Gamma-ray spectra and level structure of 55Cr from thermal neutron capture in 54Cr

Nuclear Physics A187 (1972) 12-20; Not to be reproduced

GAMMA-RAY

by photoprint

SPECTRA

OF =Cr FROM THERMAL

@ North-Holland Publishing Co., Amsterdam

or microfilm without written permission

from the publisher

AND LEVEL STRUCTURE NEUTRON

CAPTURE

IN “0

D. H. WHITE t and R. E. HOWE Lawrence Livermore Laboratory, University of California, Livermore, California 94550 ++ Received 19 November

1971

Abstract: Gamma-ray measurements have been made of the 54Cr(n,y)55Cr reaction at the Livermore reactor. Spectra were taken with Ge(Li) Compton-suppression and pair-coincidence spectrometers. A total of 83 observed y-rays are attributed to capture in 54Cr, of which 26 are assigned to specific transitions among 10 inferred levels in 55Cr. The neutron binding energy is determined to be 6246.3kO.4 keV. E

NUCLEAR

REACTIONS %r(q,y), E = thermal; measured Ev, I,; deduced Q. 55Cr deduced levels. y-branching. Enriched target. Ge(Li) detectors.

1. Introduction The “Cr nucleus should be fairly well described in the shell model as ?r(fg)“v(p+)” configuration based on the 4sCa core. The low-lying negative-parity states should then be due to simple f-p shell excitations. Both theoretical and experimental studies of the isotope are presently quite sparse. Since 55Cr is not accessible by /?-decay, the level structure studies have been limited to studying the 54Cr(d, p)” ‘Cr reaction ’ - ‘). As electromagnetic transitions must therefore be particle-induced, we elected to excite the nucleus via the 54Cr(n, y) “Cr reaction. This allows us to investigate possible correlations with the corresponding (d, p) studies. Capture of thermal neutrons in 54Cr should favor populating lowiying negative-parity + and 3 levels by strong primary El transitions, thus providing a means of selecting simple p-shell neutron excitations. 2. Experimental

procedure

The measurements were taken in the thermal-neutron beam of the Livermore Reactor. Liquid-nitrogen-cooled quartz and bismuth crystals in the beam removed essentially all fast-neutron and y-ray components, producing a thermal flux of 5 x 10’ n/cm’ * set at the target. The target (see table 1) consisted of 1.9 g 54Cr (as Cr,Os) enriched to 94.1 %. With t Present address: Oregon College of Education, Monmouth, Oregon. tt Work performed under the auspices of the US Atomic Energy Commission. 12

5SCr y-RAY SPECTRA

13

TABLE1 Isotopic composition of the enriched f5Cr target Isotope

50 52 53 54

Abundance (at. %)

Thermal cross section (b)

Contribution (%)

0.11 4.01 f0.05 1.79f0.05 94.1 +0.1

17.0 Al.4 0.76icO.06 18.2 f1.5 0.38f0.04

2.3&0.2 4.2 f0.4 44 zt4 49 +6

a cross section of 0.38f0.04 b [ref. “)I, half the y-ray contribution is attributed to capture in 54Cr, with most of the balance attributed to capture in 53Cr. The sample was encapsulated in deuterated polyethylene and centered in a 6Li2H-lined beam pipe, which is flanked on each side with separate Ge(Li)-NaI(T1) spectrometers. These spectrometers are optimized respectively for high- and low-energy y-rays “). The low-energy spectrometer consists of a single 15 cm3 coaxial Ge(Li) diode surrounded by an anticoincidence NaI(T1) annulus. A 113 h run resulted in a 4096channel Compton-suppressed spectrum spanning 300-2500 keV (see fig. 1). The intensity and energy were calibrated by a set of standard sources (IAEA, Vienna), resulting in intensity uncertainties of 5-10 % and energy uncertainties of 10B4 for good peaks. Two calibration runs were taken: one with calibration sources

1

I

400

600

800

1OCQ

1200

ENERGY (koV)

102

/ 1400

/ I600

I 1800

I 2Lw

, 2200

I 2400

ENERGY (keV)

Fig. I. Low-energy anticoincidence spectrum of y-rays from the reaction 54Cr(n, y)s5Cr.

