decaying level ; the comparison between the experimental and calculated excitation function confirms this value for the spin, while the parity remains uncertain (fig. 16). The leoel at E_ = 2009 keV. This also is a level observed for the first time in the present work. The excitation functions of the transition to the ground state and of the 1543 keV y-decay have been studied and a J-value of ~ can be proposed, but the parity remains undetermined (fig. 1 ~.
140
U. ABBONDANNO et al .
OA 0.7
16t3 617
~
o
16!!
]65 0
OA
~_ 4
E, - 16 "3 k.V
E,- 1066 keV
o~
(7/i )
~~~~-
Xy
;~~~-"--
42
1~ ~_
V 3
1.4 1.0 46
E, -16ee k.v
a4 .4 0
E,- 1473 keV LS E, - s.e Mev
~t'
MIrE4 . S " 0 .73 0 E~ 1l79 k.V
7,P7 ;
i79 liro
3~/7 >/i
Ei 1673 k.V
i~
L~
3Vh - /y ~~_!~-~
0.4 .i 0
44
len
20
23 E (/4eVl
2
E~ 1394 k.V ii
E~ 1lf77 keV
cepB
20
E - P.! M.V
1
~y23
E (M.VI
1
cesB
Fig. 15 . Excitation functions and angular. distributions of y-decays from the 1685, 1688 and 1873 keV levels The solid linesare the calculations for the spin, parity and S-values shown in thedecay schemes; thedashed curves are calculated for parity opposite to that indicated.
The level at E_ = 2021 keV A new level is proposed at this energy, which decays through three observed y-rays . The excitation function of the 2021 keV y-decay is consistent with an assignment of both J` _ -}- and ~+ (fig. 17). The level at Ex = 2067 keV. This level must correspond to the 2064 f 4 keV level observed in ref. Z) and to the 2061 ±3 keV level observed in ref. 6). The calculations ofthe excitation function of the 2067 keV y-ray for J` _ ~-, ~+ are both in sufficient agreement with the experimental data, which, however, are very poor (fig . 17). The level at E_ = 2104 keV. This level might correspond to the 2097±4 keV level observed by Moreh and Shahal Z) and assigned J = (~, ~). Wang er al. 6) assign a 2103 keV y-ray to a level at 2572 keV, but our observed threshold is at E = 2.3 MeV. In the present work the level has been attributed two decays, the excitation functions of which have been studied and confirm the J = ~ assignment. Calculations with a positive parity choice are in slightly better agreement with the experimental data (fig . 17). The level at E_ = 2160 keV. We have considered the 2160 keV y-ray as the transition to the ground state from a new level. Calculations for its excitation function have
'SAs(n, n'y)'sAs 1t99~ (~/7 ~!"1
0
aA 47
19/!
a.
7h-
a7
s/7"
!; u991~w
14 1
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o E,- 19~{ tw
~7 " (i " 4101
__
E c - I31~ 4w 7000 0
1909 QI 47
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779
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23
E (Mw)
LO M Q7
!i- 1a79 Yw ~i~
! (rw) ~.. a Fig. 16 . Excitation functions and angular distributions of y-decays from the 1899,1909,1988 and 2000 keV levels. The solid lines are the calculations for the spin, parity and 8-values shown in the decay schemes or near each curve. For the 1645 keV y-ray, two values for the branching ratio are reported : the calculated excitation functions of the 1629 keV y-ray correspond to the curves calculated for a branching of 60 of the 1645 keV decay.
been performed for J~ _ ~ t and the curve obtained for JR = ~+ is slightly more than the experimental data (fig . 17). The level at Ex = 2176 keV. This must correspond to the 2176 keV level observed in ref. z) and assigned (}, ~) and to the 2180±3 keV level observed in ref. 6). The excitation function of the transition to the ground state has been studied and calculations strongly favour J = ~, but the parity cannot be assigned (fig. 17). The level at Ex = 2228 keV. A level at E_ = 2233 keV (J = ~, ~) was first proposed in ref. 2), which could correspond to the 2228 keV level observed in the present work. We have studied the excitation function of the transition to the ground state and a J = ~ assignment seems the most probable, but JR = ~- cannot be excluded (fig . 17). The level at Ei = 2239 keV. This also could correspond to the 2233 keV level observed in ref. Z) ; we studied the excitation function of the 2239 keV y-decay in order to test their assignment J = ~}, ~ and the best agreement between experimental data and calculations was obtained for J = ~ (fig . 17).
U . ABBONDANNO er rrl.
14 2 2009~~~ J/1'' Q4
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The lege/ at Ex = 2259 keV. This is the highest level for which CN calculations
have been pt;rformed in the present work. It could correspond to the 2252± 10 keV level observed only in ref. S) ànd tentatively assigned (~-, ~-). We have tested both these values, but they yielded too large cross sections. On the other hand, the excitation functions calculated for J~ = i t are both in sufficient agreement with the experimental data, so that J = ~ can be proposed (fig . 17) . 4. Conclusions
As already stated, the energy levels of'SAs have been investigated up to an excitation energy of 2609 keV : in this energy range we observed S8 levels, twelve ofwhich had not been reported previously . These are levels at Ex = 1101, 1899, 1988, 2000, 2009, 2021, 2147, 2160, 2228, 2239, 2502 and 2508 keV. It is probable that new levels lie also at Ex = 2327, 2358, 2419 and 2609 keV. The observed y-rays totalled 157 in number and, except for a few (EY = 191, 276, 392, 507, 781, 1342, 1355, 1815, 1852,
'sAs(n, n'y)' sAs
143
1924 and2263 keV), they were all identified and enabled us to construct the decay scheme of'SAs shown in fig. 3, where the thick and the broken lines indicate, respeotively, new and uncertain levels . As regards the energy levels lying below 1873 keV, the scheme is similar to that deduced in ref. 4) ; all the y-rays reported, in fact, have been observed also in the present work, except for the transitions having an energy below our instrumental threshold. Besides, we have resolved the ambiguities concerning the assignment of the 893 and 1300 keV y-rays, favouring the levels at 1173 and 1580 keV. As for the 816 keV y-line, the existence of the decaying level seems more probable at 1080 keV than 1095 keV. The decay scheme we propose confirms most of the levels observed in refs. z - 6), as was presented in detail in subsect. 3 .3. The spin and parity of 341evels have been studied by comparing the experimental cross sections with theoretical calculations performed using the statistical model of the compound nucleus. Many uncertain spin-parities have been confirmed. New spin values are proposed for thirteen levels . In some cases the parity value can also be tentatively assigned . References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)
Nucl . Data Sheets 16 (1975) 25 R. Moreh and O. Shahal, Phys . Rev. 188 (1969) 1765 R. M. Wilenzick, V. R. Dave and J. A. Nelson, Phys . Rev. C4 (1971) 2126 W. R. McMurray, P. J. Celliers and R. Saayman, Nucl. Phys . A225 (1974) 37 R. R. Betts, S. Mordechai, D. J. Pullen, B. Rosner and W. Scholz, Nucl . Phys . A230 (1974) 235 C. W. Wang, Y. C. Liu, E. K. Lin, C. C. Hsu and G. C. Kiang, Phys . Rev. C9 (1974) 1396 M. Schrader, H. Reiss, G. Rosner and H. V. Klapdor, Nucl . Phys . A263 (1976) 193 W. Hauser and H. Feshbach, Phys . Rev. 87 (1952) 336 G. R. Satchler, Phys . Rev. 94 (1954) 1304 ; 104 (1956) 1198 E. Sheldon and D. M. Van Patter, Rev. Mod. Phys. 38 (1966) 143 E . Sheldon and R. M. Strang, Comp. Phys . Comet. 1 (1969) 35 U. Abbondanno, F. Demanins, M. R. Melissa and G. Nardelli, Nuovo Cim. Lett . 18 (1977) 97 F. Demanins and G. Nardelli, INFN Report BE-70/3 U. Abbondanno, A. Boiti and F. Demanins, Nucl . Insu . 142 (1977) 605 ; U. Abbondanno, A. Boiti, F. Demanins and M. R. Melissa, Nucl . Insu . 148 (1978) 577 T. Mukoyama, Nucl . Insu. 125 (1975) 289 C. A. Engelbrecht, Nucl . Insu . 93 (1971) 103 D. I. Gerber, L. G. StrSmberg, M. D. Goldberg, D. E. Cullen and V. M. May, Brookhaven National Laboratory report BNL 400 (1970) C. Meeker Davisson and R. D. Evens, Rev. Mod. Phys . 24 (1952) 79 M. Lagonegro and U. Abbondanno, INFN Report BE-73/4 F. D. Becchetti, Jr . and G. W. Greenlees, Phys . Rev. 182 (1969) 1190 E. H. Auerbach and F. G. J. Percy, Brookhaven National Laboratory, report BNL 765 (1962)
