The reaction 35Cl(n,γ)36Cl studied with non-polarized and polarized thermal neutrons

I.E.I

[

Nuclear Physics A264 (1976) 6 3 - 9 2 ;

~ ) North-HollandPublishing Co., Amsterdam

/',lot to be reproduced by photoprint or mierof-dm without written permission from the publisher

THE REACTION 3SCl(n, 3,)3~C1 STUDIED WITH NON-POLARIZED AND POLARIZED THERMAL NEUTRONS A. M. J. SPITS and J. KOPECKS"

FOM-RCN Nuclear Structure Group, Reactor Centrum Nederland, Petten, The Netherlands Received 15 December 1975 Abstract :The y-radiation following capture of non-polarized and polarized thermal neutrons in 35El has

been investigated. Of the 420 j-rays ascribed to the 35C1(n, 7)36C1 reaction, 236 have been placed in a 36C1 decay scheme. The branching ratios and the excitation energies (with 0.04-0.9 keV errors) of 72 bound states have been determined. Unambiguous spin assignments are given for 11 levels. The multipole mixing ratios for some primary 7-ray transitions have been determined. There exists a significant correlation between (d, p) stripping strengths and (n, y) reduced primary transition probabilities for transitions to In(d, p) = 0 levels.

El

NUCLEAR REACTIONS 35C1(i~,y), 35C1(n, y), E = thermal; measured Er, It, y-CP; deduced Q, polarization function R. 36C1levels deduced j-branching, J, 7z, 6. Natural targets.

I

[

I

I. Introduction

Capture in 35C1 of non-polarized thermal neutrons has been studied by means of Ge(Li) detectors by many authors 1-7). The picture obtained by the complicated v-ray capture spectrum, however, still lacks consistency on many points. In the present work an attempt has been made to improve the completeness, accuracy and precision of the data. Much care was devoted to achieve accurate energy and intensity calibrations. The systematic uncertainty in the energies could be reduced by calibration on 14N(n, V)15N v-rays; the 14N-tSN mass difference was recently established with a high degree of accuracy 8). Much information of spins of bound levels, and in favourable cases on their parities, the multipole admixture of primary 7-rays and the admixture of the lowestspin component in the capture state may be obtained by using a polarized thermal neutron beam. Such experiments have been performed earlier for capture in chlorine 9, 10).The present paper describes another experiment of this type, making use of the high polarized-neutron intensity available from the Petten magnetized mirror set-up. A description of this set-up may be found in ref. ~1). This paper is part of a series of two articles on thermal-neutron capture in the two stable chlorine isotopes, the first of which appeared in 1973 [ref. ~2)]. 63

64

A. M. J. SPITS A N D J. K O P E C K Y

2. Circular polarization of capture 7-rays The circular polarization of ~'-radiation after capture of polarized neutrons is given by the relation Pr = R(Jt, ,If, or, 5)P, cos 0, where P, is the neutron polarization and 0 is the angle between the directions of the 7-ray emission and the neutron spin. The polarization function R depends on the target and final-state spins Jt and Jf, respectively, on the mixing parameter ~ of the two coherently interfering spins J e = J r - ½ of the capture state (where ~ depends on the primary ~-ray transition considered), and on the ~-ray multipole mixing ratio ft. Further it is assumed ~~), that if (as in the present case) the capture state has even parity, the primary transitions to odd-parity levels have pure E1 character. The left-hand plot of fig. 1 then applies, where R is displayed as a function of for fi = 0. For the even-parity final levels the dependence of R on the multipole mixing 5 has to be taken into account. The middle and right-hand plots of fig. 1 show R as a function of ~ for 0t = 0 (J¢ = 1) and ~ = 1 (Jo = 2). Since the thermal-neutron capture cross section of the 35C1(n, T) reaction is dominated by capture via one bound state just below the neutron threshold t3), with J~ = 2 ÷, the contribution of the J~ = 1 ÷ component in the capture state is very small such that ~ may be assumed to be close to unity. On this assumption the theoretical values of R for pure dipole transitions have the values R = 0.75, 0.25 and - 0.50 for Jf = 1, 2 and 3, respectively. By a measurement

'°I

Jt =3/2 1.0 .1=1

0.5

l

\\

R

x.N\\\

0 - ----

-0.5

J,=.~

=0

=0

.1f=1

~>0 ~<0

. . . . . . 4;, - 0.~

.//

/

/

/

//"J,=3

0 J==l

-1.0

-1.0

-1.0 J==2

~,=o

5=0

d~=l

Fig. 1. The polarization R for a J~argct= 5+2 nucleus as a function ofct and Jf for negative-parity final levels (left-hand side figure; 6 = 0) and as a function of 52/(1 +62) and Jf for positive-parity levels (middle and right-hand side figures; ct = 0 and 1, respectively). The fraction of the J" = 2 + component o f the capture state is denoted by ct, the admixture o f the quadrupole strength in a primary transition by 62/(1 + 62). For 36C1, ~ may be assumed to be close to unity.

35Cl(n' ~,)36C1

65

with a sufficient degree o f accuracy, one thus can assign unambiguous spin values to negative-parity levels. For the lower-lying positive-parity levels, the possible multipole admixtures in both the primary and secondary y-ray transitions complicate the analysis. However, by combining the results o f circular-polarization measurements with those o f y-y angular-correlation 14) and/or y-ray linear-polarization 15) work, unambiguous spin assignments may be obtained, as shown by H o n ~ t k o et al. 15), who thus were able to establish the spins of the first and second excited states of 36C1. For secondary transitions the polarization function R can easily be calculated, if the intermediate level has negative parity. The two transitions involved then may be assumed to have pure E1 character. For example, for a 2 + --, J~ --, 2 + cascade ending on the ground-state one has for the secondary transition R = - 0 . 3 7 , 0.21 and 0.67 for J~ = 1-, 2 - and 3 - , respectively.

3. Experimental procedure 3.1. ACCUMULATION OF THE SPECTRA

3.1.1. Singles spectra. Part of the experimental conditions during the recording of the singles 35Cl(n, 7) spectra have already been described in ref. 12). It may be summarized that samples of about 100 mg of lead chloride o f natural composition enveloped in teflon (C2,F4~) tubes, as well as empty teflon target holders, were exposed to thermal-neutron irradiation in an external radial beam hole o f the Petten high-flux reactor. Spectra were accumulated in three energy ranges: low energy (60-800 keV), medium energy (0.2-2.5 MeV) and high energy (1-9 MeV). For accurate energy calibrationin the low-energy range additional spectra were recorded together with the 7-rays from radioactive sources and the capture y-rays from samples of chromium oxide and iron mixed with lead chloride. In the high-energy range also one run with a sample containing a mixture of lead chloride and ureum was performed. 3.1.2. Pair spectrum. Two 7.6 cm x 7.6 cm NaI crystals were used in combination with a 40 cm 3 Ge(Li) detector to function as a pair spectrometer system. A 100-600 keV window was set on the NaI spectra to suppress as much as possible the incident 7-radiation due to scattering; the resolving time for the two coincidence units was 2z = 50 ns. These rather broad windows on energy and time were justified by the fact that only triple coincidences were recorded. In a total measuring period of seven days a spectrum was accumulated with a counting rate of 15 s-1 for the coincident pulses, o f which 10 ~ was due to random events. The resolution for this spectrum, to be referred to as "pair spectrum", varied from 4.9 keV at E~ = 2.0 MeV to 9.8 keY at E~ = 8.6 MeV. 3.1.3. Circular-polarization spectra. A full description of the polarized-neutron set-up can be found in ref. 11). In the present experiment, 15 g o f PbC12 stored in a

66

A. M. J. SPITS AND J. KOPECKY

teflon holder was irradiated by thermal neutrons, polarized to (90 +_5)~o, with a flux of about 3 x 10v cm -2 • s -1. The circularly polarized capture y-rays after transmission through a permendur polarimeter were detected by a Ge(Li) detector which had an 11 ~o relative efficiency. The two spectra, accumulated with opposite neutron spin directions were routed to different halves of the memory of a Laben analyzer. Spectra have been accumulated with E~ = 1.8-5 and 4-9 MeV, with measuring times of 7 and 15 d, respectively. 3.2. ANALYSIS OF THE SPECTRA

3.2.1. Singles spectra. The background corrected spectrum in the energy range E~ = 0.28-2.25 MeV is shown in fig. 2. Peaks are identified by their energies in keV, or, for double- or single-escape peaks, by the symbols " d " or "s". Residual background peaks are marked with a "b", or, more specifically, by"38Cl" if they originate from capture in 37C1, whereas the broad peak near E~ = 480 keV labelled "TLi" stems from the reaction l°B(n, ct)TLi. The spectra in all energy ranges were analysed by means of a program ~6) which fits asymmetric Gaussian functions superimposed on a smoothly varying background to the data points. For further details about the analytical procedure and the energy and intensity calibration, see ref. 12). 3.2.2. Pair spectrum. The pair spectrum was corrected for random coincidences by subtracting a spectrum generated by random events only. The residual singleescape peaks still showed up in a ratio of 1:40 relative to their concomitant doubleescape peaks. They were almost completely eliminated from the spectrum by subtracting 2.5 ~ of the original spectrum shifted upwards by 511 keV. For illustration a part of the corrected spectrum is displayed in fig. 3. The spectrum was contaminated to a slight degree by background radiation, originating mainly from capture in Fe, H, F, A1, Pb, Cr and Li in amounts of 3', 2, 0.7, 0.7, 0.2, 0.15 and 0.15 ~ , respectively. This contamination could be accounted for. The pair spectrum was analysed in essentially the same way as the singles spectra, with the understanding that the energy calibration curve was constructed with a high degree of internal consistency by using, besides the calibration 7-rays from capture in H, Fe, Pb and A1, also the relationships between peak positions following from the many well-established two- and three-step cascades in the 36C1decay. In order to reduce the systematic error possibly resulting from a timing mismatch between medium- and high-energy transitions in the pair spectrum, and also to match low- and high-energy 36C1 y-ray intensities, the efficiency curve for the peaks in the pair spectrum was co-determined by the intensities of some strong peaks in the singles spectra. 3.2.3. Circular-polarization spectra. The data processing technique to determine asymmetries and convert these into R-values is described in ref. 11). For the absolute

°

1626

b

j

-~b

l

l

1680

1640 • 1648

1000

]

d d

I

b

17~o

~

1710 1731

I

I

~

5OO

i

l

m28

817

833

b

12~50

I

5e3

d

4

d

•

d -

"l ¢" J

I

1235

d

d

J

623

d

b

lit~

703

_d

d

~

L

~ 1426

2038d

1373

+ 1366 d

2000

I

I

A

1327

~61

d

I

633

+s 2110

J

2132

dd

1462

-

812

d

801

2179 2200 +d

b

- F_. in k e V

21;

1500

1496 • d

~

869

1

864

Fig. 2. Singles spectrum of the reaction asCl(n, 7) covering the low-energy range. The spectrum has been corrected for background radiation. Full-energy peaks are labelled with their energies in keV, whereas double- and single-escape peaks are indicated by the symbols d and s, respectively. Residual background peaks are labelled with the symbol b or, more specifically, with the symbol of the corresponding final nucleus.