14

D. H. WHITE

AND

R. E. HOWE

TABLE 2 Gamma rays Energy (keV) 104.75*0.1g 211.66f0.12 241.85 ho.06 324.03 ho.05 351.65kO.14 517.68f0.05 565.89hO.05 575.63f0.4 651.05 iO.09 840.93 10.07 846.5110.13 880.74+0.09 908.2710.08 1086.7410.21 1197.23+cO.35 1209.45*0.31 1232.21 ho.11 1283.2110.27 1298.69hO.15 1340.22kO.36 1343.5610.29 1399.86&0.24 2379.4810.15 2408.6 10.4 2444.9010.3 2454.86kO.24 2457.8 2463.9 2576.5 2652.4 2687.0 2697.4 2718.0 2782.0 2894.3 2912.3 2930.6 3019.0 3221.1 3267.1 3351.5 3478.1

10.3 +0.3 ho.7 ho.3 ho.3 kO.3 kO.6 &0.8 kO.3 ho.9 10.3 10.9 JLO.4 11.2 iO.66 10.7

“) Normalized

intensity “) 0.09 0.07 22.1 1.57 0.12 0.70 11.8 0.11 0.09 3.06 0.32 0.65 7.93 0.09 0.59 0.07 0.88 0.07 0.13 0.09 0.12 0.12 1.59 0.08 0.31 0.33 0.59 0.10 0.10 0.83 0.90 0.28 0.08 0.07 1.45 0.07 0.63 0.08 1.86 0.04 2.72 0.09

attributed to the reaction %Zr(n,y)ssCr

Assignment

242566-

0 242

518566-

0 0

8811474-

0 566

1474-

242

268?-

241

28952687-

242 0

2895-

0

c -2895 4044566

to 100 % population

Energy (keV) 1419.55&0.21 1435.6 *0.6 1474.28&0.12 1630.56+0.27 1647.03rtO.24 1660.1 50.5 1664.8750.24 1689.14kO.3 1695.9 ZtO-5 1703.1910.23 1803.65f0.62 1816.4Of0.27 1842.0510.21 1873.3 f0.4 1908.3 rtrO.3 2078.2910.18 2083.7OztO.17 2120.971rO.19 2143.6110.16 2202.00f0.14 2269.4 10.3 2328.66&0.16 3506.0 &to.8 3524.5 fl.6 3548.5 &to.5 3558.6 10.5 3659.5 ho.9 3791.17fl.l 3801.6 +0.6 3977.0 40.6 4044.1 jzo.5 4162.2 10.5 4219.5 kO.8 4466.3 AO.7 4771.8 ho.4 4868.8 kl.0 5047.3 10.7 5421.3 10.5 5586.1 11.3 5594.06+1.3 5680.4 kO.5 6004.4 10.6 6246.4 50.6

Intensity “) 0.11 0.27 6.99 0.10 0.13 0.03 0.08 0.12 0.06 0.18 0.26 0.15 0.21 0.18 0.12 0.41 0.36 0.31 0.39 0.69 0.18 0.52 0.05 0.05 0.90 1.43 0.06 0.05 0.12 0.35 0.31 0.46 0.08 0.15 13.1 0.21 0.11 2.11 0.07 0.06 2.14 13.4 55.5

of the ground state. Uncertainties approximately

Assignment

1474-

0

2269-

566

2084-

242

20842687-

0 3

C -4044 22700 2895566

C

-2687

-2270 C 4&M-0 -2084 C

C

- 1474

c C c-o

-

566 242

10 %.

only, and one including the “Cr spectrum. A total of 49 y-rays were tentatively attributed to “Cr (see table 2), after peaks that were of other origin or so weak as to be

ssCr

y-RAY

1.5

SPECTRA

of dubious existence were excluded. The y-rays resulting from the 3.59 min p- decay of the 5 %r ground state lolll) were too weak to be observed. The high-energy spectrometer consists of a single 6 cm3 Ge(Li) diode flanked on top and bottom by NaI(T1) detectors 10 cmx 15 cm in diam. A 193.6 h pair-coinci-

“I000

1500

2000

2500 ENERGY

ENERGY

Fig. 2. High-energy

pair

coincident

spectrum

3000

3500

4000

(keV)

(keV)

of y-rays

from

the reaction

54Cr(n,y)5sCr.

dence run resulted in a 4096-channel spectrum of second-escape peaks spanning 2-7 MeV (see fig. 2). The y-rays from the 53Cr(n, y)54Cr reaction 12) were used to calibrate the energy and intensity. A total of 48 peaks attributed to 5%r were observed in this interval. 3. Results 3.1.