A STUDY OF THE 'sAs(o, n'y)'sAs REACTION UGO ABBONDANNO, FERRUCCIO DEMANINS and MARIA ROSA MALISAN Istituto di Fisica dell'Universitd, Trieste and Istituto Nazionale di Fisica Nucleare, Sezione di Trieste and GIANCARLO NARDELLI Istituto di Fisica del/'Università, Padova and Istituto Nazionale di Fisica Nucleare, Sezione di Padova Received 28 December 1977 (Revised 26 April 1978) Abstract : The de-excitation gammas following the inelastic scattering of neutrons from'SAs have been studied for incident neutron energies from 1300 to 2800 keV in steps of 100 keV. The energy levels and the branching ratios of their decays have been deduced from excitation function measurements : 58 energy levels have been found in the excitation energy region below 2 .8 MeV, twelve of which are reported for the first time in this work . The experimental excitation functions and angular distributions have been compared with the theoretical predictions based on the statistical theory of the compound nucleus . Spin and parity assignments for the levels and multipolarities for the decays are proposed for excitation energies of levels up to 2300 keV. E
NUCLEAR REACTION 'SAs(n, n'y), E = 1300-2800 keV ; measured Er , a(E, E~, ~. '°As deduced levels, J, n, y-branching, 8 . Natural target .
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Information on the energy levels of °As, as reported in Nuclear Data Sheets '), is mainly based on the study of the 'SAs(y, y') [ref. Z)], 'SAs(n, n'y) [refs. 3'4)], '4Ge(3He, d) [ref. 5)] and '4Ge(p, y) [ref. 6)] reactions. Recently, new data were obtained by Schrader et al. ') who investigated the level scheme of 'SAs via the (3He, d) reaction. However, the experimentally determined structure of this nucleus still had some regions with incomplete information due to the limited bombarding energies used in the (n, n'y) reactions and the selection rules the (y, y') and (3He, d) reactions are subject to. Therefore we decided to study'SAs by means of the (n, n'y) reaction, observing the de-excitation y-rays, in order to obtain additional information on the energies, spins and decay modes of the levels of this nucleus . Neutron inelastic scattering, as a method of spectroscopic investigation, has, in 117
11 8
U . ABBONDANNO et al .
fact, some remarkable advantages, namely the absence of the Coulomb barrier (which allows the excitation of the low-lying states of the target nucleus with lowenergy incident neutrons, limiting in this way the number of competitive reactions that may complicate the y-spectra) and the absence of selection rules. For these reasons, (n, n'y) reactions have been studied intensively in recent years to gain information on the properties of nuclear energy levels. More recently, new interest in them has arisen through the advent of high-resolution and large volume truecoaxial Ge(Li) detectors. The experimental cross sections for the y-rays following inelastic scattering can be compared with the theoretical predictions of the statistical compound nucleus (Cl~ model of Hauser and Feshbach e), extended by Satchler ~ and later revised by Sheldon and Van Patter 1°). By this comparison, the spins and parities for the excited states can be determined with reasonable confidence . In the present work, the energy levels and decay modes of'SAs have been deterihined up to an excitation of 2609 keV from y-spectra obtained at incident neutron energies between 1300 and 2800 keV. Measurements of angular distributions for the decay gammas from' SAs were made at neutron energies of 1 .5, 2.0, 2.5 and 2.8 MeV. A preliminary report of the results, concerning the level scheme and the branching ratios, has been published elsewhere 12). 2. Experimental details and data analysis
The measurements were performed using the 7 MV Van de Graaff accelerator at the Laboratori Nazionali di Legnaro (LNL, Padua) . The incident neutrons were obtained with the 3H(p, n)3He reaction in a tritium-titanium target (0.84 Ci/cms) bombarded by a 3 ns pulsed beam of protons, and were incident on a cylindrical sample of powdered 'sAs metal in a thin-walled aluminium container, 4.5 cm in diaméter and 5 cm high, placed 10 cm in front of the neutron source . The energy spread of the neutrons was about 30 keV over the energy range used in the measurements and it was chiéfly due to the angular spread of the sample . The de-excitation y-rays were observed by means of a true-coaxial Ge(Li) detector with a nominal volume of 52 cm3, shielded from the tritium target by a shaped lead wedge. The full-width at half-maximum of a y-line obtained by the Ge(Li) detector during operating conditions, was x 2 keV at almost all the energies up to 2.6 MeV. The time-of-flight technique with a flight path of 60 cm was used to discriminate between the background from the neutrons scattered in the Ge(Li) detector and the y-rays produced in the de-excitation of 'sAs . For monitoring purposes, a neutron time-of-flight spectrometer using a NE 213 scintillation counter was placed at a distance of4 m from the neutron source at an angle of 20° with respect to the beam direction. The charge collection was also monitored by means ofa current integrator. The y-ray spectra were recorded in a 4096 channel analyser . All the measurements on the 'sAs sample were followed by similar measurements, at the same neutron
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energies, on a ' ZC sample, for identification of background peaks arising from neutron interactions in the Ge(Li) detector and surrounding materials. Additional measurements were made employing standard calibrated y-sources in order to determine the energy scale and the efficiency of the detector . The' 6Fe(n, n'y) 56Fe reaction was finally studied at a neutron energy of 2.5 MeV, in order to obtain absolute normalisation by comparing the y-yields from the sample with the known values 's) for the 90° differential production cross sections of the 846 and 1240 keV y-rays . The decay scheme of'SAs and the y-ray production cross sections have been deduced from the analysis of the y-ray de-e~ccitation spectra. For this purpose, measurements were performed at 90° for neutron energies from 1 .3 to 2.8 MeV, in steps of 0.1 MeV, in order to determine y-excitation functions. The y-angular distributions were observed at E = 1 .5, 2.0, 2.5 and 2.8 MeV, at angles between 30° and 150° to the incident beam direction. A part of one of the y-spectra obtained in the measurements is shown in fig. 1 . It was observed at E = 2.5 MeV ;the prominent y-rays attributed to inelastic scattering in' SAs and some background peaks (B) are labeled.
