Volume 156B, number 3,4 PHYSICS LETTERS 20 June 1985

T H E M A S S OF 35p A N D S P I N - P A R I T Y A S S I G N M E N T S F O R E X C I T E D 35p S T A T E S

S. K H A N , Th. K I H M , K.T. K N O P F L E , G. M A I R L E

Max - Planck- Instttut fur Kernphystk, D - 6900 Heidelberg, West Germany

V. B E C H T O L D and L. F R I E D R I C H

Instttut fur angewandte Kernphyslk, Kernforschungszentrum Karlsruhe, D-7500 Karlsruhe, West Germany

Recewed 26 March 1985

The 36S(d,3He)35P reacuon was studied vath 52 MeV unpolanzed and vector-polarized deuterons. The mass excess of 35p was determined to be -24859+2 keV. Angular distributions of the dffferentml cross sections do(0)/dfl and the analyzing powers ~Tl1(0 ) for strongly exc~ted states were measured. Excltat~on energies of vanous proton hole states were determined and the respectwe values of spin and panty were assigned. The DWBA analysis shows that these states exhaust nearly the complete p~ckup strength of the (2s,ld) shell

Recently, highly enriched 36S targets became avail- able [1]. This has prompted two different proton pickup experiments to investigate 35p for which only the ground state was known [2]. Its mass excess was first determined from the/3- end-point energy in 35p decay [3] with considerable error: - 2 4 9 3 6 - 75 keV. The improved values of - 2 4 8 1 9 + 10 keV and - 2 4 8 5 4 -+ 5 keV deduced from the 36S(14C, 15N)35p reaction [4] and the 36S(d, 3He)35p reaction at 30 MeV [5], respectively, are, however, incompatible within the quoted errors. This disagreement as well as a discrepancy of the measured energies o f an excited state in 35p around an excitation energy o f E x ~ 3.9 MeV [4,5] motivated us to a careful investigation of the 36S(d, 3He) reaction at an incident energy of 52 MeV. Moreover, the use of vector-polarized deuterons promised to remedy the persisting lack o f information on the spins o f excited 35p states [2,4,5].

For mass and excitation energy determinations 36S(d, 3He)35p spectra o f high statistical accuracy and fair resolution (~.90 keV FWHM) were obtained (fig. 1) using the momentum analyzed beam of 52 MeV unpolarized deuterons from the Karlsruhe cyclo- tron which provides intensities of typically 100 nA on target. The 1 mg/cm 2 thick target consisted of 208pb sulfide with highly enriched (81.1 + 0.2)% 36S

0370-2693/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

600

400

Z

0

c~ 200

I • 31p • 33p o 11B o 207TI tx 15 N

I

10

' ' ' ' l ' ' ' ' l

o, © ,,, 3t~S(d,3He)35 P 7, ® = ~,

+ ¢,, ,m ~)Lab: 1 0 ° w

Q

= ° o..,~e' . ;,~ .

5 0 Ex (MeV)

Fig. 1. Energy spectrum of 3He particles from the (d, SHe) reaction on S6S and the contaminants 12C, 160, 328, 348, and 2°8pb. The abscissa indicates 35 p excitation energies.

on 12C backing [6]. The 3He reaction products were identified simultaneously in two AE - E surface bar- tier detector telescopes (300/am, 1500 #m) arranged symmetrically about the beam axis. A total of 14 pulse height spectra was accumulated at 9 °, 10 °, 11 °, 12 ° and 22 ° in two ADCs of 8k conversion gain pro- viding a slope of ~-9 keV per channel. Kinematics and small variations of the incident energy were utilized to locate in each spectrum the peaks o f interest in dif- ferent ADC channels so minimizing the effect o f dif-

155

Volume 156B, number 3,4 PHYSICS LETTERS 20 June 1985

Table 1 Error contributions (in keV) in the mass determination of asp by the 36S(d, aHe) reaction at 51 MeV incident energy.

kinematics: scattering angle (± 0.2 °) incident energy (± 1 MeV)

t 0.6 ± 0.8

mass excesses a): a2 S -26016.18 ± 0.24 alp -24440.7 ± 0.6 a4S -29932.25 ± 0.21 aap -26338.0 ± 1.5 368 -30664.44 ± 0.25

asp (g.s.) centroid b) ± 1.1

deduced error for asp mass excess ± 2.2

a) Values taken from ref. [7]. Compared to these errors the errors of the d and aHe masses are negligible.

b) This value accounts also for the standard deviations from the calibration of the individual spectra which - on the average - amount to ~ 3.7 keV resulting thus for the 14 in- dependent measurements in a total calibration error of 0.9 keV.

ferential nonlinearities in the ADCs. The energy cali- bration relied exclusively on strong lines of the well- known excitation energy [2] of the 32,34S(d, 3He)31,33p reactions, drawing thus solely on the as- sumption of a homogeneous mixture of the different sulfur isotopes in the target. The obvious inherent er- rors of this calibration procedure are listed quantita- tively in table 1, demonstrating that the largest indivi- dual error contribution to the mass excess of 35p arises from the error in the mass of 33p. We find for the mass excess of 35p a value of - 2 4 8 5 9 + 2 keV which is in good agreement with the value of ref. [5], and, less

significant, with that of the/3- end-point measure- ments [3]. The 40 keY smaller value of ref. [4], how- ever, is ruled out by our measurements being indeed also inconsistent with the result of both ref. [3] and ref. [5], within the quoted error limits.