ENERGY

LEVELS

We assigned the y-rays to a proposed level scheme, using the Ritz combination principle and guided by the levels observed in (d, p) studies. Our assignments, in most cases, required observing direct population from the capture state. Since the neutron binding energy is quite low in 55Cr, primary transitions to levels above 2 MeV cannot be clearly distinguished by the usual criterion of their high energy. It is therefore likely that several additional levels from 2-4 MeV that were observed by (d, p) reactions are weakly populated by (n, y). A total of 26 transitions have been placed, accounting for 88 % of the observed strength. The assigned transition energies were adjusted to the proposed level scheme by a linear least-squares program, LEVEL 13), that takes account of the nuclear recoil. Precise values for the energy levels (see fig. 3) are determined with a goodness of fit (X*/degrees of freedom) of 0.40.

16

D. H. WHITE

AND

R. E. HOWE

3.2. INTENSITIES

An s-wave interacting with the 54Cr O+ ground state will capture into +’ states in “Cr. Primary El transitions will therefore populate f- and 3- low-lying states. The intensity depopulating the capture state is found to account for about 90 % of the ground state intensity (primarily the 241.85, 565.89 and 1474.28 keV transitions).

P4C,+nl---r--r

--r---a----r--6246.3+.0.4

1

'

'

lo9r!ae

'

'

'

'

'

__

'