2xX)'

oL

2 x 10", --.

lO'

d~i~l

913

93?

d

I

I

I

~

I

38~

3750

3775

~

3g01

41~4

L

I

I

~

4140

i

I

I

I

4441

L

i

L

1

I

430O

44OO

45OO

46O0

47O0

I 4945

114

4962

4990

5018

C 161

5.~05

8248

i t

J

I

l

49OO

E), in keY

nple of the high-energy part of the spectrum of the reaction 35C1(n, 7) taken in the pair escape mode. The spectrum has been c, i coincidences and single-escape peaks (see text). Peaks are labelled with the ~-ray energies in keV if the corresponding intensity exceeds

~

m

~.

.2o

|

0

i

5111+66ZO~661if

'v

4729'

4996,

,

69711'

4441.4945

3822

I

~

8820+8618

4 3

~ ,,.,~.,.:~.

.

7781

,.,.

•

;

.

~

7

I

'

, 5518'

4960,

g.,~B

+

7415'

~

.

4617

~

•

IIt 4960

7791'

•-

7415

'

F

V

Ii

t/

.

.

.

I

8

V

'

, tl',

i

,

t

,

5518 I ~ i

' ~ .

I .

.

E ~,

61166620-6628 5715

:x 2.5

.

.

5903'

, ,

.

5057','~t

.

57.48

.

Ix 2.5

I

]

5716.

'

A

-~\

I

5903 6978'

8519

,..,,.~...,..~

in M e V

~ u ~ ~-,~'~,~*"~'~"-"

m7','~~

t

'': tt , tl:

4729+5248 ' 5903'4945 5518'I

~,'

,.,.~,.~,,r ,,,.,~~.'~.:."~¢~'~¢.,..~~ .~,,,.~.,~, ~ . ~ . ~ ' - ,

,

Fig. 4. Circular-polarization spectrum of the reaction 35C1(n, y). The two spectra relating to the two opposite neutron spin directions are displayed by their sum and difference. The peaks are labelled with unprimed, primed and doubly primed energies in keV, relating to full-energy, single-escape and double-escape peaks, respectively.

lOL

2 x 10 I-

- 2 x 1 0 4~

°-

2xlO4

o

#-

2x1~

. 5X10414063'4617

t

0 "-~~ " ~ : ~ w " ~ , ~ " v " ~ ' ~ "

3X'I04]-~

70

A. M. J. SPITS A N D J. K O P E C K Y

calibration of the sensitivity response curve of the polarimeter the E~ = 5.42 MeV E 1 transition in the a2S(n, 7) reaction was used, which has R = -0.50 [ref. ~')]. The spectra of the circularly polarized 7-radiation from 36C1 are displayed in fig. 4. For each case the difference and the sum of the two spectra, accumulated for opposite neutron spin directions, are presented.

4. Energy calibration The set of calibration energies of 7-rays originating both from radioactive sources and neutron capture in contaminants, which was used for the analysis of singles and pair spectra may be found in ref. 12). The lead chloride-ureum spectrum provided a set of intense 'SN capture 7-rays, which yielded for the '4N(n, 7) reaction Q = 10833.23 keV with a statistical error of 0.09 keV. This value, together with previously measured, less precise, Q(14N) values, viz. Q(14N) = 10833.1 +0.4 keV [ref. 17)] and Q(, 4N) = 10833.2___0.2 keV [ref. , 8)] (here the errors again are purely statistical), can be compared with the value of Q('4N) = 10833.40___0.03 keV reported in an article of Smith and Wapstra 8). Here a set of highly precise Q-values of thermal-neutron capture reactions with errors of 30 eV is presented, as derived from atomic-mass measurements with a mass spectrometer. The (systematic) uncertainty of 30 eV inherent to these values is mainly determined by the uncertainty in the massenergy conversion. The Petten values, though they differ by at most 0.3 keV with the values ofref. 8), are systematically lower and far less precise. It therefore seemed appropriate to convert the Petten energies to the "atomic-mass standard;' by enlarging them with 16 ppm. This precise calibration has an accuracy which is mainly determined by the above statistical error of 0.09 keV, equivalent to 8 ppm, in the Petten Q(~4N) value. To allow also for the 0.03 keV uncertainty in the mass-energy conversion and other possible sources of systematic errors, a total systematic uncertainty of 20 ppm has been adopted, which replaces the systematic uncertainty of 60-70 ppm previously adhered to 12. , 7). Previously measured energies as those given in refs. 12, 17.18) may be enlarged TABLE 1 Energies of some intense y-lines resulting from thermal-neutron capture in nitrogen

Er + Er (keV) ")

Er + E~(keV) ~)

3532.77_ 0.07 3678.39+0.06 5270.13 4- 0.07 5299.20 _ 0.29 5534.21 4-0.19

5563.26_ 0.08 6324.5 +0.5 7300.54 4- 0.28 10833.49 4- 0.22

a) The recoil energy is denoted by E r. The energy calibration is chosen to reproduce the 14N(n, y) reaction Q-value of 10833.40 keV given in ref. 8).

35C1(n,),)a6CI

71

with the same amount of 16 ppm to yield energies adjusted to ref. 8). In such a way e.g. from ref. 17) a Q(12C) value of 4946.3_ 0.2 keV results, which may be compared with the value given in ref. 8), Q(lZC) = 4946.43___0.03 keV. The (recoil corrected) energies of the above y-rays following thermal-neutron capture in 14N are presented in table 1.

5. Results

5.1. SINGLES AND PAIR SPECTRA Gamma-ray energies and intensities in the high-energy range Er = 2.5-9 MeV were derived from the pair spectrum whereas transitions up to E --- 2.2 MeV followed from the analysis of the singles spectra. In the overlap range E~ = 2.2-2.5 MeV the results of the singles and pair spectra were combined to yield final energies and intensities. The complete list of y-rays following thermal-neutron capture in 35C1 is given in table 2. The intensities have been normalized by means of the relation ~_,i(Er + Er)ilr~ = 100 Q, in which equation the sum extends over the recoil corrected energies times intensities of the complete set of ~-rays, and Q stands for the reaction Q-value. This method is justified if there are reasons to believe that the omission of the unobserved weaker transitions does not substantially influence the result. For the present case this missing part is estimated at a few per cent only. On the whole, the 7-ray energies of the present work, though more precise, agree well with those of ref. 4), whereas a systematic difference of about 100 ppm exists with those recommended in ref. 7) as calibration standard. The discrepancy of about 300 ppm with the work of ref. 3) is even more',serious, such that one should conclude that systematic errors exist in the older work. It is for the present work only that full consistency can be claimed with the aforementioned highly precise Q(12C) value of ref. 8). As to the intensities, the best agreement is achieved with the work of ref. 5), in which an absolute intensity calibration is performed by comparison of the 36C1 y-rays with the I v = 100% ground-state transition in 2°8pb. Between the two sets of intensities a small systematic discrepancy is discernible, with the values of ref. s) about 10 % higher than those from the present work. On the basis of table 2 and the set of known 36C1 levels reviewed in ref. 19) the construction of a 36C1 decay scheme was undertaken in the way as described in ref. 12), i.e. (i) the majority of the levels claimed as being observed in the (n, y) reaction agrees within the errors with those from ref. 19), (ii) their excitation energies, however, exhibit a far greater precision, because the information contained in the energies of all y-rays to which a position in the decay scheme has been assigned yields precise excitation energies via the least-squares calculation described in ref. 12). In the intermediate stage these precisely determined excitation energies (0.04-0.9 keV

72

A. M. J. SPITS A N D J. KOPECKY Table 2

Gamut r a y s f r o m t h e 35C1(u,7)36C1 r e a c t i o n

Iy b)

Ey+Era) (keV)

I n t e r p r e t a t i o n c)

(~x

in k,v)

Ey+Er a)

ITb)

(keV)

I n t a r p r e c a t i o n c) (E x i n keV)

292.27+0.09

0 . 2 6 +0.03

2810 ÷ 2518

1730.9 +._0.2d)