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osAs(~, n'~)'sAs
121
The spectral data, i.e. energy, area and FWHM of each peak after subtraction of the background, were extracted by means of an automatic computer code ' 4 ), based on a modified non-iterative method ~ s), which allows the analysis of relatively large zones of the spectra. This analysis has been performed at the HP 2116 B computer of the LNL and at the HP 2100 S computer of the Istituto di Fisica dell'Università di Trieste. A typical result of this method is shown in fig. 2. In this figure, the points represent the measured spectrum, the straight line the calculated background, and the curves the Gaussian shapes fitted to the experimental points . Photopeak yields obtained in this way have been corrected for the attenuation of both neutrons and y-rays in the sample ; the effects of multiple scattering have also been taken into account, according ref. ' 6). The cross sections for the neutrons and gammas used in the evaluation of these corrections have been obtained from refs . 1' .18) . The y-yields were normalised to the monitor counts of the neutron detector, and then corrected for the relative efficiency of the Ge(Li) detector . Finally, as explained above, absolute values have been obtained from the corrected data by comparison with those, corrected and normalised in the same way, from the 56Fe(n, n'y) 56 Fe reaction . 3. Results and discussion 3 .1 . RESULTS
The results of the measurements described in the previous section are presented in this section and compared with the predictions of the statistical theory of the compound nucleus. The first result is the decay scheme of'SAs, which is shown in fig. 3 and table 1 . Each value reported in the second column of table 1 is the weighted average of the excitation energies determined for the corresponding level . As for the observed y-rays, the assignment to transitions in' SAs was simplified by the small energy step used in the excitation function measurements . Some ambiguities were resolved by comparing the experimental data with the calculated ones, as will be described in detail in subsect. 3 .3 . The second group of results represents the excitation functions of y-rays from levels lying above 1000 keV. The data, given in absolute values, are reported in figs. X17 : the errors have been estimated as a combination of the statistical error, the error in the y-ray detector efficiency determination, the error in neutron monitoring, and the uncertainty ( f 10 /) due to the absolute normalisation to the data from the 56Fe(n, n'y) 56Fe reaction . The third group of results consists in the y-ray angular distributions . In figs . X17 they are reported as angular asymmetries with respect to 8 = 90° w(9) = Q(~~Q(90°) .
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12 6
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The absolute values are reported in table2, as integrated cross sections Q(E, Ey) = 4aa° obtained from a Legendre-polynomial fit to the experimental values . It should be.noticed that the errors reported in table 2 and the errors of the angular distributions reported in figs . 4-17 do not take into account the uncertainty due to the absolute normalisation . 3.2. COMPOUND NUCLEUS CALCULATIONS
Since the predictions of the statistical theory of the Chi on differential cross sections have been described in detail in ref. 1 °), the presentation of the calculations and results obtained in the present work will refer to the symbols and nomenclature adopted there. All the calculations have been performed by means of the code MANDY ` 1) at the CDC 6200 computer of the Centro di Calcolo delfUniversità di Trieste. As we pointed out in the introduction, the comparison between measured and calculated cross sections can provide information on the level spins and parities and the radiation multipolarities . Our study began then by testing our calculations (in a special way, the transmission coefficient set) on a particular level (Ei = 1349 keV,fig. l1), for which the possible Jx values have been limited by previous studies. In fact, this level had been observed also in the'4Ge(3 He, d)'SAs reaction S" '), and the deduced value of lp = 1 allowed J` to be ~= or ~- . By observing the experimental angular distributions of the 1349 and 948 keV y-rays, the value J = ~ can be excluded since it would imply isotropic distributions. However calculations of the excitation function have been performed for J* _ ~-, ~+, ~- using the transmission coefficients calculated with an optical model code ' ~ adopting the parameters of Becchetti and Greenlees 2 ~ : in this framework, the experimental values have been best fitted when J~ _ ~- . Calculations performed with another set of transmission coefficients z') give, for this level and for JR = ~-, results which are less than the experimental data of about 20 ~. Calculated cross sections have shown enough sensitivity to parity to allow, in some cases, parity assignments. In our calculations, a partial wave cutoff at 1~ = 4 has been used [see ref. 1 ~]. As for theextra CN exit channels considered in the r-term, wehave taken into account all'sAs levels that could be excited at the neutron energy considered (their number varied from 22 at E = 1 .3 MeV to 59 at E = 2 .8 MeV) . The extra exit channels have been confined solely to the inelastic and elastic channels because the other possible decay channels ['sAs(n, p)' S Ge (Q = - 0.416 MeV) and'sAs(n, a)'2Ga (Q = 1 .198 MeV)] have negligible transmission coefficients at E -<- 2.8 MeV. As explained in ref. 1 ~, the measured cross sections of the observed y-rays may be affected by contributions of cascades from higher levels, which modify not only the absolute values but also the shapes of the angular distributions. In our calculations, we took into account this fact only when the cross sections of cascades were greater
'sAs(n, n'y)~ sAs
127
than about 10 ~ of the observed y-ray cross section, being 10 % of the error in the absolute value of the measured data. In what concerns the analysis of the angular distributions, which has been performed together with the study of the excitation functions, their shapes are sensitive also to the mixing ratio of the multipolarities of the y-rays . In order to extract this parameter, every experimental angular distribution has been developed in a Legendrepolynomial least-square fit. The experimental aZ and a4 values have been reported on the ellipse yielded by the programme MANDY, and in this way the value of S has been determined. If this comparison between the a* led to a double-valued solution for S, the lower value of S has been systematically adopted. If possible, excitation functions and angular distributions have been jointly studied in order to determine J, n and S. In the absence of angular distribution data, only J (and in some cases, tentatively, n) can be proposed . 3 .3 . DISCUSSION
In the excitation energy range Ex _<_ 886 keV we obtained twelve levels and confirmed all levels reported in Nuclear Data Sheets 1) except the uncertain 585 .7 keV level observed in the (3He, d) reaction S). The level reported at 304 keV was not observed in our time-gated measurements because of its 17.53 ms half-life. Since our measurements started from E = 1 .3 MeV whereas the y-ray excitation functions are sensitive to the spins and parities involved in the observed transitions especially near the threshold, it follows that the excitation functions of the levels below 1 MeV are not significant. On the other hand, neither can the angular distributions of their decays yield useful information because they are disguised by the contributions of the many cascades from the higher levels. Therefore, for the levels with E_ <__ 886 keV we give only the experimental integrated cross sections of table 2. In our CN calculations, we adopted the spin-parity values reported in column 9 of table 3, which are derived from previous works and are compatible with the cross sections of the observed transitions to these levels . For the higher levels, we report in table 3 the previous spin-parity assignments compared with those suggested from our study of excitation functions and angular distributions. The spin and parity determined for the levels involved, and, in some cases, the multipolarities and mixing ratio of the radiation, are reported in the figure concerning the observed transition . In the following, a discussion of the results obtained for each level is presented in some detail . The level at Ex = 1042 keV. In the present measurements, we observed the excitation functions of the two y-decays of this level, EY = 740 keV and EY = 643 keV, and the angular distributions for the latter. In ref. a) J = ~ is proposed whereas in ref. ') an !p = 3 or 4 transition is observed to a level at 1048 f 5 keV : from our excitation function calculations, in which the contribution of a pure cascade from
U . ABBONDANNO et al.