l/2+

2269.23i0.24 2083.83ti.18

(l/2

1474.22M.06

1/2-

~~~~-~-880.75iO.O9 565.9OiO.04 1 t t

4 I

1 ! tt

517.68i0.05 (5/2' 241.86ztO.04 I/f 0

556 24

(5/2,7/2)3/2-

‘1

3/2keV

31

uli

Fig. 3. Level scheme of 5sCr.

Moreover, most of the low-lying levels have more intensity going out than coming in. We conclude that the 10 % missing primary transition strength is distributed among a number of unresolved levels above 3 MeV. 3.3. CORRELATIONS

The (n, y) reactions have often shown a high correlation between the 2, = 1 (d, p) strengths Gdp = (2J+ I)&, and the (n, r) strengths G,, = I/E3 for primary y-rays leading to the same final states. Comparing our observed transitions to levels below 3 MeV with corresponding I,, = 1 (d, p) strengths of Bock et al. “) we obtain a correlation coefficient

[F(G,,i - %I” c(Gdpi - (3,,J21* = o’96* &

This correlation suggests the presence of direct capture in the reaction 43Ca(n, y)‘“Ca, although recent studies indicate such correlation may be attributed to a small number of common doorway states in the reaction channel I”).

s5Cr y-RAY 3.4. NEUTRON

BINDING

SPECTRA

17

ENERGY

The neutron binding energy is found to be 6246.3k0.4 keV, based on the leastsquares output. This value is in good agreement with the value 6245k8 keV in the 1966 nuclear reaction table I’). 3.5. THERMAL

NEUTRON

CROSS SECTION

Since the amounts of both 53Cr and 54Cr in the sample were known, it was possible to determine the thermal neutron cross section of 54Cr based upon that of 53Cr. The relative yields from ‘jCr and “Cr were calculated using the 54Cr decay scheme of White et al. 12). Taking the 53Cr thermal neutron cross section to be 18.2+ 1.5 b as reported by Pomerance r “) [adjusted for the more recent ’ 9 ‘AU cross section “)I, we obtained a value of 0.34f0.04 b for the thermal neutron cross section of 54Cr. This number compares reasonably well with the adjusted “) value 0.38f0.04 b reported by Bazorgan et al. “). TABLE 3

Energy levels in 55Cr

(n, Y)

present data (keV) 0 241.86&0.04 517.68fO.05 565.90&0.04 880.75 *0.09

1474.22hO.06

2083.83kO.18 2269.23 *0.24

2686.99hO.22

(4 P)

(d. P) MacGregor and Brown b,

Bock et al. “)

(keW

+W 0 1 241 S) 1 517 3 564 1 879 3 1213

3

1470

1

2008 2078 2260 2320 2545 2596 2679

2 4 0 3 3 1

(d, P) (d, P) Bjerregaard Singletary and Miller d, et al. ‘) (keV) WV)

0

245 *) 524 573 893 920 1229 1418 1487

2031 2098 2283 2341 2570 2622 2695 2710

0 246 ‘) 522 583 881

0 248 h, 523 574 890

1486 1656 1775 1979 2023 2093 2275

“) s, ‘) ‘)

2886

1

Ref. 4). b, Ref. I). l lO keV for all levels. f8 keV for all levels. f 15 keV for all levels. J) &35 keV for all levels.

(d, p) Rosalky et a/. ‘) (spin)

0

8

230 ‘)

3

600

$

1206

930 1200

1474

1520

2020

2000 2090

4

2268 2310 2480

2779 2894.52AO.20

(d, P) Bochin et al. ‘) (keW

2790

2874 2905 ‘) Ref. 3).

“) Ref. 2).

‘) Ref. 7).

‘) Ref. 5).

18

D. H. WHITE AND R. E. HOWE

4. Discussion The levels in 55Cr have previously been studied almost entirely by the (d, p) reaction ‘- ‘). These workers’ results up to 3 MeV are compared with the present results in table 3. The large correlation between the (n, y) and (d, p) reactions is due in part to the strong direct population of the ground state in both reactions. The ground state spin $- is inferred from the logft value 4.95 for the 3.5 min p-decay to the 3- ’ 5Mn ground by the j-dependence of I = 1 (d, p) stripstate ‘*). Th e sp in is further corroborated ping ‘). The extreme single-particle model for 54Cr has a ground state primarily of the (lfg)4(2ps)2 configuration, with some (lf+)’ [ref. “)I and (2p+)’ [ref. “)I neutron admixture. One would thus expect the ’ 5Cr ground state to be primarily a 2~~ state. The 242 keV level has spin a- as determined by thej-dependence of the I = 1 (d, p) reaction “) and a negative parity would be consistent with strong El population from the capture state. The 518 keV level is known to exist from (d, p) reactions ’ - “) with 1 = 3 character “). The spin is therefore 3, z’ -. Since such a level should strongly favor populating only the ground state, we have therefore tentatively assigned the 517.68 +0.05 keV y-ray to depopulate this level. As expected, no direct population from the capture state is observed. In fact, no population is observed at all, although it is not unlikely that it would be weakly populated from one or more intermediate energy levels. The 556 keV level is weakly populated from the capture state, but strongly populated from the 1474 keV $- state. This state has $- spin as determined from j-dependent I = 1 (d, p) reactions by Rosalky et al. “). It populates primarily the ground state, with a weak branch to the 242 keV level. A level has been observed at 880 keV by various (d, p) studies le4) that has an I = 3 angular distribution. Its spin is therefore limited to (3, 3)-. Although there is no