0.23+0.11

3332 ÷ 1601

337.52+0.07

0.07 +0.02

3332 ÷ 2994

1743.3_+0.2

0,24+0,02

6288 ~ 4525

358.29+0.06

0.19 +0.03

1959 ÷ 1601

1786.38+0.19

0.22+0.09

427.70+0.07

0.022+0.002

436.13+0.02

0,86 +0.07 0.09 + 0 . 0 2

468.3 + 0 . 6 516.73+0,08

22.7

+0,9

532.77+0.08

0.09 +0.02

593,2 +0.2

0.I0 +0.02

1788.36+0.14

0.28+0.11

1601 ÷ 1165

1806.41+0.10

0.15+0.02

3332 ÷ 2864

1828.40+_0.08

O. 35+0.02

2468 + 1951

1840.4 +_0.2

0.71+0.18

2492 ÷ 1959

1846.1 +_0.5

0.19+0.03

1857.3 +_0.3

0.27+0.02

4315 ÷ 2468

623.2 +0,3

0.021+0.005

3724 + 3101

1861.3 +0.4

0.13+0.02

632.75+0.09

0.32 +0.02

3101 + 2468

1870.6 +0.3

0.06+0.02

703.05+0.06

0. I0 +0.02

3599 + 2896

1936.90+_0.13

786.27+0.05

9.6

+0.5

1951 ÷ 1165

1950.99+_0.05

18.7 +0.4

1951 ÷

0

788.41+0.05

15.0

+0,3

1959.19~0.05

12. I +0.4

1959 ÷

0

788 ÷

0

0.48+0.03

C ÷ 6642

812.44+0,13

0,044+0,005

1601 ÷

788

1995,59~0,12

0.32+0.02

5205 + 3209

859.49+0.10

0.11 + 0 , 0 2

2810 ÷ 1951

2003.39+0.10

0.20+0.02

3963 ÷ 1959

863.90+0.09

0.14 +0.02

3332 + 2468

2011.27+0.15

0.11+0.02

3963 ~ 1951

890.9 +0.2

0.07 +0.02 0.09 +0.02

2492 + 1601

2021.85+0.06

0.45+0.02

2810 ~

3724 ÷ 2810

2034.42+_0.09

0.60+_0.03

C ÷ 6545 3997 ÷ 1959

913.41+0.11 936.9 +0.2

788

0.50 + 0 , 1 5

2896 ÷ 1959

2038.0~0,3

0.52+0.09

944.97+0.18

0,08 +0,02

2896 + 1951

2044.2+_0.2

0.17+0.02

3209 ÷ 1165

1035.32+_0.09

0 . 0 9 +0.02

2994 ÷ 1959

2075.22+_0.06

0.75+0.03

2864 +

1043.32+0.15

0.11 +0.02

2994 + 1951

2092.3 +0.2

0.35+0.06

1067.0 +0.2

0 . 0 6 +0.02

3963 ÷ 2896

2110.3 +0.2

0.19+0.06

1131.16+0.04

1.58 +0.09

3599 ~ 2468

2131.7 +_0.3

0.14+0.02

1162.6~0.3

1.3

+0.6

1951 +

788

2156.08~0.07

0.63+0.03

25.7

+0.8

1164.74+0.04

788

C ÷ 6487

5463 ÷ 3332

1165 ÷

0

2179.48+0.15

0.38+0.02

4139 ÷ 1959

1170.85+0.05

0 . 4 6 +0.03

1959 +

788

2200.00+_0.08

0,37+0,02

C + 6379

1234.8 + 0 , 2

0 . 0 6 +._0,02

5550 + 4315

2234.8 +_0.4

0,21+0,04

C ÷ 6345

1327,37+0,05

1.13 + 0 , 0 6

2492 + 1165

2239.7 ! 0 . 4

0.19+0.04

1356,3 + 0 , 3

0,06 +0,02

5018 + 3661

2246.3 +0.4

0.17+0.05

1372.62+0.05

0,33 +0,03

3332 + 1959

2253.0 ! 0 . 5

0,38+0,08

1425.61+0.18

0,07 +0,02

4758 + 3332

2259.5 ~ 1 . 0

0.17+0.05

6090 ÷ 3830

1446.0 +0.4

0.14 + 0 , 0 6

5972 + 4525

2266.0 +0.7

0.23+0.03

4758 + 2492

1462.45+0.18

0 . 0 6 +0.02

2275.4 ~ 1 . 4

0,18!0,08

1496.46+0.08

0 , 1 5 +0,02

2282.4 +1.4

0.20+0.08

1600.86!0.05

3.43 +0.17

2288.9~+0.9

0.34+0.09

1605.8 +0.2

0,09 +0,05

2294.7 ~ 1 . 8

0.23+0.08

1626.1 +0.3

0.32 +0,05

1628.8 +0.4

0 . 2 3 +0.04

C + 6951

1601 +

0

2311.14!0.16

1.09+0.05

2319.0 +0.6

0.22+_0.04

1o.4

5578 ÷ 3332

4758 ÷ 2468 C ÷ 6268

1639,83+0,07

0.51 +0.02

3599 + 1959

2326.1

0,40+0.04

4844 + 2518

1648.01+0.07

0.53 +0.02

3599 + 1951

2368.7 ~ 0 . 6

0.31+_0.06

5578 ÷ 3209

1679.8 + 0 , 2

0.21 +0.02

2468 +

788

2381.6 +_0.7

0.27+0.07

5246 ÷ 2864

1684.42+0.17

0.27 +0.02

3635 ~ 1951

2391.9 1o.8

0.26+0.06

1710.1 + 0 , 2

0.31 + 0 , 0 3

3661 + 1951

2418.2 +_0.2

0.57+0,05

1730,2 + 0 . 2 d)

0.36 +0.18

2518 +

2422.3 + 0 . 9

0.18+0.04

788

3209 ÷

788

aSCl(n, y)36C1

73

T a b l e 2 (continued)

ET+Er a)

I 7 b)

(keV)

I n t e r p r e t a t i o n c)

Ey+Era)

(E x i n keV)

2427.5 +0.8

0.17+0.03

2433.0 ~ 0 . 4

0.22+_0.03

2440.0 +-0.4

0.21+-0.02

2445.7 +-0.8

0.14+-0.02

Iy b)

(keV)

I n t e r p r e t a t l o n c) (E x i n keV)

2747.9 ~ 0 . 9

0.11+_.0.03

2752.5 +--0.4

0.23~0.03

5563 ÷ 2810

2759.3 + 0 . 6

0.15+__0.03

6090 ÷ 3332

5777 ÷ 3332

2766

0.11+-0.06

4034 ÷ 1601

~2

2448.6 +-0.7

0.15+-0.03

4409 ÷ 1959

2779.8 ~ 1 . 3

0.11+-0.06

2454.6 ! 1 . 0

0.14+-0.03

6090 ~ 3635

2793.2 ~ 1 . 8

0.14+-0.08

2467.6 + _ 0 . 4

0.31+_0.03

2468 ÷

0

2797.8 ~1.1

0.35~0.09

2470.6 +--0.3

0.69+-0.03

3635 ~ 1165

2801.9 ~ 0 . 5

0.59+-0.10

C ÷ 5777

2473.3 ~ 1 . 3

0.23+-0.10

5151 ÷ 2676

2809.3 ~ 1 . 0

0.30+-0.14

4409 ÷ 160 I

2480.0 ! 1 . 1

0.15+-0.05

4999 ÷ 2518

2812.6 ~ 0 . 9

0.33+_0.15

2489.3 ~ 1 . 0

0.49~0.13

C ÷

6090

2821.3 ~ 1 . 2

0.13~0.04

2493

0.35+-0.12

2492 +

0

2831.2 +-0.9

0.13+_0.03

3997 ÷ 1165

0.20+-0.09

3661 ÷ 1165

2836.9 + _ 0 . 8

0.15~0.03

5329 ÷ 2492

2503.7 ~ 1 . 0

0.12+-0.03

5972 ÷ 3469

2845.59+_0.17

1.17~0.04

2511.2 ~ 0 . 6

0.20+-0.03

6545 ÷ 4034

2864.06+0.15

6 . 0 +0.2

2520.2 +-0.7

0.19+-0.04

5329 ÷ 2810

2871.2 +_0.7

0.61+O. I1

2525.5 ~ 1 . 0

0.19+-0.04

5018 + 2492

2877.0 + 0 . 6

0.57~0.11

2530.6 + _ 0 . 4

0.37+-0.04

4999 ÷ 2468

2895.93+O.14

0.36+0.07

2538.G +-0.2

0.61+--0.03

4139 ÷ 1601

29]7.7 + _ 0 . 2

0.07+0.02

2545.5 + 0 . 4

0.23+--0.03

2930.4 +0.7

0.15+0.03

5605 ÷ 2676

2551.5 +0.6

0.23+0.04

2940.8 ~0.9

0.10~0.03

5838 ÷ 2896

2556.0 +0.7

0.17+0.04

5550 ÷ 2994

2953.3 ~ 0 . 6

0.17~0.03

C ÷ 5627

2564.4 +1.6

0.19+0.09

4525 ~ [959

2965.6 ~ 0 . 7

0.14+_0.03

5777 + 2810

2569.3 +1.9

0.18+0.09

5246 ÷ 2676

2974.68+-0.16

1.14~0.06

C ~ 5605

2581

0.15+0.07

2990.4 +-0.9

0.14~0.04

2586.8 +1.6

0.18+0.08

2994.6 +-0.5

0.85+_0,04

2593.1 +0.9

0.23+0.06

4553 ÷ 1959

3001.5 +--0.2

0.74+-0.04

2607.4 +1.6

0.19+0.09

C ÷ 5972

3009.5 ! 1 . 1

0.13+-0.04

2623.3 +0.4

0.59+0.07

C ÷ 5956

3016.07+_0.19

0.98+__0.05

C ÷

2633.5 +0.3

0.16+0.03

3026.2 +-0.9

0.14+--0.04

C ~ 5550

2640.2 +0.3

0.18+0.03

5972 ÷ 3332

3041.4 ! 1 . 2

0.11+-0.04

2647.0 +0.4

0.28+0.04

4598 ÷ 195]

3049.0 ! 1 . 4

0.09+-0.04

!2

2497.1 ! 1 . 2

+2

2650.9 +0.6

0.22+0.03

3061.85~0.15

3.5 +-0.2

2655.8 +0.8

0.08+0.02

3069.1 +-0.5

0.26+_0.04

2665.6 +0.3

0.30+0.03

2676.00+0.15

1.70+0.05

2680.4 +0.3

4734 ÷ 1951 3963 + 1165

C ÷ 5734 2864 ~

0

C ~ 5703 2896 ÷

0

C ÷ 5588 2994 ÷

0

C ÷ 5578

3830 ÷

5563 788

C ÷ 5518 5588 ÷ 2518

3077.2 ! 1 . 4

0.09~0.04

3086.8 +-0.5

0.21+-0.04

5578 ÷ 2492

0.39+0.04

3095.7 ~ 1 . 0

0.11+_0.03

5563 ÷ 2468

2687.8 +0.6

0.15+0.03

3106.5 +0.9

0.18+0.06

5972 ÷ 2864

2692.9 +0.6

0.14+0.03

3116.23+O.17

0.94+0.06

C ÷ 5463

2700.0 +1.8

0.09+0.04

3127.1 +1.3

0.07+0.03

2709.5 +1.0

0.19+0.04

3 1 3 5 . 4 +0,6

0.18+0.03

2714.5 +1.4

0.12+0.04

3152.5 +0.9

0.08+0.03

C ~ 5914 2676 ~

0

4315 ~ 1601

2724.1 +1.0

0.14+0.05

3158.2 +0.9

0.08+0.03

2733.2 +1.0

0.10+0.04

3166.3 +0.9

0.07+0.02

2736.7 +1.7

0.08+0.04

3 1 9 5 . 6 +1.1

0.08+0.03

2741.8 +0.7

0.14+0.03

3202.0 +0.6

0.13+0.03

C ÷ 5838

5627 + 2492

A. M. J. SPITS A N D J. KOPECKY

74

Table

Ey+Er a )

iyb)

(keV)

2 (continued)

interpretation c ) (Ex in keY)

Ey+Er a )

Iy b}

(keY)

(Ex in keV)

3210.7 + 0 . 6

0,14+0.03

3640.5 +0.8

0.08+0.02

3221.3 ~I.I

O. 1 2 + 0 . 0 4

3646.8 +0.5

0.12+0.03

3241.5 +1.2

0.07+_0.02

3657.7 +l.l

0.11+0.03

3250.2 +0.4

0.30+0.03

3661.6 +0.4

0.23+0.04

3256.2 +1.4

0.07+0.03

3670.0 +0.6

0.08+0.02

5703 ÷ 2492

C ÷ 5329

interpretat£onC)