For spin and pari ty determinations of 35p states we used the vector-polarized 52 MeV deuteron beam to measure both angular distributions do(0)/dI2 and an- alyzing powers iT11(0 ) of the 36S(-d, 3He)35p reac- tion. The beam transport system was now set in its achromatic mode yielding 5 nA on target and an over- all resolution of ~ 180 keV. Details of the experiment- al arrangement are described elsewhere [8]. The virtue of this approach for spin determinations has been out- lined recently [9] demonstrating the distinctly differ- ent patterns of the analyzing powers i T l l ( 0 ) f o r / = l + 1/2 proton pickup to be remarkably stable against a change of Q-value or target mass [10]. Thus, our spin assignments are essentially based on a straightfor- ward comparison of the various experimental angular distributions of i T l l ( 0 ) so that DWBA predictions are merely used for additional evidence.

We observed besides the ground state six excited states of 35p, three of which were hi therto unknown. For the five strongly excited states we determined spins, parities and spectroscopic factors from their an- gular distributions and analyzing powers shown in fig. 2. We used standard local, zero-range DWBA calcula- tions with parameters which have been shown to de- scribe the (d, 3He) data on 32S and 34S [11,12]. The results are summarized in table 2. Below we present some detail.

"-c' E d : 52 MeV

~ ' 100 2

10_1 *

0.5

: o .o ' ,

-0.5

10 0

10-1

10 -2 }

10-3 0.5

0.0

-0 .5

2.39 MeV

f " ~ iT I I]-

101

10 0

10-1

10-2 0.5

0.0

-0.5

&86 MeV-

101

10 o

10 -1

10 -z

0.5

0.0

-0 .5

i i , , i

4.67 MeV

101 . . . . .

10 0 t 5.191,4eV

10-

10-"

• /

-0.5 r . . . . .

. . . . . . 0 . . . . . . ' 0 ° 200 4.0 ° 60 ° 0 ° 2 * 4.0* 60 = 0 ° 20 ° ° ' 60" 0 ° 20 ° 4.0 ° 60 ° 0 ° 20 ° 4.0 ° 60 ° ec.m.

Fig. 2. Measur_~d angular distributions of the differential cross sections do(O)/d~2 and analyzing powers iT11(0 ) for indicated states from the a6S(d, aHe)aSP reaction and the comparison with DWBA predictions.

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Volume 156B, number 3,4 PHYSICS LETTERS 20 June 1985

Table 2 Spectroscopic results from the S6S(d, 3He)SSP reaction.

Ex (keV) lp ] n C Z S

g.s. a) 0 1/2 + 1.63 2386 ± 6 2 3/2 + 0.31 3857 ± 2 2 5/2 + 2.91 4474 ± 21 <0.2 b) 4665 ± 3 2 5/2 + 1.06 5189 ± 13 2 5/2 + 1.38 7520 -+ 30 <0.4 b)

a) The S6S(d, SHe) Q-value was measured to be -7601 ± 2 keV corresponding to a aSp mass excess of -24859 ± 2 keV.

b) lds/2 transfer assumed; for 2Sl/~ pickup the corresponding spectroscopic factors would be reduced by a factor of ~10.

The 35pground state. We assign ,pr = 1/2 +, in agree- ment with the l = 0 transfer observed in ref. [5].

The state a t2386 +- 6 keV. We assign jTr = 3/2 +. This state appears weakly excited (C2S = 0.31) in agreement with shell model predictions of Wildenthal (E x = 2652 keV, C2S = 0.12; see ref. [5] ). It was not observed in the other pickup reactions [4,5].

The strong state at 3857 + 2 keV. This has spin and parity j~r = 5/2 + according to the observed analyzing power (fig. 2). This is compatible with the l = 2 assign- ment for a state observed at 3864 -+ 10 keV in 36S(d, 3He) at 30 MeV incident energy [5]. The excitation energy of 3950 -+-+ 20 keV measured in the 36S(14C, 15N)35p reaction [4], however, is at variance with the results of both (d, 3He) investigations - as is the 35p mass excess deduced with this reaction. This casts doubts on the error assignments to the energy calibra- tion procedures in ref. [4].