128
TABLE 3
Comparison between spin and parity values reported in the literature and obtained in the present work Level no.
E, (keV)
J. a)
0 1 2
0 198 265 279
}}}}_
400 468 572 617 821 860 864 886 1042 1063 1075 1080 1095 1 1 1128 1172 1204 1301 1309 1349 1371 1420 1430 1503 1581 1606 1654 1685 1688 1873 1899 1909 1988 2000 2009 2021 2068 2104 2110 2147 2160 2176 uz8 2239
~}+ }, }
3
4 5 6 7 8 9 10 11 12 13
14 15 16 17 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 a7 48
303
J * .)
J . r)
~+
~+ }_
J * b)
J'r)
}
_}
}+
y~+
_
J` (present work)
~+
.)
~_
')
}_ }
_
(} , ~ )
~ } }
} ,~
(} , ~ )
}
} ,~
(}, ~)
~
}~+ , ~~+
(~, ~, ~, i, (}, ~) (~, ~) (~, ~)
~, ~+
b)
~
b) b)
~
~+
`)
}
u)
_ _ } ,~
}, ~
C)
b)
}+
(~~ +) _ _ } ,~
}
4_ ~
b) b)
~
~
`)
}
b)
(} ) }c_r (}, ~) }+
}+
}, }-
(}, })
},$
}+
b)
`) ~)
b)
(}, }+) `) } b) } } _} (} , }+)
`) `) `) `)
}c+~
b)
}+ }
.)
(},})
}
b) b) s ~ b)
'sAs(n, n'y)'sAs
129
T~sce 3 (continued) Level no.
E, (keV)
49 50 51 52 53 54 55 56 57 58
2259 2303 2327 2358 2379 2419 2502 2508 2571 2609
J*
°)
J` e )
}, }-
~
J` (present work)
, ~, ~ +, ~~, i+
}, ~-
`) The values simply adopted from the literature in our calculations for the transitions to these levels. b) The assignments among the previous ones that fit best our data . `) The newly proposed assignments. > Ref. '). °) Ref. '). `) Ref. Z) . ') Ref. `) . ~ Ref. s). .)
N
Ei - M7 Yq
vo
E " LSM .v
ne
E,- ~sot.v
E - 20 M~V
r
Q°
S 1 -
I
3
E - 27 M~V
LO
Ei 6I7 Y~V
Ob
E - 2 " AbV l0
E (NbV)
us "
Fig. 4. Excitation functions and angular distributions of both y~ecays from the 1042 keV level. The solid lines are the calculations for J` _ ~- . the dashed lines those for J` _ ~+ .
13 0
U. ABBONDANNO et at. l06]
~~
47q
7!4 k~V
]/s_
MhE2
C~-a67
106]
_
E~ " 106] k~V
p
M1+E2
~/2 C " -L2s ]/
a2
u
Ls E " LSMN
2 .0
2.s
E(M~V)
0 .1 LS E " 2SM~V
LO
u
20
2.3
E (M~V)
LO
E " 20M "V
QS
u
! " 23 MrV
! - 2.~Mdv
Lo
Lo E ~ 2! M~V
a5
L1 av 1
0
-1
E~~ ~92 k~V 1 t
106]
1
I
o
372
MI+E2 1
C " a"1
~/2 -
QhI -1
ro. s ~e . s Fig. 5. Excitation functions and angular distributions of three y-decays from the 1063 keV level. The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes; the dashed line is the excitation function calculated assuming the opposite parity for the decaying level.
the 1301 keV level has been taken into account, good agreement is found (fig . 4) for J = ~. The level at Ex = 1063 keV. The excitation function and the angular distributions for the prominent decay from this level (EY = 784 keV) have been studied. The experimental points for this y-ray compare better with the calculations for Jx = ~-, confirming thespin assignment given in ref. 4). Calculations have been performed also for other decays (E7 = 1063 keV and 492 keV) : in the limit of statistical errors, they fit the experimental data (fig . 5). The level at Ez = 1075 keV In the present work, two gammas are found which can be assigned to a level of this energy . The excitation function and the angular distributions for the decay to the ground state have been studied and confirm the assignment J = ~ proposed in refs. z " 4 " ') (fig . 6). The levels at E_ = 1080 keV and Ei = 1095 keV. The existence of levels at these energies was first proposed by McMurray et al. °) to account for the presence of a 816 keV y-line, observed at E < 1130 keV. This gamma was attributed to either a transition from a 1080 keV level to the 265 keV level (J~ _ ~-) or a transition from a 1095 keV level to the 279 keV level (J` _ ~-). We tested both hypotheses in our
'sAs(a, a'y)'sAs a7s
E~ 107s 4~Y
s t
-0
MNIY
ye'a
f -Wf
;f!-
~~51-5~i+ E - tSM~V
u
~
~
E
m
3
Lo
u LO
u u
ns k v
~i~
w as
~~
LO
u
E~-
131 _________
T f f ~
tow
"
E - L! M.V
xes
20
__ T Et+w
~.s
t
y`t' f-tm
3h-
E (M~l
LO 2
E - 20 M~V
T
' '
E - 7A MrV
1 T
t
E - ?3 M .V
T
E - 2f M1r
E - 23 M.V
T
u
r
to
Fig. 6. Exçitatioa functions and angular distributions of y-decays from the 1075 and 1080 keV levels. The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes; the dashed lines arb the excitation functions calculated assuming the opposite parity for the decaying level.
