Proton spin-flip in inelastic scattering 52Cr and 54Fe

2.B: 2.L [

Nuclear Physics A97 (1967) 561--566; (~) North-Holland Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilmwithout written permissionfrom the publisher

PROTON

SPIN-FLIP

I N I N E L A S T I C S C A T T E R I N G 52Cr A N D 54Fe R. BALLINI, N. C1NDRO, J. DELAUNAY, J. FOUAN, M. LORET and J. P. PASSERIEUX Centre d'Etudes Nucldaires de Saclay, France

Received 17 February 1967

Abstract: Proton spin-flip at 11 MeV was measured by the PP'7 technique on the first 2 + level of ~Cr (Q ~ -1.43 MeV) and the three lowest 2 ~ levels in 54Fe (Q : -1.41, 2.96 and 3.16 MeV). Comparisons of the measurements are made with the predictions of the collective model. Only the fit for the second 2+ in 5~Fe (Q ~ -2.96 MeV) is acceptable. E

NUCLEAR REACTIONS ~'2Cr,SaFe(p, P'7), E = 11 MeV; measured a(Ep,, Er, 0); deduced proton spin-flip probability for 2+ states. Enriched 54Fe target.

I

I

I. Introduction Recent experiments on p o l a r i z a t i o n 1,2) a n d c o r r e s p o n d i n g calculations 2,3) at i n t e r m e d i a t e energies show that p o l a r i z a t i o n a n d spin-flip m e a s u r e m e n t s are m o r e sensitive to nuclear structure (reflected by the f o r m factor) t h a n the inelastic p r o t o n scattering which d e p e n d s m a i n l y on the spin a n d p a r i t y changes involved in the transition. W e m e a s u r e d previously 4) p r o t o n spin-flip for the first 2 + levels on N i a n d Zn isotopes a r o u n d 11 M e V essentially to study the r e a c t i o n m e c h a n i s m a n d to see if the scattering m a t r i x calculated by the v i b r a t i o n a l m o d e l (which accounts fairly well for the inelastic scattering on the first 2 + levels) is able to fit the data. F o l l o w i n g a suggestion o f Satchler a n d Perey 5), we m e a s u r e d the spin-flip p r o b a bility for 11 M e V p r o t o n s scattered, respectively, on the first 2 + level o f 52Cr a n d on the three lowest 2 + levels o f S4Fe. These two nuclei have 28 neutrons, a n d a (lf._)" p r o t o n configuration seems a r e a s o n a b l e way to describe the low-lying states. This assumed simple structure should be reflected on the spin-flip p r o b a b i l i t y s). W e also c o m p a r e d o u r spin-flip m e a s u r e m e n t s with those o f a s y m m e t r y in inelastic scattering o f G a r i n et al. 1) on the same nuclei. T h e spin-flip m e a s u r e m e n t s were p e r f o r m e d using the PP'Y m e t h o d 6) which consists in m e a s u r i n g the a n g u l a r c o r r e l a t i o n between p r o t o n s scattered inelastically f r o m a 2 + excited level a n d the E2, 2 + ~ 0 + ( g r o u n d state) de-excitation 7-ray emitted p e r p e n d i c u l a r l y to the r e a c t i o n plane. A very general s y m m e t r y principle due to Bohr a n d the p o l a r d i a g r a m o f the r a d i a t i o n intensity for pure l = 2, m multipoles show that only p r o t o n s which flip their spin can p o p u l a t e the m = + 1 substates c f 561

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the 2 + state and only the corresponding radiation contributes along the quantization axis perpendicular to the reaction plane.