observed (n, 7) population, we find a y-ray at 880.74kO.15 keV that is tentatively attributed to its decay. The 1474 keV level is strongly populated in the present study. The resonance in the as the analog state in elastic scattering of protons from 54Cr has been interpreted 5 ‘Mn of the 1474 keV ’ 5Cr state “). Its +- spin is consistent with the + value deduced from j-dependence of 1 = 1 (d, p) stripping by Rosalky et al. ‘). It depopulates to both low-lying $- states, with a weak branch to the 242 keV &- state. The 4162.2 keV y-ray is assumed to directly populate the level at 2090 keV ‘, ‘, 4*‘). Moreover, this state is assumed to decay with appropriate intensity to the ground state and probably also to the 242 keV state. Although a spin of 5,s is therefore indicated, Bock et al. find an I = 4 angular dependence to the stripping reaction, indicating a 3 e+ state “). It is therefore likely that more than one level is involved. If the order o;ihe 4162.2 and 2083.7 keV transitions are reversed, this would imply a level at 4162 keV, consistent with a level observed at 4.18 MeV by the (d, p) reaction “). The 1842 keV transition could also bz assigned to its decay to a level at 2320 keV [also observed

sSCr y-RAY

SPECTRA

19

by (d, p) “)I, which finally could decay to the 241 keV level by the observed 2078.29 keV y-ray. However, the intensity balance is rather poor, and the lack of ground state population by the 2320 keV level would be hard to explain. The correct placement of these transitions remains an open question. The 3977.0 keV y-ray is assigned to populate the 2269 keV level ’ - 4), which is found to decay with the appropriate intensity to lower +- states. As Bock et al. find an I = 0 (d, p) stripping pattern, the spin of that state must be ++, corresponding to a (Ml, E2) transition from the capture state. The 2687 and 2894 keV levels are both populated from the capture state, and both decay to the ground and 242 keV state. The 2894 keV state also decays to the 566 keV state. Rosalky et al. “) find a preference for aj = -$fit for the I = 1 (d, p) distributions for both levels, consistent with our results. This spin is further eorroborated for the 2894 keV level by sum rule assignments ‘). The 2202 keV y-ray is tentatively assigned to populate the level observed at 4044 keV lm4). This level depopulates to the low-lying $- states, indicating a probable spin of +,3. Other levels have been observed by (d, p) stripping, such as at 1215 keV. However, their I = 3 angular distribution indicates a spin high enough to preclude direct capture state population. Moreover, these states are not observed to decay to other lowlying states, indicating that they are not appreciably populated from higher intermediate energy levels. A large number of weak y-rays are observed, which have not been assigned. Although a few Ritz combinations can be found that correspond to observed (d, p) levels within their uncertainties, such assignments are felt to be too unreliable to be included. 5. Conclusions

The capture-state decay is found to preferentially populate low-lying 2p., and 2p, neutron states, as corroborated by the high correlation with I = 1 (d, p) strengths. It therefore seems reasonable that the capture mechanism is mediated by a small number of doorway states (perhaps one). It would be very useful to be able to compare the 5‘Cr experimental results with an appropriate shelf-model calculation, where the ground state is described by the (lf~)4(2p~)3 configuration, with appropriate 2p+ and lf& mixing. The authors are indebted to M. C. Gregory for helpful discussions. References 1) A. MacGregor and G. Brown, Nucl. Phys. 88 (1966) 385 2) L. D. Singletary and M. R. Miller, Phys. Rev. X38 (1965) B569 3) J. H. Bjerregaard, P. F. Dahl, 0. Hansen and G. Sidenius, Nucl. Phys. 51 (1964) 641

20 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18)

D. H. WHITE

AND R. E. HOWE

R. Bock, H. H. Duhm, S. Martin, R. Rude1 and R. Stock, Nucl. Phys. 72 (1965) 273 D. M. Rosalky, D. J. Baugh, J. Nurzynski and B. A. Robson, Nucl. Phys. Al42 (1970) 469 G. Brown, A. Den&g and A. E. MacGregor, Nucl. Phys. Al53 (1970) 145 V. P. Bochin, K. I. Zherebtsova, V. S. Zolotarev, V. A. Komorov and L. V. Kra~nov, NucI. Phys. 51 (1964) 161 G. A. Bazorgan, J. W. Irvine, Jr. and C. D. Coryell, Phys. Rev. 95 (1954) 781 D. H. White and R. E. Birkett, Nucl. instr. 73 (1969) 260 John C. Hill, Nuel. Phys. Al!@ (1970) 89 W. H. Zoller and W. B. Walters, J. Inorg. Nuci. Chem. 32 (1970) 246.5 D. H. White, D. J. Groves and R. E. Birkett, Nucl. Instr. 66 (1968) 70 D. H. White, R. E. Birkett and T. Thomson, Nucl. Instr. 77 (1969) 261 A. M. Lane, Phys. Lett. 31B (1970) 344 J. C. Maples, G. W. Goth and J. Cemy, Nucl. Data 2 (1966) 429 H. Pomerance, Phys. Rev. 88 (1952) 412 Brookhaven National Laboratory report BNL-325, 2nd ed., 1958 K. Way el al., Nucl. Data B3 (3,4) (1970) 8