CN calculations : the assignment J = ~ is excluded, for both levels, by the trend of the experimental angular distributions, which exhibit a remarkable anisotropy . Assuming J = ~ for the 1080 keV level, and a branching ratio of 55 ~ for the $15 keV decay, the calculated angular distributions have too weak an anisotropy to fit the experimental data, whereas the calculated excitation functions are in reasonable agreement with the experimental yield: Therefore, we performed the calculations with J = ~ for this level and good agreement was found between experimental data and theoretical curves (positive parity, see fig. 6). With the hypothesis that the 815 keV y-ray is a decay from a 1095 keV level with a branching of 100 ~, the assignment J = ~ for such a level cannot account for the experimental W(~ considered, since a ~ ->' ~ spin sequence implies distributions with a peak at 90° or with too slight a concavity upwards . A J = ~ assignment to this level describes well the angular distributions for the 815 keV y-decay, but is in total disagreement when the absolute values of the excitation function are considered (see fig. 7). The level at Ex~ = 1101 keV. In ref. a) a y-ray with E7 = 1102 keV (first observed at E = 1720 keV) was assigned to the 1503 keV level. In the present work, a 1101
13 2
U. ABBONDANNO et al. lon
E,- els t.v
~s-
sn __ i
u
_m 3
~
s.o
Ls E-LSMa
as
E (M .v)
a: E-2sM .v
T
~ E - 20 A4V
no
s.s
E (M.v)
Ls i
Q3
1
Z
n~
Ls
La 4
~
~+
1 1 i ~ i i ~ 1 ~ i
E,- 1101 4~V
E-23Mw .
I
I
I
I
I
Ls LO I
0 cw i
-1
1
0
-1
co~ Y
Fig. 7. On the left-hand side, the excitation function and angular distributions of the 815 keV y-ray if considered as a decay from a 1095 keV level. The solid lines are the calculations for !` _ } -, the dashed one is the excitation function calculated for J` _ }+ . On the right-hand side, the excitation function and angular distributions of the 1101 keV y-ray: solid lines are the calculations for J` _ }+, the dashed one is the excitation function calculated for J` _ }- .
keV y-ray was observed at E = 1 .3 MeV together with one at 796 keV : assuming that both decays come from the same level, we propose a level at 1101 keV. The 1102 keV y-decay observed by McMurray et al. a) corresponds rather to a 1105 keV y-ray that we first identified at E = 1 .8 MeV. We studied the excitation function and the angular distributions of the 1101 keV decay, because the other y-line, although stronger, overlaps the 799 keV y-line too much to give good data. From the flat trend of the angular distributions and from the values of the excitation function, for which we took into account the E,, = 403 keV cascade from the EX = 1503 keV level, J = ~} is proposed (see fig. 7). The level at Ex = 1128 keV. The previous assignments are J = ~, given in ref. °), and J~ _ ~-, given in refs. z . s" '). The assignment J = ~ is confirmed (fig . 8) by the flat structure ofthe angular distributions. The excitation function suggests a positive parity for this level . The level at E_ = 1172 keV. This level decays to the 279 keV level with the 893 keV y-ray and to the 572 keV level with the 600 keV y-ray. In ref. a) the suggested spin values are ~, ~ and The ~ assignment being excluded by the anisotropy of the
u.
"As(n, n'y)'SAs n]e
!~~ I1]~t~V
El
nn
I~i+
_____ ° _
LO
133
L0
/ 4]
sn
Yri
E E ~ i9] bV
4S
L]
1 .3 ! ~ 1.] M~V
7 .0
_s 3
"
"
u
].o
Ls
E - ] .0 M~V
E tM.v)
E ~ 23 M~V E ~ 2! AwV
LO OS
Ol
LZ
E (M~V) 1.0 .S 0
4!
LZ
] .S
M] +EI i~-0 .P
-"- 5-~
~3 ~
/
n/ta
M3+EI i~-QA]
E ~ 23 M~V
"
"
"
"
" "
! ~ 7.OM~V
Od
!~~ 600 4~V ! ~7 .tM~V
E ~ 2.eM~V 1 .3 L0
cep B
"
cep 0
Fig. 8. Excitation functions and angular distributions of y-decays from the 1128 and 1172 keV levels . The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes ; the dashed curves are calculated assuming the oppositie parity for the decaying level.
angular distributions, the calculations of excitation functions were performed for the ~+, ~-, i+ and i- assignments. The best agreement (see fig. 8) was found for J i. The level at Ex = 1203 keV. Between the previous assignments, J = ~ [refs. a ~ S)] and J = ~ [refs. the angular distributions of the 1203 keV y-ray favour J = ~. function suggests (fig . 9) negative parity for this level. The excitation The level at E_ = 1301 keV. A level at this energy was deduced by McMurray et al. to attribute the 1020 keV y-ray and was assigned J = ~. Betts etal. s) observed a level at 1297 keV which could correspond to this level, and for it they proposed ~ - ,~+~ ~+ = 2) . We have studied the excitation function and the angular distri(lv butions of the 1020 keV decay and calculations have been performed for J~ _ ~+, ~-, ~+ ; as for the excitation function, good agreement has been obtained both for Jx = ~ - and J~ _ ~+ (see fig. 9), but the experimental angular distributions favour the latter assignment . -The level at E_ = 1309 keY. McMurray et al. °) suggested the existence of. two levels at this energy, each decaying with a pair of y-rays, which we too have observed . 2
2,
4)
s)],
~
13 4
U. ABBONDANNO et al. E,-,2o2
~.v
,w,
-~--(ai27 II+MY i - -L73 279 ~ ~,,Z_
E r- 1020 t.V T 120~ 0
e E - LS M.V L2
2A
1
a"
M1+E2
3,/2 C-QS!
2 ,
13 ~ E (M.V)
~ ~~~
E - 20 M .V
20
2a
E (Mrlr) '
L2
L2 .-. _s
4!
3
E-13M.V
E-10A4V
L2
E-2AM.V
0 .!
L2
u
! -2" M.V L2
1f
a"
E-1" M.V
p
~ .. " ~.. " Fig. 9. Excitation functions and angular distributions of y-decays from the 1203 and 1301 keV levels . The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes or near each curve; the dashed curve is calculated for parity opposite to that indicated.
To the one decaying with the 1309 and 1044 keV y-rays they assigned J = ~, ~, ~, to the other, decaying with the 1006 and 908 keV y-rays, they tentatively assigned J = ~, -~. In our work, we have attributed all the observed decays to a unique level the experimental angular distributions of the 1309, 1044 and 1006 keV y-rays are well fitted by the calculations with J = ~. The 1044 keV y-ray excitation function is in sufficient agreement with this assignment (fig . 10). The level at Ex = 1349 keV. This level has already been discussed in subsect. 3.2 ; in fig. 11 the excitation functions and the angular distributions for its decays are reported . The level at Ex = 1371 keV. Three decays (with EY = 242, 549 and 1371 keV) are assigned to this level . The excitation function and the angular distributions of the transition to the ground state have been considered and the calculations favour J` _ ~- (fig . 12) as also McMurray et al. °) suggested. The level at Ex = 1420 keV. The previous assignments for this level were J = ~, ~ [ref. a)]. We studied the excitation function and the angular distributions of the 1420 and 1142 keV decays and the best agreement was obtained with an assignment J` _ ~- (fig . 12) .
