Levels in 36Ar and 38Ar from alpha capture in 32S and 34S

Nuclear Physics A171 (197t) 298-304; Not to be reproduced

LEVELS

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~or~~-Ho~~aadPu~Z~shiagCo., Amsterdam

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IN 36Ar AND 38Ar FROM ALPHA

CAPTURE

IN 32S AND 34S R. E. CLARKE,

F. E. DUNNAM

and H. A. VAN RINSVELT

Department of Physks and Astronomy, University of Florida, ~a~nesv~l~e~ Florida 32401 Received 13 May 1971 Abstract:

The 32*J4S(a,y)36* 38Ar reactions have been studied over an energy range ~5~= 32-4.3 MeV. Of the thirteen resonances observed in this energy region, three are attributed to 32S and ten to 34S. The excitation energies and resonance strengths are reported. The main decay of all levels is directly to the ground state, permitting definite spin and parity assignments to all resonances from angular distribution data. The resonance strengths support the isobaric spin selection rule for electric dipole transitions in self-conjugate nuclei.

E

NUCLEAR

MeV; measured REACTIONS 32*34S(a, y), E = 3.243 w 3*Ar deduced levels, J, 7t, I’,, , Natural and enriched targets.

u(E; 8).

1. Introductiou

The usefulness of radiative a-capture as a tool for nuclear spectroscopy in the 2sId shell has been clearly demonstrated - several (a, y) studies are cited in the energy level compilation “). The chief d~~dvautage of using a 4EIe beam lies in the fact that the carbon contaminant present in most accelerator collimator-target systems leads to the production of sizeable neutron backgrounds, arising mostly from the 13C(a, n)‘% reaction. When NaI(T1) detectors are used, the neutron background appears in the y-ray pulse-height spectra as an exponentially decreasing phenomenon out to 7 or 8 MeV y-ray energy. Various methods ‘- “) have been suggested for minimizing the background but none is totally successful, although it now appears that large Ge(Li) detectors (> 50 cm3) offer a means of reducing the problem considerably 6), Until the latter comes into wider use, however, NaI(T1) detectors can still be used in cases where the excitation energies in the compound nucleus are well above 8 MeV and also are for the most part befow the neutron binding energy so that relatively few neutrons are observed from the target itself. Studies of compound excited states in 36Ar and 38Ar in several different ranges of excitation energies have been reported: the group at Utrecht ‘) examined the 32134S (4 Y)36*38Ar reactions for E, = 2.2-3.2 MeV and Phillips “) has published results of a similar study of 38Ar with a 3.0 to 3.6 MeV cl-beam. In addition, the literature includes results of proton capture data in chlorine 1g9l lo> and ah.0 of (p, CX)experiments 11, 12). 298

299

36*38Ar LEVELS

In the present paper we report the results of the a-bombardment (E, = 3.243 MeV) of 32S and 34S, leading to excitation energies of 9.5 to 10.5 MeV in 36Ar, and of 10.1 to 11.1 MeV in 3sAr. Preliminary results were briefly reported earlier lJB14). 2. Experimental method The a-beam used in this experiment was provided by the 4 MV Van de Graaff accelerator at the University of Florida. The beam was first magnetically deflected through a 90” angle and then focussed on target by means of an electromagnetic quadrupole lens. The field of the 90” analysing magnet was determined by the usual nuclear magnetic resonance technique. 8 I500

-

1000

-

500

-

3e’34S (a,y)s*‘mAr Target: S (“S enriched

to 35%)

Y 8 K r 3 s % P 2

3p’54S (a,y)w’MAr Target: ZnS (natural

E,

S)

(MeV)

Fig. 1. Gamma-ray yield curves for the 32*34S(a,y)36-J8Ar reactions. The target thickness at Ea = 3.6 MeV is 12 keV for the natural target and 26 keV for the enriched target. The axis of the detector forms an angle of 55” with the beam direction. The distance from the target spot to the face of the crystal is 3 mm.

R. E. CLARKE

300

et al.

The naturai sulfur targets were prepared by evaporation in vacua of ZnS onto thin copper backings, previously cleaned with a dilute solution of ammonium hydroxide. Targets enriched in 34S to 35 % were prepared by evaporating the enriched elemental sulfur powder onto a thin warm silver backing Is) cleaned with alcohol. Target thicknesses varied between 30 keV and 12 keV for 3.6 MeV cc-particles. In order to inhibit deterioration of the water cooled targets, the energy dissipation during all runs was kept below 6 W. The carbon buildup and consequently the overall background was considerably reduced by the use of a liquid-nitrogen trap located in front of the target. Two NaI(T1) assemblies (12.5 cm diameter by 12.5 cm long crystals, resolution 9 % for the 0.662 MeV 13’Cs line) were used for the detection of the y-radiation. The spectra were stored in a 512~channel pulse-height analyser. 3. Results and discussion 3.1. YIELD