States at 4665 + 3 and5189 + 13 k e K We assign spin and parity j~r = 5/2 + (compare fig. 2) in agree- ment with the l = 2 transfer observed in the previous (d, 3He) study for states at 4664 -+ 10 and 5202 -+ 10 keV [5]. In addition, we found two weakly excited states at 4474 and 7520 keV (C2S < 0.2 and C2S < 0.4 with 1 d5/2 transfer assumed).

The total sum of our deduced spectroscopic factors for proton pickup from the (2s, ld) shell amounts to Z C2S ~ 7.9 which is - within the usual errors of spectroscopic factors of ~20% - the complete ex- pected shell model strength (~ C2S = 8). This obser- vation is at variance with the result of the 36S(d, 3He)

i i i

325 1 ~

~¥ 3& S v

A

X e ~ _ •

365

I I I

-20 -10 Ej/MeV 0

Fig. 3. Proton occupation probabilities in the (2s. ld) shell for indicated sulfur isotopes versus the single-particle energies. Solid lines represent a fit to the BCS occupation probability v] = (1/2)[1 - (E] - X)/e/] with e/= [(£1 - X) ~ + A s ] 1/2 using the following parameters (h[MeV, ~/MeV): S2S (-5.9, 3.5), ~S (-8.9, 2.5) and ~S (-10.9, 1.3).

experiment [5] at 30 MeV where only 50% of the ex- pected spectroscopic strength was deduced. On the other hand, our summed spectroscopic factors for pickup from 36S are consistent with those obtained for 32S and 34S measured also at 52 MeV incident en- ergy [11,12] and analyzed by the same DWBA proce- dure. From the agreement o f our 32S results with the findings of several different pickup experiments [ 13], we conclude that we have identified for even sulfur isotopes the complete (2s, ld) pickup strength. The strength is, however, rather differently shared by the three subshells in 32S, 34S and 36S. This is obvious from fig. 3 where the proton occupation probabilities are plotted as a function of the single-particle ener- gies, the latter being calculated as the first moment of the proton strength distributions deduced from both proton pickup and available stripping data according to the prescription of Baranger [14]. The data points are well reproduced by a two.parameter fit to the BCS occupation probability function 02 [15] (solid lines). The rapid fall-off of this function in 36S com- pared to 32S reveals a corresponding reduction of the amount of ground-state correlations, indicating an in- crease of blocking of contributing neutron configura- tions which is most pronounced for 36S with a magic neutron configuration.

157

Volume 156B, number 3,4 PHYSICS LETTERS 20 June 1985

References

[ 1] P. Maier-Komor, Proc. 1 lth World Conf. of the Inter- national Nuclear Target Development Society (Seattle, October 1982) p. 164.

[2] C.M. Ledered and V.S. Shirley, eds., Table of isotopes (Wiley, New York, 1978); P.M. Endt and C. Van der Leun, NucL Phys. A310 (1978) 1.

[3] G.P. Goosman and D.E. Alburger, Phys. Rev. C6 (1972) 820.

[4] W.A. Mayer, W. Henning, R. Holzwarth, H.J. Korner, G. Korschinek, W.U. Mayer, G. Rosner, and H.J. Scheerer, Z. Phys. A319 (1984) 287.

[5] C.E. Thorn, LW. Olness, E.K. Warbnrton and S. Raman, Phys. Rev. C30 (1984) 1442.

[6] J. Alexander and H.L. Wirth, Max-Planck-Institut fur Kernphysik Annual Report (1983) p. 45.

[7] A.H. Wapstra and G. Audi, The 1983 Atomic Mass Evaluation, NucL Phys. A432 (1985) 1.

[8] T. Kihm, G. Malrle, P. Grabmayr, K.T. Kn6pfle, G.J. Wagner, V. Bechtold and L. Friedrich, Z. Phys. A318 (1984) 205.

[9] V. Bechtold, L. Friedrich, P. Doll, K.T. Knbpfle, G. Mairle and G.J. Wagner, Phys. Lett. 72B (1977) 169.

[10] G. Mairle, G.J. Wagner, K.T. Knbpfle, Liu Ken Pao, H. Riedesel, V. Bechtold and L. Friedrich, NucL Phys. A363 (1981) 413.

[11] H. Mackh, G. Malrle and G.J. Wagner, Z. Phys. 269 (1974) 353.

[12] S. Khan, Diplomarbeit (Heidelberg, 1985), and to be published.

[13] P.M. Endt, At. Data Nucl. Data Tables 19 (1977) 23. [14] M. Baranger, NucL Phys. A149 (1970) 225. [ 15 ] D.J. Rowe, Nuclear collective motion (Methuen,

London, 1970) Chapter 11.

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