6510 ÷ 2864

3661 ÷

0

3272.1 +I.0

0,12+0.04

3677.3 +0.5

0.09+0.03

3282.0 +0.9

0.06+0.02

3683.8 +0.8

0.07+0.02

3291.4 +0.5

0.17+0.03

3696.8 +0.9

0.07+0.02

3296.2 +I.I

0.07~0.02

3707.5 +0.4

0.15+0.03

3311.7 +0.7

0.12~0.03

5263 ~ 1951

3715.2 +0.8

0.07+0.02

3316.6 +0.5

0.21+_0.03

C ÷ 5263

3727.9 +0.7

0.06+0.02

3320.3 +I.I

0.10+0.03

5838 ÷ 2 5 1 8

3735.8 +0.3

0.16+0.02

C ÷ 4844

3332.98_+0.18

0.74+_0.O3

C ~ 5246

3744.4 +0.4

0.13+0.02

5 7 0 3 ÷ 1959

3340.6

+1.4

0.05~0.02

3750.2 +0.2

0.30+0.04

C ÷ 4829

3348.1

+0.5

0.15+0.03

3757.1 +0.5

0.09+0.03

3352.6

+1.0

0.07+0.02

3359.7

+0.4

0.10+0.03

3371.8 +1.6 3 3 7 5 . 3 !0.4 3382.2

+1.0

3388.7 +0.9

C ~ 5307

5 3 0 7 ÷ 1959

C ÷ 4884 4496 +

788

5734 ÷ 1959

3775.0 +0.2

0.23+0.03

3784.7 +I.0

0.05+0.02

0.10+U.07

4525 ÷ 1165 6268 + 2 8 9 b

3792,1 +0.5

0.I0+0.02

0.48~0°II

C ÷ 5205

3800.5 + 0 . 6

0.07+0,02

0.06+0.02

3810.3 +_0.6

0.11+_0.03

0.07+0.02

3821.67+0.18

0.99~0.05

C ÷ 4758 5 7 7 7 ÷ 1951

C ÷ 5151

3428.87+0.16

0.81+0.04

3826.2 +0.2

0,73+0.05

3437.3 +0.4

0.06+0.02

3834. I +0.7

0°09~0.02

3446.5 +0.9

0.06+0.02

3842.4

+1.0

0.08~0.02

3459.6 +0.5

0.07+0.02

3848.5

+1.0

0.06~0.02

3861.2

+0.7

0.I0~0.03

3868.4

~1.5

0.05~0.02

3916.2

+0.5

0.09~0.02

3469 ~

0

3469.6 +0.4

0.10+0.03

3488.2 +0.5

0.08+0.02

3500.2 +0.3

0.24+0.03

C ÷ 5079

3504.0 +0.5

0.18+0.03

5 4 6 3 + 1959

3513.0 +0.4

0.09+0.02

4956 + llb5

4598 ÷

788

C ÷ 4734

6379 ÷ 2518

3962.6!0.2

0.40~0.05

3963 +

0

3976.1

0.13~0.02

5578 ÷

1601

!0.6

3526.0 +0.6

0.06+0.02

3981.34+0.17

0.92+0.06

3538.9 +0.3

0.14+0.03

3996.4

+0.9

0.09+0.02

3 5 5 1 . 0 +0.3

0,06+0.02

3558.2 + 0 . 3

0.22~-0.03

3561.5

C ÷ 4598

4003.3

+0.8

O. 1 I + 0 , 0 3

5605 + 1601

5 5 1 8 ÷ 1959

4025.9

+l.1

O. 13+0.05

C ÷ 4553

0.59+0.05

C ÷ 5018

4030.2

+0.9

O. 15+0.04

3566.15+0.19

0.25+0.03

5 5 1 8 ÷ 1951

0.09+0.02

3573.64+0.17

0.18-~-0.03

3581.4 +1.4

O. 2 0 + 0 . 0 7

3588

+2

0.4

3599.5

+0.5

3604.6 3614.3

+0.2

4040.9

+0.8

4054.4

+0.2

0.55+0.05

4061.5

+0.5

O. 20+0.02

C÷

4999

4071.2

+1.4

0.06+0.02

O. 54~0.08

3599 +

0

4077.4

+1.0

O. 08+ O. 02

+1.2

0.35~0.09

5 5 6 3 ÷ 1959

4083.0

+0.3

0.67+0.04

+1.5

0.15¥0.06

5 5 6 3 ÷ 1951

4088

+2

0.08+0.03

4092

+3

O. 0 7 + 0 . 0 3

¥0.2

3622.9

+0.5

0.15+0.02

C ÷ 4956

3629.3

+0.6

O. 17+0.02

5 5 8 8 ÷ 1959

4097.3 +1.0

O. I 1+0.03

3634.7

+0.4

O. 29 +0.03

3635 ÷

4105.1 + 0 . 8

O. 07+ O. 03

0

C ÷ 4525

C ÷ 4496

5263 ÷ 1165

75

3SCl(n, 7)3°C1 Table 2 (continued)

Ey+Era)

IT b)

(keV)

I n t e r p r e t a t i o n c) (Ex i n keV)

Ey+Er a)

IT b)

(keV)

I n t e r p r e t a t i o n c) (E x i n keV)

4127.3 + 0 . 6

0.13+0.02

4633.7 +1.5

0.05+0.02

4133.9 ~1.0

0.11+_0.03

5734 ÷ 1601

4650.8 +I . 0

0.06+0.02

4139.6!0.4

0.29+0.03

4139 +

4675.9 +l.0

0.05+0.02

4150. I +I.1

0.06+0.02

4685.1 + 0 . 7

0.08+0.02

4163.4 !0.6

0.11+0.02

5329 ÷ I165

4712.3 +1.0

0.06+0.02

4169.6 !1.5

0.05+0.02

C ÷ 4409

4721.4 + 1 . 4

0.05+0.02

4176.0 !1.3

0.0610.02

4729.3 +0.2

0.67+0.05

5518 ÷

788

4190.9 !1.1

0.06!0.02

4735.4 + 0 , 7

0.12+0.03

4734 ~

0

4198.4 ~1.2

0.06+0.02

4748.8 + 0 . 5

0.13+0.03

4206.7 !0.4

0.1610.03

4755.9 +0.4

0.22+0.03

4228.9 ~1.2

0.05!0.02

4761.6 +0,9

0.09+0.03

4263.8 +_0.5

0.07!0.02

4770.9 + 0 . 7

0.07+0.02

4284.4 !0.5

0.07~0.02

4781.0 +0.7

0.07+0.02

4294.1 !0.6

0.10!0.02

4792.1 +0.8

0.06+0.02

4299.1 ! 0 . 3

0.30+0.03

4813.6 +0.6

0.20+0.03

4307.2 ! 0 . 6

0.08+0.02

4818.6 +1.4

0.08+0.02

4321.0 +0.8

0.05+0.02

4829.1 +0.4

0.24+0.04

4332.9 ! 0 . 8

0.07+0.02

4835.6 +1.8

0.05+0.02

4354.8_+0.5

0.16+0.03

4844.3 +].3

0.06+0.02

4360.9 +1.4

0.05+0.02

4856.1 +0.5

0.06+0.02

4394.8 +0.6

0.05+0.02

4 8 6 7 . 0 +0,6

0.06+0.02

4404.2 +0.5

0.07+0.02

4884.5 +0.5

0.11+0.02

4414.4 +0.3

0.27+0.03

4906.9 +0.6

0.07+0.02

4418.6 +0.8

0.14+0.03

4933.5 +0.5

0.14+0.03

4424.6 +1.1

0.08+0.02

4944.7 +0.3 e)

0 . 7 +0.3

4431.0 +1.1

0.06+0,02

4945.6 +0.5 e)

0.4 +0.2

4440.74+0.19

1.00+0.05

4952.4 +0.5

0.23+0.04

4446.8 +1.2

0.09!0.03

4958.3 + 1 . 3

0.13+0.04

4458.4 _+0.6

0.17+0.03

4980.3 +0.2

3.53+0.15

4464.4 +1.7

0.06+0.02

4989.6 +0.6

0.34+0.05

5777 ÷

788

4473.1 +1.6

0.05+0.02

4999.2 +0.6

0.12+0.03

4999 ÷

0

4491

0.07+0.02

5008.1 +1.I

0.07+0.02

4500.4 +1.9

0.06+0.02

5 0 1 8 . 3 +0.2

0.50+0.04

5018 +

0

4509.6_+0.9

0.07+0.02

5 0 2 7 . 4 +l.O

0.07+0.02

4517.9 ! 0 . 5

0.17+0.03

5307 ÷

788

5040.7 +l.l

0.06+0.02

4525.2 ! 0 . 2

0.49!0.04

4525 ÷

0

5055.0 +1.3

0.05+0.02

4533.0!0.9

0.07+0.02

5079 ~

0

4546.4 +0.7

0.25+0.08

4550.1 +0.6 4557.4 +0.8 4582.3 +1.9

0.05+0.02

4587.2 +0.5

0.27+0.04

4592.6 +1.3

0.11+0.03

6545 ÷ 1951

4597.7 +1.2

0.08+0.03

+2

0

5463 ÷ 1165

5956 ÷ 1601

C ÷ 4139

5246 ÷

788

C + 3830

829 ÷

0

C ÷ 3724

4884 ÷

0

C ÷ 3635 5734 ÷

788

4956 +

0

C ÷ 3599

5 0 7 8 . 8 +0.6

0.13+0.03

C ÷ 4034

5086.2 + l . ]

0.06+0,02

0.35+0.07

6510 ÷ 1959

5109.8 +0.5

0.12+0.02

0.10+0.02

6510 ÷ 1951

5124.4 +0.8

0.08+0.02

C ÷ 3997

5141.9 +.._0.8

0.11+0.02

6545 ÷ 1959

5 1 5 0 . 5 +0.4

0,24+0.03

5150 ~

0

5204.7 +0.4

0.24+0,03

5205 ÷

0

5243.2 +1.1

0.11+0.04

4604.4 +1.1

0.05+0.02

4616.9 +0.2

0.65+0.05

4626.1 +1.4

0.05+0.02

+0.3

0.53+0.05

5260.2 +1.0

0.09+0.02

5266.0 +0.8

0.12+0.03

5247.7

C ~ 3963

C ÷ 3469

C ~ 3332

A. M. J. SPITS A N D J. KOPECK'~"

76

T a b l e 2 (continued)

Ey+Era )

iyb)

interpretation

c)

Ey +E r a )

(E x in keY)

(keY) 5290,1 +1,1

0,05+0,02

5300.9 +0.6

0.10+0.02

6090 ÷

788

iTb)

interpretation

5733.7 !0.4

0.52+--0.04

5757

0,09+0,05

+2

c)

(E x in keY)

(keY)

5734 ÷

0

5777 ÷

0

5316.7 +1,0

0,05+0,02

5778.1 ~1,0

0,19+0,04

5331.6 +-0.7

0.08+-0,02

5903,36+-0,19

1,11+0.04

5346.6 ~l.3

0.05+_0.02

5911,8 +_.0.9

0.12+-0,03

5914 ~

0

5357,8 ~ l . I

0.06~0.02

5957.0 ~0.4

0.24+-0.04

5956 ÷

0

5371.4 ~ 1 . 2

0,07+-0,02

6087.0 +0.2

1.08+0.11

5378,7 ~ 1 . 2

0.07+_0.02

6111.39~0.18

5407,0 ! l . O

0,06+-0,02

6186.1 +--0.8

0.13+0,03

5420.3 +-0.5

0.15+-0.02

6268,5 ~0,3

0,40~0,03

5428.6 ~ 1 . 2

0.06+--0.02

6289,8 ~1,1

0,09+-0,02

19.7 ~0.2

C ÷ 2676

C ÷ 2492 C ÷ 2468 6268 ÷

0

5447.5 ~ 1 . 0

0.05+-0.02

6344,8 + - 0 . 7

0.15+0,03

6345 ÷

0

5465.3 +-0.8

0.07+_0.02

6379,7 +-0,5

0,31+-0.04

6379 +

0

5473.3 +_0.5

0.14+_0.03

6417,8 ~1,5

0,13+-0,06

6422 ÷

0

5481.3 ~0.6

0.09~0.02

6422.1 +-0,3

0,31+._0,05

5492.8 ~0.4

0.17~0.03

6432.3 +1.8

0.06+0.02

5500,7 +0.6

0,10+0,02

5517.74+-0.19

1.59~0.08

5561.6 +_0.6

0.08~0.02

5579.5 ! l . l

0.07+-0.02

5585.5 +-0.3

0.45+-0.04

5599.7 ~1.1

0.08+-0.02

5604.8 +_0.3

0.32+-0.04

5634.5 +I.1

0.06+0.02

5703.6 +0.7

0.36+0.06

5715.69+-0.19

5.14+_0.06

a)

energy is written

The r e c o i l error

b)

5518 ÷

0

6488.4 +_0.8

0.14+_,0.03

6487 ÷

0

6544.9 + 0 . 8

0.16+0.03

6545 ÷

0

6620.07+-0.19

8.10+_0.10

6628.16+-0.19

4.64+_0.10

6640.8 + 1 . 0

0,37+0.08

6642 ÷

0

6951.5 +-0,9

0.21+--0.04

6951 ÷

0

6978.29~0.19

2.23+-0.09

C ÷ 1601 C + 1165

C ÷ 2994

C ~ 1959 C ÷ 1951

5605 ~

0

7414.5 + 0 . 2

10.00+0.10

5703 +

0

7790.96+0.19

8.61+0.08

C÷

788

C ÷ 2864

8579.31+0.19

2.94+0.06

C÷

0

a s E r . The e r r o r s

quoted a r e p u r e l y s t a t i s t i c a l ;

a systematic

o f 20 ppm s h o u l d be added t o e a c h e r r o r ,

The intensities have been normalized such that

~Iy(E+E r) -

100 Q, where Q denotes the

neutron binding energy. The errors again are purely statistlcal; a systematic uncertainty of 8% is believed to be associated with each of them. c)

The capture state is denoted by C.

d)

Doublet structure not observed in the spectral analysis, but assumed to follow from the branchings given in ref.21).

e)

Doublet structure not observed in the spectral analysis, see text.