"As(n, n'yf SAs
135 1309
E
5 b
E~ IOM keV
307
LO 17ov
~,
463
LS
0
-_
20
E ~ LS /AeV
Ml+E4
y`t" " ~-0 .37 ~/4
4,S
E (MV)
LS LO
M4+E3 i~-0 .7 M1+E4
i ~ 0.76
ftf1 + ~1~1-
E ~ 20h~V
Ey 17091uV E ~ 23 A~V
43 E ~ 4 .0 NoV
LS
E ~ 2SMoV IA
LO
03
u
E~2" MeV IA
E~2" MV
'
+
.3 0 cec "
1
LO
E~4. " MV
Er ~ 1006 YeV
coy "
Fig. 10 . Excitation function and angular distributions of y-decays from the 1309 keV level. The solid lines are the calculations for the spin, parity and b-values shown in the decay schemes; the dashed curve is calculated for parity opposite to that indicated.
The level at Ex = 1430 keV. The spin values previously suggested 2' a, s) for this level were } and } ; the angular distributions of two y-rays attributed to this level exclude a ~ assignment since they are not isotropic. The choice of J = ~ is in better agreement with the experimental data and a positive parity assignment seems slightly favoured (fig . 13). The level at Ex = 1503 keV. In refs . s,a) J = ~, ~ are suggested for this level . We have studied the excitation function of the 1303 keV y-decay and the theoretical curves calculated with JR = ~t are in reasonable agreement with the experimental data . Parity assignment is not unequivocal, but near the threshold better results are obtained with J` _ ~+ . The angular distribution of the 403 keV y-ray at E = 2.8 MeV confirms this assignment, even if the associated errors are large (fig . 13). The level at Ex = 1580 keV. A y-ray with E,, = 1300 keV was first observed at E = 1720 keV by McMurray et al. a) who considered it a decay from either a 1565 or 1580 keV level. They suggested a spin value of ~ for the decaying state. In the present work a weak y-ray with EY = 1580 keV was also observed (from E = 2.1 MeV) which could be the transition to the ground state from a level with the same energy. So we based our calculations for the 1300 keV y-ray on the assumption that
l36
U . ABBONDANNO et al.
EY 1749keV
lu9
Er 91 "keV
r68
IA
r00
!~..~.__ ._
.___-~_ 1319~__ MI+E4 C~0 .i0 0
4A
E-LSA4V
25
as
MI+E4
C- L73
~h
1
F
-
_ 1/2
C-L2S
~
,~r
~
1
e
E (MeVI
-~ii T~
u
M1+E4
LO
E - 2.SMeV
20
23
E (MeV)
as
E - 20MeV L4
_m
E - 2"MeV ds
3
Lo
E-uM .v L2
ns
t
I
T
as E - 2"MeV ü
E - Ze MeV F
!1- ""1keV
ce~ "
cec
Fig . 11 . Excitation functions and angular distributions of y-decays from the 1349 keV level . T'he solid lines are the calculations for the spin, parity and b-values shown in the decay schemes ; the dashed curve and the dotted one are the excitation functions calculated, respectively, for Je = }+ and J` _ }- .
it is a decay from a 1580 keV level and the theoretical excitation function calculated with Jx = ~- is in good agreement with the experimental data (fig . 13). The level at Ex = 1606 keV. A level at this energy was observed by Moreh and Shahal Z), McMurray et al. 4) and Betts et al. s) who suggested, respectively, J = (~, ~), J = ~ and J* _ (~-, ~-). In the present work, four y-decays are assigned to this level, of which the cross sections of two have been studied. The anisotropy of the observed angular distributions exclude the J = ~ assignment, whereas the excitation function of the 1606 keV y-ray is well fitted by the calculations with JR = ~- (fig . 14). The level at Ei = 1654 keV. A level at this energy was observed in almost all the previous works, but information on it has been contradictory. In ref. ~), a 1655 keV y-ray was observed, but not assigned . McMurray et al. 4) assigned 1656 and 1083 keV y-rays to the decay from this level and their CN calculations gave a best fit to the excitation cross section for J = ~. A level near this energy (at 1669 ± 7 keV) was deduced by Betts et al. s ), to which they assigned J~ _ ~-, ~- ( lp = 3). Finally, Schrader et al. ') found a level at 1660 f 10 keV, to which there is a corresponding mixed lP value of (1 +4). In our work, we attribute three y-decays to a level at 1654 keV. The excitation function calculations for J = ~ give values too low, whereas we
1371 E~ 177l Y~V
v
0
"~
OS
E1 ~1470t~V
1470
73
E (M~VI
MI+E7
0
2A
E ~ 73 M~I
0 1 .0
MI+E7
779
03
--~__~_-~T` 20
E ~ 20 M~V
;/i i ~ 44~_
M1+E7
i~-47~-
i ~ -Q37
2d
E (MN)
E ~ i" M~V
QS ET~ 1147 E~V "
E ~ 23 /MV 3
0 l-
1
E ~ 73 M~V
4
7A
"
1 7A
E (MJl)
1
E~2" MN E~2 " MN LO
T 43 a~ "
a~ "
Fig. 12. Excitation functions and angular distributions of y-decays from the 1371 and 1420 keV levels . The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes; the dashed curves are calculated for parity opposite to that indicated.
obtain a more satisfactory agreement for J = ~ . The parity assignment is uncertain, but the cross section of the most intense decay (E7 = 1083 keV) favours the choice of J` _ ~+ (fig . 14). The level at Ex = 1685 keV. A 1686 keV y-ray was observed, but not assigned in ref. 3). The existence of a level at this energy was first proposed in ref. a) but no indication was given as to its spin. In the present work, we also observed a y-ray with this energy at E = 1 .8 MeV, together with five other y-rays, which can be considered as decays from this level. The excitation functions of the 1685 and 1066 keV y-rays have been studied and J` _ ~- can be tentatively proposed (fig . 15). The level at E_ = 1688 keV. This level must correspond to the 1690 keV level observed in ref. 4). We attribute to it three possible decays, of which we studied the excitation function of two (EY = 1688, 1423 keV) . Both the experimental excitation functions are well fitted by the calculations for J = ~ (fig . 15). The level at E_ = 1873 keV. A level at this energy was observed in refs. a. a . e) and assigned J = (~, ~) in ref. Z). In the present work, it has been attributed five decays. The spin assignment has been deduced from the study of the excitation function of the 1873, 1675 and 1594 keV decays and a spin value of ~ can be proposed, which is
13 8
U. ABBONDANNO et al. E! ~ 1170 kcV
_~
1303
__~ ~~-
Ey~ 17W kW
1101
l1+M3
e~-QS~ i
19e /
b v
/
/
0
E ~ 7.3 MeV
Yz.H
1470
!