CURVES

AND SPECTRA

The excitation functions for both the natural and enriched targets are shown in fig. 1. The yield was measured for y-rays between 8.0 MeV and 11.0 MeV. Thirteen resonances were observed in the energy region investigated here. Ten resonances were attributed to 34S and three to 32S. This distinction could be made by inspection of the relative heights of the resonances in the excitation functions and by examination of the y-ray spectra which were measured at each resonance. The spectra were taken

Resonances Resonance number 1 2 3 4 5 6 7 8 9 10 11 12 13

TABLE 1 in the reactions 32S(a,y)36Ar and 34S(a, ~f~~Ar; energies, excitation and parities, strengths, and radiative widths in Weisskopf units

Isotope (izV) 3.324 3.408 3.498 3.563 3.603 3.778 3.889 3.989 4.024 4.082 4.168 4.256 4.278 all &5 keV

‘) The radiative widths r

Jr

(h&) 3% 3% 3% 34S 34S 3% 3% 3%S 3% 345 3% 3% ?S

10.183 10.259 10.339 10.397 10.433 10.590 10.689 10.189 10.220 10.862 10.348 11.017 11.037

used in the calculation

(2J+l)r,ry/r (eV)

11i1I1112+ 12+ 11-

energies, spins /iqZ( 2: 103) “)

8 2.7 2.1 6 14 6 0.9 0.3 0.05 2.0 0.5 3.0 4.5 all k.SO%

of IM12 were extracted

2.2 0.7 0.6 1.4 3.5 1.4 0.2 0.1 6.5 “) 0.4 6.1 b) 0.6 0.9

from the resonance

strengths assuming rr and E p +z r,. “) Radiative widths for E2 transitions Ey in MeV.

in Weisskopf units; I&.(EZ)

= 1.2AbEy5 x lo-’

eV, with

36*3*Ar LEVELS

301

at 55” and a target-to-detector face distance of 2 cm. All resonances showed a ground state transition and there was no evidence for the presence of capture y-rays to lowlying excited states. Table 1 gives the energy of the resonances and the corresponding excitation energies in 36Ar and 38Ar. The a-particle energy calibration is based on the 3199.8+ 1.0 keV 24Mg(a, y)“Si resonance 16). Excitation energies in 36Ar and 38Ar were calculated with the reaction Q-values from ref. ‘): Q = 6.644 MeV for the 32S(a, y)36Ar reaction and Q = 7.210 MeV for the 34S(a, y)38Ar reaction.

32,34s

(o,r)

At-3638

Ground State Transkion Angular distributions

cos2 8 Fig. 2. Ground-state angular distributions measured at the thirteen 32*j%(a, y)36* 38Ar resonances. The solid lines represent the theoretical angular distributions for the assigned resonance spins. The solid angle attenuation has been taken into account: Q3 = 0.90 and Q4 = 0.70.

302 3.2.

R. E. CLARKE ANGULAR

et al.

DISTRIBUTIONS

An angular distribution of the ground state transition was measured at each resonance. For these data, a monitor detector was kept fixed at 90” with respect to the beam direction and the other detector was moved in the horizontal plane between 0” and 90”. For both detectors the distance from the face of the crystal to the target spot was 10 cm. Eccentricity of the system was checked with the 3199.8 keV resonance in 24Mg(a, y)28Si. In both reactions 32S(a, y)36Ar and 34A(~, y)38Ar, only natural parity excited states can be observed. Also, if a ground state transition is detected the possible J” values are essentially restricted to l- or 2+ and the corresponding ground-state angular distributions are given by: w(e) cc 1 -P,(cos e>, W(0) cc 1 ++P,(cos e)-~~4(cos

t?),

for I- +O+ for 2+ -+ Of

where P,(cos 0) and P4(cos 0) are Legendre polynomials I’). The data, corrected for background measured just below or above the resonance, are displayed in fig. 2. The two possible angular distributions are completely different and spin and parity could thus easily be assigned to each excited state, even for the weak resonances. The results for the thirteen resonances are listed in the fifth column of table 1. The five resonances numbered 1 through 5 were reported earlier by Phillips s), and his spin assignments are in agreement with the results presented here. It appears that the resonances 12 and 13 in the 34S(cr, y)3sAr reaction can be identified with the resonances at Ep = 797.6 and 824.0 keV in 37Cl(p, y)3*Ar [ref. ‘)I. The corresponding excitation energies are within the experimental error; however, there is no further evidence for this result. There is also some indication that resonances 8 and 9 in the reaction 32S(c(, y)36Ar can be identified with the resonances at E, = 1715 and 1743 keV in the reactions 35Cl(p, y)36Ar [ref. ‘)] and 35Cl(p, CQ,)~~S [ref. “)I. In this case, the spin assignments are in agreement with the (p, rz,,) data and the resonance (90 %) ground-state data.

at E, = 1715keV in the proton-capture data transition, as does the corresponding resonance

exhibits a large in the a-capture

3.3. YIELDS

The relative strengths of the resonances were calculated from the areas under the peaks observed in fig. 1. These areas (since the measured widths of the resonance peaks are purely experimental) are proportional to the resonance strengths, after correction for the appropriate isotopic abundance, the detector efficiency, and an angular distribution factor for the 2+ resonances (for which the average yield is not observed at 55”). No further correction was felt to be necessary. Similar calculations were also performed with the y-ray yield determined from the spectra measured at each resonance; the results obtained with both methods were comparable within 15 %.