35C1(n, ~)36C1

77

errors) were used to decide where a y-ray would fit in the 36C1 decay scheme. Also intensity considerations have been taken into account. The interpretation of certain y-rays in previous (n, y) work was rejected on the basis of an improved precision for y-ray energies or excitation energies or a better resolution of y-ray doublets, whereas many other y-rays not observed or interpreted befbre could be placed in the decay scheme. In order to make possible the fitting of some rather intense y-rays, a number of levels have been accepted next to those given in the Endt and Van der Leun compilation 19). Seven of them could be identified with levels given in ref. 32). The E x -5017.95+0.19 keV level has also been observed in ref. 4) and may possibly be identified with the 5000_+ 8 keV level of ref. 19). Most of the levels not given in ref. 19) have been adopted on account of both primary and secondary y-rays, whereas two of them (given in brackets) have been accepted in virtue of the observation of their ground-state decay only. The latter transitions have such large energies that they do not reasonably fit somewhere else in the decay scheme. The y-lines which have not been placed in the level scheme all have an intensity smaller than 0.5 ~ but for the y-lines E~ = 1840.4_+0.2, 2156.08_+0.07, 2418.2_+0.2 and 2871.2 _+0.7 keV with percentage intensities of 0.71 _+0.18, 0.63 _+0.03, 0.57 + 0.05 and 0.61 + 0. l l, respectively. In table 3 the excitation energies of the 36C1 levels found in the present work are listed together with those from ref. 19). The statistical errors have been enlarged with a 20 ppm part which accounts for the systematic contribution to the final error listed, as explained in sect. 4. For each level also the summed intensities of incoming and outgoing y-rays are given. In the last column the intensities of the y-rays interpreted as primaries are tabulated. From the least-squares calculation mentioned above a 35Cl(n ' y) reaction Q-value 8579.39 keV resulted, with a statistical error of 0.06 keV. By adding to this error quadratically the above 20 ppm error which allows for systematic uncertainty, one obtains Q = 8579.39__0.18 keV. It is in reasonable agreement with, but markedly more precise than the value reported in the 1971 atomic-mass table of Wapstra and Gove 20), Q = 8580.5 -+0.9 keV. Further it is in decidedly better agreement with the value given by Ishaq and Kennett 4), Q = 8579.1 _+0.5 keV than with the result of Fubini et al. a), Q = 8581.5_+0.2 keV. The error in the latter value should be considered unrealistically small. In table 4, the y-ray branching ratios of the 36C1 levels populated in the 35C1(n, y) reaction as observed in the present work, are given in per cent. For the levels below E x = 4 MeV the present values can be said to be in reasonable agreement with the results from previous less elaborate measurements 19, 21), but because to the latter no experimental errors had been assigned a detailed comparison is impossible.

A M. J. S P I T S A N D J. K O P E C K Y

78

Table 3 Excitation energies and intensity balance of 36CI levels, observed in the 35CI(n,7)36C1 reaction

Ex(keV) present work e)

0

0

Primary y - r a y branching ratlo(%)

Intensity b) ref.]9~ -

in

97

+4

14.0 23.8 5.1 30.7 13.5 22.7

+-0.9 +_1.1 +-0.3 +-I .5 +-0.6 +._0.9

out

15.0 25.7 4.3 29.6 12.7 23.2

+-0.7 +1.3 +-0.3 +1.5 +-0.7 +1.3

3.16+0.15 9.2 +0.4 10.7 +_0.5

1164.75+-0.04 1600.88+.._0.05 1951.04+-0.06 1959.21_+0.06 2467.89+-0.08

789.2+--0.2 1164.9+_0.2 1601.1+-0.2 1951.6+-0.2 1959.6+_.0.2 2468.6+--0.2

2492.08+-0.08 2518.19+_0.12

2491 +2 2517 +5

2.18+-0.15 1.28+-0.10

I. 65+-0 . 15 0.36+-0.18

l. 16+--0.13

2675.98+-0.16 2810. 37+-0,I0 2863.71+0.10

2677 +2 2812 +-5 2864 +2

1.67+._0.16 O. 65+-0.06 5.8 +_0.2

1.70+-0.07 O. 82+-0.05 6.8 +0.2

I. 19+--0.07

2895.97+-0.10 2994.37+-0.10 3100.63+-0.13 3209.0 +_0.2 3331.83+-0.10 3469.4 +-0.4 3599.04~0.09

2896 +_5 2995 +-2 3100 +5 3207 +6 3334 +--2 3468 +--5 3601 +-2

0.37+-0.11 0.69+-0 . 08 % 0.02 0.63+_0.07 1.37+-0.09 0.24+-0.04 3.5 +__0.2

0.94+._0.15 I. 06+-0.07 0.32+--0.02 0.35+-0.04

3635.28+_0.12 3661.4 +--0.2 3723.79+-0.16

3633~ 6+_0.2 3666 +--6 3724 +5

3830.5 +-0.5

788.42+_0.05

2.39+--0.14 4.9 _+0.2 8.7 +--0.4 21.1 +-0.9

5.5 +-0.2

0.49+_0.05

0.85+0.12

O. 56+-0.06

0.10+-0.03 3.3 +-0.2

O.

0.8 +--0.3 0.06+0.02 0°06+0.02

1.2+--0.2 0.74+-0.10 0.11+-0.02

0.8 +-0.3

3825 +5

0.31+-0.06

0.11+_0.04

0.14+-0.04

3962.56+-0.12 3997.2 +0. 3 4033.8 +-0.4 4138.78+-0.15 4315.1 +-0.4 4408.5 +0.7 4496.3 +_0.3 4525.00+0.14

3964.5+0.5 3994 +_2 4035 +--5 4139.2+-0.4 4316 +7 4406 +8 4497 +--2 4524 +4 c)

0.65+-0.05 0.05+-0.02 0.45+-0.08 1.00+-0.05 0.06+._0.02 0.05+0.02 0.67+-0.03 0.79+0.06

1.11+-0.11 0.65+_0.10 0.22+-0.03 1.28+-0.07 0.31+.0.05 0.45+0.14 0. !5+-0.03 0.78+0. I0

O. 70+-0.07

4552.9 +-0.9

4553 +_8

O. 13+--0.05

O. 23+_0.06

O. 13+-0.05

4598.1 +-0.2 4734.5 +_0.8 4757.58!0.19

4599 +-2 4726 +-8 4757 +-2

0.92+0.07 0.08_+0.02 0.99+-0.06

0.39+_0.04 0.23+_0.07 0.63+-0.09

O. 98+._0,07

4829.2 4843.9 4834.5 4956.2

4826 +8 4849 +_8 4879 +_8 4957 +-8

0.30+-0.04 O. 16+-0.03 0.07+0.02 0.15+-0.02

0.24+-0.05 0.40+-0.04 0. I I+_0.02 0.23+_0.05

0.32+-0,04

5000 +-8 5018 d)

0.20--+0.07 0.59 +_0.04

0.64+_0.07 0,74+0.06

+-0.3 +0.3 +_0.6 +0.2

4998.6 +-0.4 5017.95+-0.19

13+-0.02

3.8 +0.2

O. 07+0.02

O. 06+._0.02 0.27+-0.09 1.08+__0.06 O. 06+0.02 O. 72+0.05

0.59+0.05

0.09+-0.02 I. 06+-0.07 O. 17+-0,04 0.08+_0.02

O. 16+-0.02 0.2 I+-0.08 0.63+0.03

3sCl(n, y)36C1

79

Table 3 (continued)

Ex(keV ) present work a)

Intensity b) ref.19)

in

Primary T-ray branching ratio(%)

out

5079.! +--0.4

5082

5150.5 +_0.2

5152

+8 !2

0.24+--0.03 0.81+-0.05

0.1310.03 0.47+_0.]]

0.26!0.03 0.87+_0.06

5204.6 +--0.3 5246.4 +-0.2 5262.7 +--0.5

5205 5249 526l

!2 +5 c) !8

0.48+-0.]] 0.74+-0.04 0.2]+__0.03

0.56!0.04 0.62+-0.12 0.23+--0.04

0.5|10.12 0.79+__0".03 0.23+-0.03

5306.9 +-0.4 5329.1 +-0.3 5463.4 +_0.2

5306 +--8 533] +8 5461 !2

0.]210.04 0.30+_0.03 0.94!0.07

0.32+_0.05 0.46!0.06 0.62!0.05

0.]3+-0.04 0.32+-0.03 0.96+-0.08

5517.53+-0.]6

5517 5550

3.5 +_0,3 0.14+--0.04 0.98!0.06 0.74*0.05 0.14!0.04

2.7 +-0.2

3.8 !0.3 0.15+-0.04 1.07+-0.07 0.80+0.06 0.14+_0.05

5550.1 +-0.5 5563.3 !0.2 5578.0 ~0.2 5588.0 !0.4 5604.8 5626.7 5703.2 5733.9 5777.3 5837.6

!0.2 !0.4 !0.4 +0.2 +0.2 ~ +0.6

)5584

!2 !8

!8

0.23+_0.04 0.84!0.]3 0.82+0.09 0.43!0.05

5605 5622 5701 5734

!5 c) !8 ~8 +6 c)

5836

+8

1.14+-0.07 0.58!0.07 0.17!0.03 0.18!0.03 0.57!0. II 0.63!0.07 1.17÷0.06 1.25÷0.21 0.59*0.10 1.54+0.08 0.14*0.03 0.21+0.04

5913.6 +0.3

5906

+8

0.30*0.02

0.12+0.03

0.31÷0.02

5956.3 +0.3

5952

+8

0.59*0.07

0.40*0.05

0.63+0.08

5972.0 +0.4

5972

+8

0.19+0.09

0.48+0.07

0.21+O. lO

6090.2 +0.4

6090

+8

0.49*0. J3

0.56+0.07

0.52+0.]5

6268.3 ~ 0 . 2 °

1.09_+0.06

0.51~0.08

].16~0.06

6344.6 ! 0 . 5 a

0.21!0.04

0.15~0.03

0.23!0.04

0.56~0.06

0.40!0.02

6379

+5 c)

(6422.1 ~ 0 . 4

6379.3810.17

6423

! 5 c)

6487.2 Z 0 . 3

6480

+7 c)

(6509.7 ! 0 . 4

6510 ~8

6545.0 ! 0 . 4

6546 ~ 8

0.37!0.02 0 0.35!0.06

0.63+0. l !

0.15+0.03

0.31+_0.05

)

0.1410.03

0.37+-0.06

0.57!0.11

)

0.60!0.03

0.7410.07

0.64+_0.04

6642.5 ! 0 . 2 ~

0.48!0.03

0.37!0.08

0.51!0.04

6950.7 ! 0 . 4 •

0.23+_0.04

0.21!0.02

0.25!0.05

8579.39!0.18 e)

a) b)

0

1.22!0.07 0.18!0.03 0.61!0.12 1.26÷0.06

93

~4

Values obtained from least-squares analysis (see text). The errors include a systematic 20 ppm error. A level not observed before is denoted by an asterisk. The normalization follows from ~ 17(Ey+Er) - ]00 Q, where Q stands for the neutron binding energy.

c)

R,f 32).

d) e)

Ref. 4), no error given. Capture state.