nse 0 20
+
Ml+E7 e ~-0 .3
EI+M2
1h
e~-133 ~_ ZS
E (MeV) LS E ~ 2e MeV
LO
2A ,
?}
E(M~VI
LO
OS
LO
~
E~
2e MeV
!~
2e MeV
E~ 107 keV
!!- 303 k~
OS 1
0 ce~ 9
-1
LS
7A
if'
E (M~V~
Fig. 13 . Excitation functions and angular distributions of y-decays from the 1430, 1503 and 1580 keV levels. The solid lines are the calculations for the spin, parity and b-values shown in the decay schemes ; the dashed curves are calculated for parity opposite to that indicated.
consistent with the concavity of the 1873 keV y-angular distribution at E = 2.8 MeV (fig . 15). The level at E_ = 1899 keV. A y-raywith this energy was observed, but not assigned by Wilenzick et al. 3). In the presentwork, it isproposed as the transition to the ground state from a level at the same energy, which may decay through two other y-rays . The excitation function and the angular distribution of the 1899 keV decay have been studied : calculations for J-values of ~ and ~ have been performed and all seem to be compatible with the experimental data (fig . 16). The level at E_ = 1909 keV. This level must correspond to the 1908 ± 7 keV level observed in ref. s) and to the 1903 ± 5 keV level observed in ref. '). In both these works, the level was assigned -~+ . We attribute two decays (the 1645 and 1629 keV y-rays) to a level at this energy, which can be considered to confirm the suggested spin value. The agreement between the experimental data and the calculations for J = ~ is not good, because of the very small values of the excitation functions, as can be seen in fig. 16 where, besides parity, two values of the branching ratio have also been considered for the 1645 keV y-ray (obviously a branching ratio of 100
"As(n, n'y)"As 1606 ~ 0
â
EI+M2 MI+E2
i ~-QS
i
43E
3,/'1 _ _ I~
1634 617 37Y
~7
L0
44 as E~~ 16061aV YA
E ~ 20 M~V 1.3
1
LO
I
i.S
E (IMV)
1 .0
1,0
7,/i-
/ r
____
i
T
,
.
~"
a4 .4 0
Ei ~1137 4~V
~!_
C " _a75
E i -1634 4~V
.4 0
E, ~ 1037 hV LS E ~ 7 .e AMV
E ~ 2! M~V
as
0
E,~10l7 4~V E ~ 2! M~V
1 .3
3/2~ ~ EI+M7
.4 0
.3 0
3
139
~ 20
T
' ~ - -23
E (AbV)
1 .7 ae
E~ ~ 1003 NV
.o . o ~~. e Fig. 14 . Excitation functions and angular distributions of y-decays from the 1606 and 1654 keV levels. The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes; the dashed curves are calculated for parity opposite to that indicated.
for this y-ray leaves the 1629 keV y-ray unassigned). Consequently very little can be said definitely about this level, but a spin value of ~ can be adopted, because other spin values would yield cross sections too large for these decays . The leoel at Ex = 1988 keV. A level at this energy is proposed for the first time in the present work . The excitation functions of the 1988 and 1518 keV y-decays have been studied and the calculations with J = ~ give curves which are in sufficient agreement with the experimental data (fig. 16). The level at E_ = 2000 keV. A y-ray with this energy was first observed at E = 2.3 MeV and was considered as the transition to the ground state of a new level . Its angular distribution at E = 2.8 MeV has an evident concavity downwards which can be reproduced well by assigning J = ~ to the decaying level ; the comparison between the experimental and calculated excitation function confirms this value for the spin, while the parity remains uncertain (fig. 16). The leoel at E_ = 2009 keV. This also is a level observed for the first time in the present work. The excitation functions of the transition to the ground state and of the 1543 keV y-decay have been studied and a J-value of ~ can be proposed, but the parity remains undetermined (fig. 1 ~.
140
U. ABBONDANNO et al .
OA 0.7
16t3 617
~
o
16!!
]65 0
OA
~_ 4
E, - 16 "3 k.V
E,- 1066 keV
o~
(7/i )
~~~~-
Xy
;~~~-"--
42
1~ ~_
V 3
1.4 1.0 46
E, -16ee k.v
a4 .4 0
E,- 1473 keV LS E, - s.e Mev
~t'
MIrE4 . S " 0 .73 0 E~ 1l79 k.V
7,P7 ;
i79 liro
3~/7 >/i
Ei 1673 k.V
i~
L~
3Vh - /y ~~_!~-~
0.4 .i 0
44
len
20
23 E (/4eVl
2
E~ 1394 k.V ii
E~ 1lf77 keV
cepB
20
E - P.! M.V
1
~y23
E (M.VI
1
cesB
Fig. 15 . Excitation functions and angular. distributions of y-decays from the 1685, 1688 and 1873 keV levels The solid linesare the calculations for the spin, parity and S-values shown in thedecay schemes; thedashed curves are calculated for parity opposite to that indicated.
The level at E_ = 2021 keV A new level is proposed at this energy, which decays through three observed y-rays . The excitation function of the 2021 keV y-decay is consistent with an assignment of both J` _ -}- and ~+ (fig. 17). The level at Ex = 2067 keV. This level must correspond to the 2064 f 4 keV level observed in ref. Z) and to the 2061 ±3 keV level observed in ref. 6). The calculations ofthe excitation function of the 2067 keV y-ray for J` _ ~-, ~+ are both in sufficient agreement with the experimental data, which, however, are very poor (fig . 17). The level at E_ = 2104 keV. This level might correspond to the 2097±4 keV level observed by Moreh and Shahal Z) and assigned J = (~, ~). Wang er al. 6) assign a 2103 keV y-ray to a level at 2572 keV, but our observed threshold is at E = 2.3 MeV. In the present work the level has been attributed two decays, the excitation functions of which have been studied and confirm the J = ~ assignment. Calculations with a positive parity choice are in slightly better agreement with the experimental data (fig . 17). The level at E_ = 2160 keV. We have considered the 2160 keV y-ray as the transition to the ground state from a new level. Calculations for its excitation function have
'SAs(n, n'y)'sAs 1t99~ (~/7 ~!"1
0
aA 47
19/!
a.
