Energy levels in 74As from the 75As(p, d) 74As reaction

Nuclear Physics Al88 (1972) 632-540;

@ North-Holland Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilm without

writtenpermission

from the publisher

ENERGY LEVELS IN 74As FROM THE =As (p, d) 74As REACTION R. FOURNIER, University

T. H. HSU, J. KROON and B. HIRD Ottawa, Ontario, Canada

of Ottawa,

and G. C. BALL Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada Received 7 February 1972 Abstract: The ‘5As@, d)74As reaction has been used to identify 23 excited states in 74As. The I, values, or mixture of 2, values, to 19 levels have been determined from DWBA fits. The spectroscopic sum rules show a neutron shell filling which is consistent with other N = 42 isotones. E

NUCLEAR

REACTION

‘5As@, d), E = 20.0 MeV; measured o(8). 74As deduced levels, i,, spectroscopic factors.

1. Introduction

The energy levels of the odd nucleus 74As are not well established. The 8 s y-decay of energy 283 keV which Schardt and Goodman ‘) interpreted as the decay of a metastable state of this excitation in 74As has been attributed by Weirauch and SchmidtOtt “) to the decay from 1g7mAu. ReLently Finckh et al. “) identified 42 levels in 74As from the time-of-flight neutron spectrum of the reaction 74Ge(p, n)74As. No spinparity assignments have been reported for any of the levels except for the ground state and the possible 8 s metastable state. This paper describes an investigation of the ’ 'As(p, d)74As reaction at Ep = 20 MeV in which I,, transfer values and absolute spectroscopic factors were determined from a DWBA analysis of the deuteron angular distributions obtained between 12” and 90”. “fhe neutron subshell filling in 75As was calculated from the pick-up spectroscopic factors. 2. Experimental procedure The measurements were carried out with a 20 MeV proton beam from the Chalk River MP Tandem accelerator. Deuterons were detected in a dE(50 pm), E( 1000 pm), E(200 pm anticoincidence detector) counter telescope used in conjunction with an on-line method of particle identi~cation described previously “). The target was made by evaporating arsenic onto a thin zapon backing. A silicon counter at a fixed angle 632

74As ENERGY

633

LEVELS

of 140” to the beam direction was used to monitor the elastic protons scattered from the arsenic in the target. A frequent comparison between the number of counts in this monitor and the integrated beam current established that no significant loss of arsenic occurred during the measurements. The deuteron spectra were analyzed using the peak fitting program of Tepel ‘) and an accurate energy scale was established by making separate measurements of the 27Al(p d)’ ‘Al reaction at regular intervals. In aider to obtain absolute differential cross sections, the target thickness was measured in a separate Rutherford scattering experiment of 5 MeV a-particles at 40” and 60”. At these angles the elastic scattering peak from arsenic was separated from those of the other light elements in the target backing. A target thickness equivalent to A similar geometry 71 pg * cm-’ of arsenic was obtained from these measurements. to that of the reaction measurements was used in order to reduce systematic errors.

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spectrum of deuterons from the 75As(p, d)74As reaction. from 15” to 65” have been added together.

The measurements

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PRESENT ME-EMS

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Fig. 2. The energy levels of 74As obtained from the ‘sAs@, d)74As reaction compared levels identified in the “‘Gee, n)74As reaction by Finckh et al. s).

with the

3. Results The energy spectra which were obtained at each angle were converted to Q-value spectra, assuming the kinematics of the ’ 'As(p, d)74As reaction. The deuterons from light elements in the target could then be identified because their apparent Q-value

74As ENERGY

LEVELS

635

changed with angle. The Q-value spectra at most of the angles of measurement were added together in order to reduce the statistical uncertainties in the identification of weakly excited levels. This summed Q-value spectrum is shown in fig. 1. In fig. 2 the energy levels populated in the 75As(p, d)74As reaction are compared with those observed in the 74 Ge(p, n)74As reaction by Finckh et al. “). The bracketed 160 keV level which is not apparent in the summed Q-value spectrum of fig. 1, is visible in some of the energy spectra as a small bump on the side of the strong 198 keV peak. In our spectra there is no evidence for the 384 keV and the 511 keV levels of Finckh et al “). With an overall resolution of 40 keV, we cannot resolve the numerous levels reported by Finckh in the region from 683 to 831 keV. There is however evidence for a high-level density in this region, resulting in an apparent background and peaks which are broad compared to those in other regions of the spectrum. The only peaks observed in this region are the 726 keV and 779 keV levels. Many of the states observed by Finckh et al. “) between the energies of 1021 keV and 1300 keV were only weakly populated in the (p, d) reaction.