36*38Ar LEVELS

303

The absolute resonance strengths were finally calculated from the known ‘) strength of the resonance at E, = 3.116 MeV in the 34S(a, y)38Ar reaction: (2J+ I)r,r,/r = 3.0 eV. The results are given in the sixth column of table 1. 3.4. ISOBARIC SPIN SELECTION RULE

The results obtained here give evidence in support of the isobaric spin selection for El radiation in self-conjugate nuclei, as was observed in the reactions 32*34S(~, y) 36938Ar for E, < 3.2 MeV [ref. 7)] and in the reactions 28,3oSi(cr, y)32B34S [ref. ‘“)I. This rule states that El radiation is inhibited in self-conjugate nuclei unless AT = f 1. So in the self-conjugate nucleus 36Ar the 1 - resonances which exhibit an El ground state transition should have a small strength while in the non-self-conjugate nucleus 38Ar the d-capture resonances (2+, E2 radiation) should be naturally weaker than the p-capture resonances (l-, El radiation). This statement can be quantitatively checked by comparing the radiative widths of all observed El transitions. The transition strengths in Weisskopf units lM12 = ry/ryw where ryw is the single-particle shell-model estimate 19) are given in the last column of table 1 ( a nuclear radius of 1.5 A* fm was used in the calculations, in which case one gets T&El) = 0.11 A”E: eV, with Ey in MeV). The ten El transition strengths observed in 38Ar are typical of the El transition strengths found in 2sld nuclei and are all larger than the only El transition strength observed in 36Ar. This is also true if we use the data from ref. ‘) and examine the entire energy region E, = 2.243 MeV. The argument is even stronger if we take into account the fact that only the strongest l- resonances in 36Ar have probably been observed in these experiments and that the weaker El transitions have been missed. In conclusion the isobaric spin selection rule for El transitions seems to be very effective in this mass region.

References 1) P. M. Endt and C. van der Leun, Nucl. Phys. A105 (1967) 1 2) J. A. Weinman, Nucl. Instt. 24 (1963) 390 3) R. J. Jaszczak, R. Nutt and R. L. Macklin, Rev. Sci. Instr. 41 (1970) 28 4) A. Adams, M. H. Shapiro, W. M. Denny, E. G. Adelberger and C. A. Barnes, Nucl. Phys. A131 (1969) 430 5) R. J. Jaszczak, R. L. Macklin, F. E. Dunnam, R. H. Bloomer and H. A. Van Rinsvelt, Rev. Sci. Instr. 42 (1971) 44 6) P. M. Endt, private communication 7) F. C. Eme and C. van der Leun, Nucl. Phys. 52 (1964) 515 8) W. R. Phillips, Nucl. Phys. 60 (1964) 544 9) F. C. Erne and P. M. Endt, Nucl. Phys. 71 (1965) 593 10) L. Simons, E. Spring, A. Fontell, I. Forsblom, P. Holmberg and H. Jungner, Sot. Sci. Fennica, Comm. Phys.-Math. 30 (1965) 6 11) B. Bosnjakovic, J. A. van Best and J. Bouwmeester, Nucl. Phys. A94 (1967) 625 12) B. BoSnjakovic, J. Bouwmeester, J. A. van Best and H. S. Pruys, Nucl. Phys. All0 (1968) 17 13) F. E. Dunnam and P. N. Carlton, Bull. Am. Phys. Sot. 11 (1966) 737

304 14) 15) 16) 17)

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et al.

H. A. Van Rinsvelt, R. E. Clarke and F. E. Dunnam, Bull. Am. Phys. Sot. 13 (1968) 1699 D. D. Watson, Rev. Sci. Instr. 37 (1966) 1605 A. Rytz, H. H. Staub, H. Winkler and F. Zamboni, Nucl. Phys. 43 (1963) 229 P. B. Smith, in Nuclear reactions, vol. 2, ed. P. M. Endt and P. B. Smith (North-Holland, Amsterdam, 1962) ch. V 18) C. van der Leun and G. Wiechers, Nucl. Phys. 52 (1964) 104 19) V. F. Weisskopf. Phys. Rev. 83 (1951) 1073