7h-
a7
s/7"
!; u991~w
14 1
~"
f
o E,- 19~{ tw
~7 " (i " 4101
__
E c - I31~ 4w 7000 0
1909 QI 47
1/7 0
cm B
a(h-
}
M 47
! - 7.! Mw
I a
-1
E1 "M7 C--Q]4~-
43
779
T
4- Iars tw
~
Ec -70004w
(1/7 :601
(lh ": eo)
E-7. " Mw
7A
23
E (Mw)
LO M Q7
!i- 1a79 Yw ~i~
! (rw) ~.. a Fig. 16 . Excitation functions and angular distributions of y-decays from the 1899,1909,1988 and 2000 keV levels. The solid lines are the calculations for the spin, parity and 8-values shown in the decay schemes or near each curve. For the 1645 keV y-ray, two values for the branching ratio are reported : the calculated excitation functions of the 1629 keV y-ray correspond to the curves calculated for a branching of 60 of the 1645 keV decay.
been performed for J~ _ ~ t and the curve obtained for JR = ~+ is slightly more than the experimental data (fig . 17). The level at Ex = 2176 keV. This must correspond to the 2176 keV level observed in ref. z) and assigned (}, ~) and to the 2180±3 keV level observed in ref. 6). The excitation function of the transition to the ground state has been studied and calculations strongly favour J = ~, but the parity cannot be assigned (fig. 17). The level at Ex = 2228 keV. A level at E_ = 2233 keV (J = ~, ~) was first proposed in ref. 2), which could correspond to the 2228 keV level observed in the present work. We have studied the excitation function of the transition to the ground state and a J = ~ assignment seems the most probable, but JR = ~- cannot be excluded (fig . 17). The level at Ei = 2239 keV. This also could correspond to the 2233 keV level observed in ref. Z) ; we studied the excitation function of the 2239 keV y-decay in order to test their assignment J = ~}, ~ and the best agreement between experimental data and calculations was obtained for J = ~ (fig . 17).
U . ABBONDANNO er rrl.
14 2 2009~~~ J/1'' Q4
46C --~~ 1/i 0
~ r
42
E+ -2009k~V /~ -
Q4
2021-T-
42
0.4 42
0 EY -
Q4 E i -ISIJkoV * 21
42
,
01
i067T
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0 ~ 3/2 -
42
(1/2 ) 2104-~~~- 1/2~1 .1 0 42
44 42
860~ 1/2 + 0
éY -
2160
9/Z
2104 k~V ~
0
EI
Ei - 2160 k~V
1.2
E c - 1243 koV /~
1/~1 _ 3./2
2.2
~~~~ -_
1
.
J/2
(J/'l~ (a/2~ (1/27
E, - 2228 koV 2279 0
7/Z~~ MI+E26 --447 ~2
E ~ - I279 k~V E - 2! M~V l~-~~
~
I
4e
23 44 ~i
(1/2~ (1/2+)
E r -2176koV
0
(3/27 (1/27
2021 k~V
1~2
0 -y- 3/i
2228
23
7/2 -
2176
0.Z
2239-~ 1/~
0
co~9
-1
0 ~ ~2 E c - 2239koV
E (MoV)
E (M~V)
Fig. 17 . Excitation functions and angular distribution of }~-decays from the 2009, 2021, 2067, 2104, 2160, 2176, 2228, 2239 and 2259 keV levels . The solid lines are the calculations for the spin, parity and S-values shown in the decay schemes or near each curve ; the dashed curves are calculated for parity opposite to that indicated.
The lege/ at Ex = 2259 keV. This is the highest level for which CN calculations
have been pt;rformed in the present work. It could correspond to the 2252± 10 keV level observed only in ref. S) ànd tentatively assigned (~-, ~-). We have tested both these values, but they yielded too large cross sections. On the other hand, the excitation functions calculated for J~ = i t are both in sufficient agreement with the experimental data, so that J = ~ can be proposed (fig . 17) . 4. Conclusions
As already stated, the energy levels of'SAs have been investigated up to an excitation energy of 2609 keV : in this energy range we observed S8 levels, twelve ofwhich had not been reported previously . These are levels at Ex = 1101, 1899, 1988, 2000, 2009, 2021, 2147, 2160, 2228, 2239, 2502 and 2508 keV. It is probable that new levels lie also at Ex = 2327, 2358, 2419 and 2609 keV. The observed y-rays totalled 157 in number and, except for a few (EY = 191, 276, 392, 507, 781, 1342, 1355, 1815, 1852,
'sAs(n, n'y)' sAs
143
1924 and2263 keV), they were all identified and enabled us to construct the decay scheme of'SAs shown in fig. 3, where the thick and the broken lines indicate, respeotively, new and uncertain levels . As regards the energy levels lying below 1873 keV, the scheme is similar to that deduced in ref. 4) ; all the y-rays reported, in fact, have been observed also in the present work, except for the transitions having an energy below our instrumental threshold. Besides, we have resolved the ambiguities concerning the assignment of the 893 and 1300 keV y-rays, favouring the levels at 1173 and 1580 keV. As for the 816 keV y-line, the existence of the decaying level seems more probable at 1080 keV than 1095 keV. The decay scheme we propose confirms most of the levels observed in refs. z - 6), as was presented in detail in subsect. 3 .3. The spin and parity of 341evels have been studied by comparing the experimental cross sections with theoretical calculations performed using the statistical model of the compound nucleus. Many uncertain spin-parities have been confirmed. New spin values are proposed for thirteen levels . In some cases the parity value can also be tentatively assigned . References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)
Nucl . Data Sheets 16 (1975) 25 R. Moreh and O. Shahal, Phys . Rev. 188 (1969) 1765 R. M. Wilenzick, V. R. Dave and J. A. Nelson, Phys . Rev. C4 (1971) 2126 W. R. McMurray, P. J. Celliers and R. Saayman, Nucl. Phys . A225 (1974) 37 R. R. Betts, S. Mordechai, D. J. Pullen, B. Rosner and W. Scholz, Nucl . Phys . A230 (1974) 235 C. W. Wang, Y. C. Liu, E. K. Lin, C. C. Hsu and G. C. Kiang, Phys . Rev. C9 (1974) 1396 M. Schrader, H. Reiss, G. Rosner and H. V. Klapdor, Nucl . Phys . A263 (1976) 193 W. Hauser and H. Feshbach, Phys . Rev. 87 (1952) 336 G. R. Satchler, Phys . Rev. 94 (1954) 1304 ; 104 (1956) 1198 E. Sheldon and D. M. Van Patter, Rev. Mod. Phys. 38 (1966) 143 E . Sheldon and R. M. Strang, Comp. Phys . Comet. 1 (1969) 35 U. Abbondanno, F. Demanins, M. R. Melissa and G. Nardelli, Nuovo Cim. Lett . 18 (1977) 97 F. Demanins and G. Nardelli, INFN Report BE-70/3 U. Abbondanno, A. Boiti and F. Demanins, Nucl . Insu . 142 (1977) 605 ; U. Abbondanno, A. Boiti, F. Demanins and M. R. Melissa, Nucl . Insu . 148 (1978) 577 T. Mukoyama, Nucl . Insu. 125 (1975) 289 C. A. Engelbrecht, Nucl . Insu . 93 (1971) 103 D. I. Gerber, L. G. StrSmberg, M. D. Goldberg, D. E. Cullen and V. M. May, Brookhaven National Laboratory report BNL 400 (1970) C. Meeker Davisson and R. D. Evens, Rev. Mod. Phys . 24 (1952) 79 M. Lagonegro and U. Abbondanno, INFN Report BE-73/4 F. D. Becchetti, Jr . and G. W. Greenlees, Phys . Rev. 182 (1969) 1190 E. H. Auerbach and F. G. J. Percy, Brookhaven National Laboratory, report BNL 765 (1962)