4. DWBA analysis The distorted-wave program DWUCK “) was used to generate the theoretical fits to the angular distributions which are shown in fig. 3. Proton optical-model parameters were taken from Perey ‘) for proton elastic scattering from 64Zn at 22 MeV. The different parameters that have been found to fit the proton scattering in this energy and mass region all gave substantially the same DWBA results. The deuteron optical-model parameters were taken from the fit of Perey and Perey “) to the 15 MeV deuteron scattering from Cu in which all six parameters were adjusted for a minimum x2. The spin dependence of both optical potentials was ignored in the calculations. Some adjustment was made to the bound state parameters in order to find the best simultaneous fits to all the angular distributions that have clear pick-up shapes. The final choice of parameters for all three potentials is shown in table 1. It was found that a 5 % to 10 % variation of the bound state radius could change the magnitude of the theoretical differential cross sections, and hence the experimental spectroscopic factors obtained from them, by about 25 %. However, the relative cross sections to different levels remained reasonably constant. Nearly all of the experimental angular distributions showed two maxima that were separated in angle by about 30”, which is the value predicted by the DWBA theory. These angular distributions were interpreted as pure I,, = 1, 2, 3 or 4 transitions, depending on the angular position of the maxima. The angular distributions to the 265 keV, 626 keV, 958 keV and 1098 keV levels were found to be abnormal in that their minima were significantly more filled in than usual. They were interpreted as mixtures of angular distributions having two different 1, values. These transitions showed no signs of being doublets in the energy spectrum, so that only odd, or only even I,, values were assumed for each transition. Reasonable fits were obtained in all cases

R. FOURNIER

636

et al.

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BCM. Fig. 3. The angular distributions of the prominent peaks in the 75As(p, d)74As reaction, compared with the predictions of the DWBA pick-up theory. The optical potentials are listed in table 1. TABLET

Optical-model

parameters

used in the DWUCK

program

ros (lb&) (fm) proton deuteron neutron

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1.25 1.07 1.25

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0.47 0.668

74As ENERGY

LEVELS

637

except the 1098 keV transition where an I = 1 + 4 admixture was required. Further, there was some indication that the angular distribution to the 541 keV level required a small I,, = 2 contribution to the mainly I,, = 4 shape, which otherwise had too small an intensity at forward angles. The angular distributions for weakly excited 1, = 1 transitions could also contain mixtures of higher 1, values but it was not possible to estimate this from the shape of the curves. 5. Discussion The neutron occupation numbers for the outer subshells in 75As can be obtained from pick-up sum rules. Since Coulomb energy differences predict that the lowest T = 5 level in 74As should lie near 6.7 MeV, all levels which are observed in the present work should have T = 4. A sum rule of Macfarlane and French ‘) predicts that all the spectroscopic factors for neutron pick-up outside the If3 core to T = 4 states should have a sum cjSj = 13.5. The distribution of the spectroscopic factors among the subshells for the most prominent levels listed in table 2 were found to be TABLET

Energy levels and spectroscopic

factors

Peak excitation (MeV fkeV) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

0.000 0.198& 5 0.265-1 5 0.337& 5 0.4421 5 “) 0.5411 5 0.578&10 0.626& 10 0.75 b) 0.845f 5 0.895f 5 0.958+10 1.022flO 1.098*10 1.301*10 1.3751 5a) 1.435+10 1.536&15 1.633+15 1.676f15 1.764&15 1X76&10 2.058,tlO

“) Broad peaks, possibly multiplets. “) Sum of 0.726 MeV&S keV and 0.779 partially unresolved.

MeVfS

2 1 2+4 4 1 4+(2) 1 1+3 1+3 1 1 1+3 1 1+4 1 1+3 1

0.01 0.24 0.11, 1.11 1.46 0.60 1.22 (0.07) 0.09 0.04,0.17 0.51, 1.19 0.49 0.19 0.13,0.29 0.22 0.03,0.19 0.11 0.16,0.40 0.12

1 1

0.09 0.06

keV levels, both I,, = 1+3

but broad and

638

R. FOURNIER

et al.

xs2ds

,= 0.19,~~~~ = 2.93, ~SIQ = 2.05 and c slet = 3.98 which have a total sum of only 9.15. It is probable that some of the missing spectroscopic strength exists in the weaker, unresolved levels at higher excitation energies than those shown in table 2. In order to make an estimate of this missing spectroscopic strength, the counts which occurred in the energy spectra in the region between 1.5 MeV and 2.7 MeV excitation were summed at each angle. The angular distribution of these many weak transitions is shown in fig. 4, where it has been fitted by a mixture of I,, = 1 and l,, = 3 angular distributions. The shape of the theoretical angular distributions was not very sen-