Table 4 Gamma-ray branch£n 8 r a c i o s ~n per c e n t ) o f 36C1 bound s t a t e s

From l e v e l : a) Ex

0

To l e v e l : a)

~n

jR

(0+)2

2+

0

0.79

1.16

1,60

O. 79

2

3+

100

1.16

0+2

1+

I00

1.60

0

79.2+1.5

i.0+0.2

19.8+1.5

1.95

3

2-

63

5

32

1.96

0

2+

95.0+0.4

3 • 6+0.3

<0,6

I.~+0.2

2,47

1+3

3-

1.3+-0.2

0.9+-0,2

<0.1

<0.2

2.49

0

2+ •

2.52

3

5- c)

2.68

0+2 d)

( I , 2 ) + b)

21

+2

(2,3)+ •

88.9+._0.5

2.90

I

(2,3)-

38

+_6

2.99

1

(I-3)

80

+2

3.10

I+3

(2-4)

3.21

l

3.33

I

i)m

+2

3.33

<6

69

<0.5

+2

+5

4.0+0.8

O'2

97.8+0.2 <0.6

> Z

<2 5.8+1.1

14

+2

8

+2

54

+7

I!

+2

9

+2

(32*_3 ÷ 2.52 HeV)

1 I. 1+_0.5

100

2- i)m •

( ] , 2 ) + 8) 3- e

2.86

>

54 +3

4" a)

2 f)

1

2.68

©

3

3.60

2.49

I00

2.81

0+2 g)

2.47

<0.2

+6

2.86

3.47

1.96

I00

( t , 2 ) + e)

(I,2)-

1.95

51

+6

49

( 8+3 ÷ 2.99 H.eV)

+6 27

+I0

39

+6

|6

+3

48

~2

I0

÷2

I00

17 ÷2

16.3+_0.9

15.7+_0.9

(3.2+_.0.6 ÷ 2.90 MeV)

Table 4 (continued)

F¢om l e v e l : a)

Ex

To l e v e l : a)

Zn

jT

I

1- •

3

(1-5)-

3.96

I

(I,2) -e

4.00

1

(I,2)-

4.03

i

(o-3)

4.14

I

(I-3)-

4.32

I

(0-3)-

3.64 3.66 3.72

0.79

0

23

+2

31

+5

1.16

55

+2

27

+9

(81_+5 ~ 2.81 HeV;

3.83

4.52

+4

!

20

÷2

42

+6

2.47

2.68

2.86

3.33

4.60

1

3- •

4.73

3

(I-5)-

1

3- •

10

+2

+5

18 4-2

(5.0+_.0.8 ÷ 2.90 MeV)

8O +~

+2

48

+2

38

+9

67

+11

30

?

÷2

33

+11

24

+9

62

+_9

53

+7

I00 63

(1,2) +

+8

13

+4

~OO 28

52

(1+3) h)

+6

+15

72

÷6

48

+15 36

+6

II

+3

I00

4.84 4.88

2.49

I00

22

(1,2) -e

0

4.83

22

1.96

19+5 ÷ 3.10 MeV)

31- +6

i)m

(1÷3) h)

4.55

4.76

1.95

I00 38

4.41 4.50

1.60

( I00 + 2.52 HeV) 0+2 h)

( i , 2 ) + h)

100

4.96

1

(0-3)"

55

+9

5.00

1

(0-3) °

19

÷4

45

+_9 58

+6

(23+--6 -* 2.52 HeY)

O0

Table 4 (cont£nued)

From r e v e l : a) Ex

To l e v e l : a) 0

~

J~

5.08

1

(0-3)-

5.15

I

(I-3)-

51

+12

5.20

I

(2,3) - a

42

+_4

67

5.02

5.25

0.79

(0-3)-

5.31

I

(0-3)-

I 1+3 h)

1.9~

2.47

2.49

+5

2.68

2.86

3.33

( 8+2 ÷ 3 . 6 6 ~ V )

+6 47

52

•

59

+2

49

+12

29

+11

(58+--4 ÷ 3.2! HeY)

24

+9

53

+6

49

+4 9

O3

+2

> z

+7 34

+2

+9

O3 48

24

44

+9

+7

(1,2)-° 3-

1.95

25

5.33

5.52

1.60

+5

27

1

1.16

I00

(I+3) h)

5.26

5.46

~-J

28

+4

8

+2

(42+--7 + 2.81 MeV)

+6

23

*3

© r~

5.55

(76+-5 ÷ 2.99 MeV; 24~5 ÷ 4.32 MeV) 18

5.56 16

5.58

+7

0+2 h)

(1,2)+ h)

I

(0-3)-

5.70

I

(0-3)-

5.73

3 h)

5.63

5.78 5.84

1

(0-3)-

55

+4

19

+9

40

+5

13

(27+_.3 + 2.81 MeV)

+3 26

+2

5.59

5.60

42

+--3 (37+6 " 3.21 KeY)

21

*--5

9

i2

(60*-5 ÷ 2, 52 geV) 26

+3

+5

100 58

+4

41

+10

13

+3

32

+16

22

+3

(49+--9 "~ 2.52 HeY;

9

+3 47

51+-9 ÷ 2.90 MeV)

+fJ

21

+3

18

+5

21

+3

(9±2 -*- 2 . 8 ] HeY)

Table 4 (contlnue~)

From l e v e l : a) Ex

To l e v e l : a)

~n

J~

5.91

0+2 g)

(1,2) + 8)

5.96

0+2 h)

(1,2)+ h)

0

0.79

1.16

1.60

1.95

1.96

2.47

2.49

2.68

61

39

+4

+4

(25+--6 ÷ 3.47 MeV) 0÷2 g)

( l , 2 ) + 8)

6.27

3,33

100

5.97 6.09

2.86

18 -+2 79

6.34

+lO

37 +8 (25-+4 + 3.64 MeV; 31+--6÷ 3.83 MeV)

38 --+6 26

-+4

(21-+lOf- 2.90 He',/)

IO0

6.38

0+2 h)

(1,2)+ h)

6.42

1+3 h)

(0-3)- h)

56

100

+4

6.49

t+3 h)

(0-3)- h)

100

26 +4

(18+__3 -~ 2.52 MeV)

c~

6.51 6.55

0+2 h)

( i , 2 ) + h)

21

+_4

6,64

|00

6.95

I00

a)

18

+3

60

+7

15

+3

37

+._4

22 (27+_.4 -* 4.03 MeV)

E x c i t a t i o n energies are i n HeV. Spins and p a r i t i e s are from r e f . 19), as are Zn valuess except when noted otherwise. Levels are msrked with an a s t e r i s k i f the present work has added new £nforaatlon on s p i n s or p a r i t i e s .

b) c)

l ~ f . 26) furnishes s t r o n g arguments f o r J~= 1+. In r e f . Ig) the J~= 1+ p o ~ s l b i l i t y was r u l e d out unduly. Ref. 34). d) ~d~f. 32). A pure ~n~2 value is reported in refs. 21,~0,~I,~3). e) Based on refe. 26,32).

f) Eels. 31-33).

An odd parity is reported in refs. 25,29).

Whenm indeed, there is a doublet with members of opposite

parity at this excitation energy, it is the even-parity level which is excited in the (n,7) reaction. g) h)

Ref. 32). An Zn(d,p ) value of I, however, is reported in ref. 25), limitln; the J~ value to J" = (0-3)-. Ref. 32).

i)

Based on present branchings and l l f e t i m e s

from r e f . 21).

+3

c

84

A. M. J. SPITS A N D

J. K O P E C K ' Y

5.2. CIRCULAR-POLARIZATIONSPECTRA The polarization function R has been determined from the measured asymmetries for 21 primary and 4 secondary transitions. The results together with previous and present spin assignments and mixing ratios are given in table 5. Most of them are Table 5 Circular-polarlgatlon r e s u l t s for 36C1 bound levels Previous work EX a) (HEY)

E¥ a) (keY)

~n b)

j~ b)

61

R

Present work jw c)

o. 13÷0.04

(1-3) +

3+ e)

0.210+0.004 e)

-0.25+0.02

3+ f)

0.22+0.02

0+2

I + e)

-0.43 ¢-0.12 e)

0.65+--0.02

I + £)

-0.14+-0.03

6978

0

( i , 2 ) ÷ 8)

6628

3

0

8579

0.79

7791

1.16

7415

1.60 1.95

(0)+2 2 /

2+

2-

1951 h) 1.96

6620

2.47

6111

0.53+_0.07

61

-0.12+_0.04 oF -0+2 d)

(1,2) +

0.20+_0.06

2-

0.16+0.04

2"

62a 0.10+_0.10

2+

-0.19+-0.06 oz" -5.2+ 1.6

0

2+

-0.06+-0.05 0.04-+0.05

(1,2) +

I+3

3-

-0.52+-0.02

3"

-0.02+_0.02

0

(I ,2) +

-0.12+0. -- I i

2+

+0.16 oP -2.3--+1.2 -0.43_0.26

O. 29+0.19

(1,2) +

1959 h)

2.49

6087

2.60

5903

0+2 8)

2.86

5716

2 8)

(1,21 + 8)

(o-4) +

2864 h)

-0.10+_0.05

(2,3) +

0.42+0.12

(I-31 +

3.33

5248

(I-3)-

0.7 +_0.3

0,2)-

3.60

4980

(2,3)-

-0.43+-0 • 07

3-

3.64

4945

(I,21-

0,69+0.13

1

3.96

4617

(I-31-

O. 38+0.18

(I , 2 ) -

4.14

4441

(I-31-

-o,2 +-0,4

(1-31-

4.50

4083

(I-31-

0.2 +0,2

(I ,2)-

4.60

3981

(1-3)-

-0.5 +-0.2

3-

4.76

3822

(i-31-

-0.37+-0,14

3-

5.20

3375

(i-31-

-0.2 +-0.3

(2,3)-

5.46

3116

(I-3)-

0.7 _+0.4

(1,21-

5,52

3062

(2,3)-

-0.69+-0. I0

3-

0.55+_0.14

(2,3)-

1+3 8)

5518 h)

62- -0.20+-0.]0 oF-5+_3 d)

62= -0.14+_0.16 d)

a)

P~esent work.

b)

Ref.19), except where noted otherwise.

c)

Baled on R and Zn.

d)

Based on the Jw=2+ assisr~ent for the Bx= 0 and 1.96 }~V levels and the Ow-3- assiSnmant for the Ex= 5.52 MeV ~evel

e)

Based on the combined analysis of r e f s . 9,1~,15).

f)

The A2 c o e f f i c i e n t from r e f . I~) and the P - c o e f f i c l e n t from r e f . 15) were used together with R and l n values.

8)

See footnotes of table 4.

h)

Secondary ground-state t r a n s i t i o n s .

35C1(n, ),)36C1

85

based upon the R-values measured and the knowledge o f the ln values. For two M 1 + E2 transitions as additional information the angular-correlation work of Van Middelkoop and Spilling 14) and the linear-polarization work of Honz~tko et al. 15) have been used. A detailed discussion of the new information is given in the next section.