“As ( p,d 1 “As

Fig. 4. The angular distribution of the many unresolved levels with excitation energies in the range 1.5 MeV to 2.7 MeV, compared with the DWBA pick-up theory assuming a mean excitation energy of 2.1 MeV.

sitive to level excitation energy, and a mean excitation of 2.1 MeV was assumed in the DWBA calculation. The spectroscopic strength from the levels in this unresolved region was then determined as ~SZ, = 0.87 and csir+ = 2.34. The total spectroscopic strength of 12.4 then approaches reasonably near to the sum rule limit of 13.5. Assuming that the difference is due to the inaccuracy of the DWBA normalization of the type described previously, then the neutron filling coefficients for 75As obtained from the c Sj are as follows: Vz”, = 0.73, V:f, = 0.83, V&g = 0.43 and V&t = 0.04. In fig. 5 are shown the subshell filling coefficients which have been reported for neighbouring nuclei. The coefficients for the nickel isotopes were found by Turkiewicz et al. 1 O); f or zinc, stripping measurements by von Ehrenstein and Schiffer 11) have been reported, and also pick-up measurements by McIntyre ’ “). The 7 ‘Ge filling coefficients were obtained from the stripping data of Goldman 13), and the Se iso-

14As ENERGY

LEVELS

639

topes were investigated by Lin 14). All the measurements were normalized to the sum rule limit for the total number of available neutrons or neutron holes. There seems to be good agreement in the overall trend for the subshell filling as the neutron number is increased through this region. The 14 and 2p subshells appear to fill preferentially and about equally in the lower half of the shell, and by N = 40 they are both almost full. In contrast, the Ig+ subshell contains few neutrons in this region, and fills rapidly above N = 40. Ni(d,pf Zn(d,p)

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Fig. 5. The subshell filling of the IV = 42 neutron configuration in 7JAs determined from neutron pick-up in the present measurements compared with subshell fillings obtained by neutron stripping and pick-up reactions on neighbouring nuclei.

Since the valence neutrons in 75As primarily occupy the N = 28-50 shell the negative parity levels which are strongly populated in the 75As(p, d)74As reaction must have large components of the configuration [(g+);’ x 75A~g.s.] with 1” = 3-, 4-, 5- or 6-. As a result, a (p, d) transition to the 2- ground state of 74As is forbidden to first order. The weak I, = 2 transfer observed to this level results from a small admixture of the configuration [(d%); ’ x 7 ‘As,.,.], -, 3-, 4-. Experimentally, three strong and one weak I, = 4 transitions were observed. The transition to the 337 keV levei was found to be pure I, = 4 so that this level has a probable spin and parity of 5- or 4-. The levels at 265 keV and 541 keV have mixed

640

R. FOURNIER

et al.

I,,= 4 + 2 distributions, and are either unresolved doublets or single levels with probable spins and parities of 3 - or 4-. The 1, = 1 and 3 transitions observed in the (p, d) reaction populate levels which have components of one of the following configurations: [(p,); ’ x ‘5Asg.s.]0+,1+,s +,s+ , Nf& ’ x 75As,.s.lI +,2+,3+,4+ or [(p& ’ x ’ 5As,,.] 1+,z +. However, these configurations are expected to mix strongly with each other and with configurations consisting of a neutron hole coupled to low-lying excited states of 7‘As. Experimentally, no pure 1, = 3 distributions were observed and 44 % of the observed 1, = 1 strength resulted from mixed I,, transitions while the remaining strength was distributed among nine states. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)

A. W.‘Schardt and A. Goodman, Phys. Rev. 123 (1961) 893 W. Weirauch and W. D. Schmidt-Ott, Z. Phys. 222 (1969) 474 E. Finckh, U. Jahnke, B. Schreiber and A. Weidinger, Nucl. Phys. A144 (1970) 67 B. Hird and R. W. Ollerhead, Nucl. Instr. 71 (1969) 231 J. W. Tepel, Nucl. Instr. 40 (1966) 100 P. D. Kunz, The program DWUCK, unpublished F. G. Perey, Phys. Rev. 131 (1963) 745 C. M. Perey and F. G. Perey, Phys. Rev. 132 (1963) 755 M. H. Macfarlane and J. B. French, Rev. Mod. Phys. 32 (1960) 567 I. M. Turkiewicz, P. Beuzit, J. Delaunay and J. P. Fouan, Nucl. Phys. Al43 (1970) 641 D. von Ehrenstein and J. P. Schiffer, Phys. Rev. 164 (1967) 1374 L. C. McIntyre, Phys. Rev. 152 (1966) 1013 L. H. Goldman, Phys. Rev. 165 (1968) 1203 E. K. Lin, Phys. Rev. 139 (1965) B340