6. Discussion 6.1. SPIN ASSIGNMENTS A N D DECAY MODES OF SOME INDIVIDUAL LEVELS

In the following the multipole mixing phase convention of Rose and Brink 22) is adhered to. Spin exclusions are made on a 99.9 % confidence level. The J~ assignments ofref. 19) have been accepted, but for the states at E x = 1.60, 2.68 and 2.86 MeV. The 9 r o u n d s t a t e . The present work is in agreement with a J~ = 2 + assignment to the 36C1 ground state. The E = O. 79 M e V level. In linear-polarization work Honz~tko et al. 1 s) established J~ = 3 + for this level and determined the mixing ratio of the primary ?-ray transition as 61 = 0.210 + 0.004. The present R-value, - 0.25 + 0.02, agrees within the errors with the value of ref. 9), R = - 0.30 _+0.06. A mixing ratio of ~ ~ = 0.22 + 0.02 may be deduced from the former R-value. The smaller error given by Honzfitko et al. takes account of the information contained in refs. 9, 14), but not of the fact that the 1 + admixture in the capture state could be non-negligible. The E x = 1.16 M e V level. The previously found R-value, 0.45 _+0.06 [ref. ~)], together with the results of linear-polarization 15) and angular-correlation work 14) led to J~ = 1 + [ref. 15)]. In the present work, an appreciably different R-value of 0.65 + 0.02 was found, resulting from more sophisticated experimental and analytical procedures. The sets o f possible 61 values following from refs. 14, 15) and from the present work, however, exclude each other at the 99.9 % confidence level, if the assumption is made that the capture proceeds 100 % through the 2 + capture state. The discrepancy vanishes if one admits a small (~ = 0.994) 1 + admixture in the capture state. The small 1 + admixture is not necessarily unphysical, if one bears in mind that thermal-neutron capture to a certain extent always proceeds by potential capture. As an estimate for the potential part o f the 35C1 capture cross section the experimental value o f the 37C1 capture cross section, O'eap : 0.428_+0.005 b [ref. 23)] may be taken, because potential capture dominates the 37C1(n, ?) reaction mechanism 12) and because the potential capture cross section only slowly varies with mass number 14.12). This 1 ~o potential capture contribution to the total thermal-neutron capture cross section of ~5C1, acap = 43_+ 2 b [ref. 23)], will be distributed in a 3:5 ratio over the two spin channels, such that about 0.4 % of thermal-neutron capture in 35C! may be expected to take place via the lowest-spin channel. For each individual transition this amount of 0.4 % may fluctuate somewhat according to whether a level

86

A. M. J. SPITS AND J. KOPECKY

is more or less strongly excited in the (d, p) reaction and because of the interference between potential and resonance capture. T h e E x = 1.60 M e V level. The In(d, p) = 0 value of this state 25), implies J~ = (1, 2) + [in ref. 19) the J~ = 1 + possibility was unduly ruled out]. This is in agreement with a recent (7, ~ ) investigation 26), in which branchings from the 36S ground-state isobaric analogue in 36C1 have been reported to the E x = 1.16, 1.60 and 2.68 MeV levels, by which the spin of these levels is limited to J~ = (1, 2+). A J~ = 1 + assignment is favoured, because J~ = 0 +, T = 2 states in other nuclei, like 28A1 and 32p, exclusively decay to 1 + states. Shell-model calculations 27) have explained this behaviour. The R-value of 0.53+0.07, measured in the present work for the primary y-ray transition, does not discriminate between J -- 1 and 2. The y-ray branchings (table 4) for the decay of the 1.60 MeV level deviate from that given in ref. 19) (100 % Yo). The 20 % branching to the 1.16 MeV level also has been observed by Fubini et al. a). In the present work also a weak (1.0 + 0.2) % branch to the 3 + first-excited state was observed. If, indeed, the E x = 1.60 MeV level has J~ = 1 +, this transition has E2 character with a strength of 4 W.u. as found from the mean life of z m = 0.80 + 0.15 ps reported in ref. 21). It was calculated by Wildenthal et al. 28) that the E2 transition from the s e c o n d J~ = 1 + state to the lowest J~ = 3 + state should be much stronger than the one from the f i r s t J~ = 1 + state. The latter transition indeed escaped observation in the present work. T h e E x = 1.95 M e V level. The ln(d, p) = 3 value 29) implies J~ = (1-5)-. The fact that the level is excited by means of a strong primary (n, 7) transition excludes J~ = (4, 5)-. The R-value of 0.20+0.06 measured in the present work in addition excludes the J~ = 1- and 3- possibilities and thus establishes the spin as J~ = 2 - . In the angular-correlation measurement of ref. 14) the spin was also determined as J = 2, based, however, on the incorrect assumpti~ons of positive parity and pure E1 character of the strong 2.47 ~ 1.95 MeV transition. A 35 % branch to the first excited state is reported in the (d, py) coincidence work of ref. 21), in disagreement with the present study, in which an about equally strong branch is observed to the s e c o n d excited state. The discrepancy most probably originates from the fact that the energies of the two y-rays in the 1.95 ---, 0.79 -~ 0 MeV cascade only differ by 2 keV from those in the 1.95 ~ 1.16 -~ 0 MeV cascade. The doublet structure of the E~ = 0.79 MeV y-line had been recognized already in refs. 2-4)with intensities for the low-energy component of 29, 26 and 55 %, respectively, to be compared to ( 3 9 . 0 + 0 . 4 ) % in the present work. The E x = 1.96 M e V level. The ln(d, p) = 0 value 29) leads to J~ = (1, 2) +. The J~ = 1 + value is ruled out in the present experiment by the R-value of - 0.06 + 0.05. Like for the 1.95 MeV level, the J = 2 assignment of ref. 14) is invalidated by the incorrectness o f the premise that the 1.95 and 1.96 MeV levels have positive and negative parity, respectively. T h e E x = 2.47 M e V level. Spin and parity ofthislevel had been determined before

35C1(n,y)36C1

87

as J" = 3- from the circular-polarization result, R = -0.42_+0.09, of ref. 9) and the In(d, p) = 3 value of ref. 25). The present R-value of - 0 . 5 2 + 0 . 0 2 agrees with that of ref. 9). By combining the results of the present work with those of Van Middelkoop and Spilling 14) one finds 61 = - 0.02 _ 0.02 and 62(2.47 ~ 1.95 MeV) = 0.01 _+0.02. The E x = 2.49 M e V level. A limitation to Jn = (1, 2) + results from stripping 24, 25, 29) and pick-up 3o, 32) angular distribution measurements. The R-value of -0.12___ 0.11 found in the present work for the primary transition excludes the J = 1 possibility. The E~ = 2.68 M e V l e v e l . The l, = 0 + 2 value in ref. 32), which seems to overrule the ln(d, p) = (1) assignment of ref. 29) established J~ = (1, 2) +, in agreement with the (~, ~)') investigation 26), already mentioned in the discussion of the E x = 1.60 MeV level, in which the spin of the E = 2.68 MeV level was restricted to J~ = (1, 2+). For the same reason as for the 1.60 MeV level, the 2.68 MeV state most probably has J" = 1 +. The E = 2.86 M e V level. In refs. 31-33) the l, = 2 value has been determined; in refs. 2s, 29), however, an ln(d, p) = 3 has been found. The possibility of a doublet with opposite-parity members has been mentioned in ref. 32). The data presented in neither of the (d, p) publications, however, seem to unambiguously exclude an l, = 0 + 2 f i t . In the present investigation an R-value of R = - 0 . 1 0 + 0 . 0 5 is found for the primary E~ = 5715 keV 7-ray transition. This value conflicts with negative parity as all possible spins (J = 0-4) are excluded by more than three standard deviations, unless one of the following less probable assumptions holds: (i) the E~ = 5715 keV v-ray is a doublet; (ii) the cascade E~ : 5 7 1 5 - 2 8 6 4 keV has to be inverted to proceed through an E x = 5.71 MeV level; (iii) the level is 5-8 ~ populated via spin channel J~ = 1 + of the capture state, in which case the level must have J~ = 2- ; as about 0.4 ~ of the total capture is believed to proceed via the 1 + channel (see discussion of the E = 1.16 MeV level), this channel should then almost exclusively feed this particular level; (iv) the E = 5715 keV ),-ray contains a large M2 admixture (6 ~ 0.2). In the most probable case that the level has positive parity, the present R-value leads to J'~ = (2, 3) +. This is consistent with the identification by K r o o n et al. 33) and Rice et al. 32) of this state with the second J~ 3 + 36C1 state from the shell-model calculation of Wildenthal et al. 28). The E = 3.33 and 5.25 M e V levels. The R-value found for the primary 7-ray transition to the E = 3.33 MeV level, R = 0.7_+0.3, together with the l,(d, p) = 1 assignment, limits the J~ possibilities to J" = (1, 2)-. The E = 3332.98 _+0.18 keV transition does not fit between the E x = 3331.83 + 0.10 keV level and the ground state. Neither is a ground-state transition from this level reported in ref. 21). AS this stron~ tt~ 74+0.03"~3 7-ray could not be positioned else=

88

A. M. J. SPITS AND J. KOPECKY

where in the decay scheme, it was interpreted as a primary, exciting a 5246.4-t-0.2 keV level. The E x = 3.60 M e V level. The In(d, p) = 1 value 19), together with the present value of R = - 0.43 _ 0.07, unambiguously yield J " = 3 - , consistent with the J" = (2, 3)- limitation ofref. 19). The E x = 3.64 and 5.73 M e V levels. In the work of Hughes and Kennett 1) a coincidence between the ),-rays E~ = 788, 2846 and 4945 keV (I s = 15, 1.2 and 1.1 ~ , respectively) is reported. They were assumed to constitute a three-step cascade through the E x = 3.64 and 0.79 MeV levels. There are, however, reasons to believe that their order has to be changed to form a cascade through an Ex = 5.73 MeV level: (i) no branching from the level at E x = 3.64 MeV to the first excited state is reported in ref. 21), (ii) the observation of an Er = 5734 keV ),-ray in the spectrum, (iii) the energies of the three ),-rays mentioned above sum up to a value below the reaction Q-value, as was already pointed out in ref. 4). It m a y be concluded that the E~ = 4944.99_ 0.19 keV ),-ray is associated with a doublet structure. To bring energies and intensities into balance an E~ = 4944.7_ 0.3 keV (I V = 0.8 + 0.3 ~o) ),-ray was conceived as the primary to the E x = 3.64 MeV level and an E~ = 4945.6_+0.5 keV (I v = 0.3 _+0.2 ~ ) y-ray as a secondary ),-ray deexciting the E x = 5.73 MeV state to the first excited state at E~ = 0.79 MeV.Then from the R-value of 0.69 _+0.13 found for the doublet a lower limit of R --- 0.56_+0.09 m a y be deduced for the R-value of the I~ = 0.8_+0.3 ~ component. This R-value combined with the ln(d, p)value of ln = 1 in this case leads to an unambiguous J" = 1- assignment for the 3.64 MeV level. The Ex = 3.96, 4.14, 4.50, 4.60, 4.76, 5.20, 5.46 a n d 5 . 5 2 M e V levels. These levels, rather strongly excited in the (n, 7) and (d,p) reactions, were not observed in the (p, d) reaction, except for the E x = 3.96 and 5.52 MeV levels. The R-values measured, combined with the ptzviously established In(d, p) = 1 (or 1 + 3) values 25) lead to unique spin assignments o f J " = 3 - for the E x = 4.60, 4.76 and 5.52 MeV levels and set limits on J for the other levels.

6.2. GENERAL PROPERTIES OF LEVELS ARRANGED ACCORDING TO THE 1. VALUE The l,(d, p) = 1 levels. In order to investigate whether a correlation between (n, y) E1 primary transition probabilities and (d, p) stripping strengths exists for In(d, p) = 1 36C1 levels, the correlation coefficient p between the two sets of quantities was calculated, but found to be vanishingly small (p = 0 . 0 9 _ 0.19) as opposed to the value p = 0.98, found for the ln(d, p) = 1 levels of 3sC1 [ref. 12)]. Another notable feature of the primary y-radiation following thermal-neutron capture in 35C1 proceeding to In(d, p) = 1 levels is that its summed intensity is only about 50 ~ , as compared to 93 ~o in the 37C1(n, y) reaction 12). Both features straightforwardly follow from the fact that the 35C1(n, y) reaction,

3SCl(n ' ~)36C1

89

unlike the 37C1(n, y) reaction, is not governed by a direct capture mechanism, but is characterized by resonance capture through one negative-energy 2 ÷ resonance, which presumably has a configuration different from the configuration "target ground state + s-wave neutron". The l,(d, p) = 0 ( + 2) levels. Table 5 shows that the 6 ~ values for the transitions to the five 36C1 levels with appreciable l.(d, p) = 0 strength are typically 61 ,~ - 0 . 2 , if solutions of 16~1 > 2 are excluded (from the J~ = 1 ÷ assumption for the E x = 1.60 MeV level follows 61 = -0.31 _0.10). As had been previously shown for In(d, p) = 1 levels 12), the correlation between (d, p) stripping strengths and (n, y) primary reduced intensities can often be improved by varying the power of Er by which the intensities are reduced. For In(d, p) = 0 levels the analysis may be expected to be complicated by the M1 + E 2 character of the primary y-radiation. If, however, the E2 admixture indeed is rather small, one might endeavour to take E~-3 as an effective reduction factor. In remarkable contrast to the non-correlated (d, p) and (n, 7) strengths for 36C1 l,(d, p) = 1 levels, for In(d, p) = 0 levels a correlation coefficient is found which for k = 3 already is quite high (p = 0.94) and even rises to 1.00 for higher powers in the reduction factor (k = 6-8), which implies that the correlation is significa~nt at confidence levels of 95 ~ and 99.9 ~ , respectively. In fig. 5 this correlation is shown graphically. The (d, p) stripping strengths were

U.I p=0.94

I ;t,

1,

"~.n(d,p) = 0

i

i

1

0.1

(2l+l)S n

~-

0.01

36CI

i

i

0.1

1 ~

Ii.El 3

Fig. 5. Comparison of the (d, p) stripping strengths (2J+ 1)Sn (black lines) and the (n, 7) reduced transition probabilities I J E 3 (open lines) for 36C1 l,(d, p) = 0 levels. The (2J+ 1)Sn values have been adopted from ref. 29). For the (d, p) stripping strength corresponding to the ground state the upper limit given in ref. 29) is denoted by the dashed line. The (n, 7) partial radiation widtlls taken from the present work have been normalized such that the value for the level at E~ = 1.96 MeV equals the value of the corresponding (d, p) stripping strength.

90

A. M. J. SPITS AND J. KOPECKX[

adopted from ref. 19). The upper limit for the ground-state (d, p) strengthis represented by a dashed line. The normalization for the (n, 7) primary reduced intensities as taken from the present work is performed such that its value for the level at E = 1.96 MeV equals the value of the corresponding (d, p) strength. It may be seen from the figure that the (n, 7) strengths follow the fluctuations of the (d, p) strengths within 40 ~ , i.e. almost within the experimental errors. A similar analysis carried out for a few other doubly odd product nuclei in the same mass range, for which data were available (28A1 and 32p) and for which correlation coefficients of p = 0.77 and 0.84 were found with confidence levels of 99.9 and 90 ~o, respectively, revealed that the above correlation for 36C1 ln(d , p) = 0 levels cannot be regarded as an isolated phenomenon. This is quite surprising, since a transition of 3s ~ 2s character is forbidden in the single-particle model. A possible explanation may be found in the doorway-state mechanism, inwhichquasiparticle pairs in d-orbits are involved. The pick-up strength of fifteen 36C1 levels with In(p, d) = 0 following from the (p, d) work of Rice et al. 32) and the corresponding (n, 7) radiation widths were found to be anticorrelated (p = - 0 . 4 ) at a 93 ~ confidence level. The l,(d, p) = 3 levels. The four ln = 3 levels at E x = 1.95, 2.47, 2.52 and 2.81 MeV with J~ = 2 - , 3 -, 5- and 4 - , respectively, of which the J~ = 4 - and 5- states were identified by Nolan et al. 34), may be readily identified with the ld~lf~ quadruplet of the simple single-particle shell-model picture. Of the spectroscopic factors, 0.85, 0.77, 0.85 and 0.46 [ref. 29)], respectively, the first three are the same within the errors, but the last is unexpectedly small. As in the 37C1(n, 7) reaction 12) the primary 7-radiation to the Jn = 4 - and 5- pure ln = 3 levels escapes observation, whereas primary 7-radiation does strongly populate the J~ = 2 - and 3- levels. F r o m this one may conclude either that the J" = 2 - level at E x = 1.95 MeV has a non-negligible (undetected) ln = 1 admixture like the J ' = 3- level at E X= 2.47 MeV, or that the J " = 2 + negative-energy ln = 0 resonance through which the 35C1(n, 7) reaction proceeds, has a certain ln = 2 admixture, which may be obtained by capture of s-neutrons via a two-step capture mechanism (doorway state). The strong M1 + E 2 primary 7-ray transition to the J~ = 3 + pure In = 2 level at E x = 0.79 MeV seems to confirm the latter possibility. 7. Conclusions and summary

Recently published highly accurate measurements on mass differences s) have been used to achieve a 20 ppm accuracy in the energy calibration of the 3SCl(n, 7) reaction. This reaction, and by consequence the 7-ray energies established in the present work, are well suited for high-energy calibration of spectra, because of the high thermal-neutron capture cross section of 35C1 and the set of strong wellseparated capture y-rays present in the high-energy range.

3SCl(n, y)36C[

91

This accuracy finds its equivalent expression in the construction of a better balanced and more elaborate decay scheme comprising 236 fitted 7-rays and 72 levels, for which the excitation energies together with branching ratios have been determined. Spin determinations or restrictions for twenty levels together with the multipole mixing ratios of some primary y-ray transitions have resulted from circularpolarization measurements of the 7-radiation following polarized-neutron capture in asC1. There is evidence that the primary transition to the second excited state of 36C1 proceeds by a small percentage ( ~ 0.6 ~ ) via the J~ = 1 + capture channel. Finally, it has been found that the multipole mixing ratios of the primary transitions to the In(d, p) = 0 levels probably do not deviate much from an average value of 6 = - 0 . 2 . The (n, 7) and (d, p) stripping strengths to these levels are significantly correlated, which at first glance seems to tally with the two-step "doorway" capture mechanism. It is a pleasure to thank Prof. P. M. Endt, Dr. C. van der Leun and Dr. K. Abrahams for their interest in this work and for their criticism of the manuscript. This work was performed as part of the research program of the "Stichting voor Fundamenteel Onderzoek der Materie" (FOM) and the "Stichting Reactor Centrum Nederland" (RCN) with financial support from the "Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek" (ZWO).

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22)

L. B. Hughes and T. J. Kennett, Can. J. Phys. 48 (1970) 1130 J. Honz~itko, J. Kajfosz and K. Konecn3~, Czech. J. Phys. B20 (1970) 1059 A. Fubini, M. Popa, D. Prosperi and F. Terrasi, Nuovo Cim. 2A (1971) 109 A. F. M. Ishaq and T. J. Kennett, Can. J. Phys. 50 (1972) 3090 G. D. Loper and G. E. Thomas, Nucl. Instr. 105 (1972) 453 C. Larsson, L. Broman, J. P. Roalsvig and A. S. Alwash, Forskningsradens Laboratorium Studsvik report LFF-46 (1973) P. de Wit and C. van der Leun, Proc. Fourth Int. Conf. on atomic masses and fundamental constants, ed. J. H. Sanders and A. H. Wapstra (Plenum Press, New York, 1972) p. 131 L. G. Smith and A. H. Wapstra, Phys. Rev. 11 (1975) 1392 J. Kopeck~ and E. Warming, Nucl. Phys. A127 (1969) 385 J. Eichler and F. Djadali, Z. Phys. 233 (1970) 154 F. Stecher-Rasmussen, K. Abrahams and J. KopeckS, Nucl. Phys. A181 (1972) 225 A. M. J. Spits and J. A. Akkermans, Nucl. Phys. A215 (1973) 260 U. N. Singh e t al., Phys. Rev. C10 (1974) 2138 G. van Middelkoop and P. Spilling. Nucl. Phys. 77 (1966) 267 J. Hon~tko, J. Kajfosz and Z. Kosina,l~e~ Nuclear Research Institute report UJF 2772.F (1972) V. Haase, Kernlorschungszentrum Karlsruhe report KFK-730 (1968) A. M. J. Spits and J. de Boer, Nucl. Phys. A224 (1974) 517 A. M. J. Spits, A. M. F. Op den Kamp and H. Gruppelaar, Nucl. Phys. A145 (1970) 449 P. M. Endt and C. van der Leun, Nucl. Phys. A214 (1973) 1 A. H. Wapstra and N. B. Gove, Nucl. Data A9 (1971) 303 A. S. Yousef, E. L. Sprenkel-Segel and R. E. Segel, Phys. Rev. C8 (1973) 684 H. J. Rose and D. M. Brink, Rev. Mod. Phys. 39 (1967) 306

92

A. M. J. SPITS AND J. KOPECKY

23) S. F. Mughabghab and D. I. Garber, Brookhaven Nat. Lab. report BNL-325 24) H. U. Gersch and W. Rudolph, Proc. Int. Symp. on neutron capture gamma-ray spectroscopy, Studsvik, 1969 (IAEA, Vienna, 1969) p. 527 25) A. M. Hoogenboom, E. Kashy and W. W. Buechner, Phys. Rev. 128 (1962) 305 26) S. Fortier et al., Proc. Int. Conf. on nuclear structure and spectroscopy, vol. 1, Amsterdam, 1974 (Scholar's Press, Amsterdam, 1974) p. 81 27) B. H. Wildenthal, as quoted in ref. 26) 28) B. H. Wildenthal, E. C. Halbert, J. B. McGrory and T. T. S. Kuo, Phys. Rev. C4 (1971) 1266 29) P. Decowski, Nucl. Phys. A169 (1971) 513; A196 (1972) 632 (erratum) 30) L. Broman, C. M. Fou and B. Rosner, Nucl. Phys. A l l 2 (1968) 195 31) G. Ronsin et al., Nucl. Phys. A187 (1972) 96 32) J. A. Rice, B. H. Wildenthal and B. M. Preedom, Nucl. Phys. A239 (1975) 189 33) J. Kroon, B. Hird and G. C. Ball, Nucl. Phys. A204 (1973) 609 34) P. J. Nolan et al., Proc. Int. Conf. on nuclear structure and spectroscopy, vol. 1, Amsterdam, 1974 (Scholar's Press, Amsterdam, 1974) p. 82