2. Experimental set-up Targets of 52Cr and 54Fe (natural chromium and isotopically enriched iron ( ~ 99 %) approximately 1 mg/cm 2 thick were bombarded by 11 MeV protons. The beam was defined with a 2 mm diana tantalum diaphragm followed by antiscattering 3 mm diam graphite slits. The whole inside of the 36 cm diam scattering chamber described earlier 7) was lined with 0.3 mm thick tantalum to reduce bremsstrahlung and avoid activation. The targets were mounted without frame in a tantalum holder at 45 ° with respect to the beam direction. The gamma background was reduced to low level in the following manner: (i) The beam was carefully focussed through the defining diaphragms which were surrounded with several cm t>f lead. (ii) Every part seen by protons and/or scattered particles was covered with tantalum. (iii) A Faraday cup situated 3.5 m from the chamber was shielded with a lead and concrete block assembly as to form a beam dump. The set-up was found to be quite effective; no difference was found in the gamma detector counting rate when the beam passing through the target holder hole was turned on or off. I11 addition to the Faraday cup current integrator, a fixed position silicon detector was used to monitor the beam intensity. A chance coincidence counting system was set between the two gamma detectors in order to control the loss in counting rate due to beam ripple. The scattered protons were detected in about 2 msr solid angle with a 1.3 mm t h i c k surface-barrier silicon detector (La Radiotechnique). This detector was polarized with 400 V and cooled to - 3 0 ° C by means of a Peltier effect thermocouple as to reduce the reverse current and increase the carrier mobility in the junction. The signals from the detector were led through a time pickoff unit (ORTEC model 260) into a charge sensitive preamplifier and then amplified with a 1 izsec double differentiation C R - C R pulse shaping. The overall energy resolution of this set-up was approximately 50 keV mainly due to the target thickness and solid angle effects. The gammas were detected with a 10.2 cm x 10.2 cm Nal(TI) crystal followed by an XP 1040 fast photomultiplier. This detector was situated above the chamber in a lead pit as to define a _+ 10 ° acceptance solid angle at 90 ° with the reaction plane. The fast output of the photomultiplier was directly connected to another time pick-off unit. The energy resolution was about 10 % for 6°Co. The beam intensity was kept down to approximately 0.02 pA in order to keep the total gamma counting rate below 3 • 104 counts/sec, in these conditions the coincidence rate for a single level was of the order of a few per rain. The slow output was connected to a charge sensitive preamplifier and the signal amplified with a 0.3 izsec CR-CR pulse shaping. The signals from the proton and gamma time-pick-off units, respectively, started and stopped a time to pulse-height converter (ORTEC model 263). The time resolution

PROTON SCATTERING

563

was better than 3 nsec F W H M for a single level coincidence peak, this width being mainly due to transit time fluctuations in the photomultiplier. The proton energy pulses were fed directly into a fast analogue-to-digital converter (maximum deadtime 60 ys) in order to get the pure inelastic scattering spectrum and the three pulses (proton and g a m m a energies and the P-7 time difference) into three analogue-to-digital converters (ADC), respectively, which were gated by a slow (1 ysec) coincidence followed by a dead-time unit. In the experiment on S2Cr the outputs of the A D C were connected to separate 1024-channel memories and single-channel analysers were used on each parameter -L A c c I

r

] CONVERTERI --~ L=MT~I';FIER

--]

-{~Er,~oRYJ

,-~

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MULTIPL'I

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Fig. 1. Block diagram of the electronic arrangement showing schematically the multi-parameter set-up. (proton and g a m m a energies and the P-? time difference) to take into account only the first 2 + level. The data was then analysed by integrating the time spectrum peak and deducing chance coincidences which appeared on this spectrum as a fairly uniform background. In the experiment on 54Fe, we used a multiparameter system for information storage 8). This enabled us to get simultaneously the necessary information for spinflip probabilities on the lowest three 2 + states of 54Fe at 1.41, 2.96 and 3.16 MeV excitation, respectively. Here the single-channel analysers were set wide open. The outputs from the three coincidence A D C were connected to the input units of the

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multiparameter system in order to characterize every nuclear event by a digital word with the three above parameters as syllables in order to form a three dimensional space of 1024 x 1024 x 512 channels. The words were first stored in a derandomizer and then recorded in two halves on a 16-track constant speed tape, thus giving us an unlimited capacity (number of events) for the three-dimensional space. The recorded information was monitored by integration and cathode ray tube display of the spectrum of each parameter separately in 1024 channel memories. Fig. 1 shows the block diagram of the electronic arrangement. Spin-flip probab,hty alon 9 k I A kf 2 + levels

I

!T + I

52

p(11MeV)- Cr q = - 1.45M eV

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15d °

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Fig. 2. Spin-flip p r o b a b i l i t y for the first 2 + level o f ~2Cr. The curve shows the pre di c t i ons o f the collective model.

The data processing was as follows: The tape was read through conditioner units into a CAE 510 computer. The conditioners acted as single-channel analysers in coincidence on proton energy and time syllables. In this way we fed into the computer g a m m a spectra in coincidence with different proton peaks and time channels. The computer program then delivered g a m m a spectra corresponding to true coincidences with different proton peaks.

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3. Results Experimental results for the angular distribution of the spin-flip probability on 52Cr and 54Fe are shown in figs. 2 and 3, respectively. The theoretical predictions

added for comparison need some comment. We have used the coupled equations code of Buck ( O R N L ) modified by Perey in order to include the spin-flip. The calculations used a coupling potential close to zero (V02 ~ 0.1 MeV); thus our calculations were essentially equivalent to the DWBA. Further, we have verified that for at least the first 2 + state in 54Fe (Q = - 1.41 MeV), the spin-flip probability is not very sensitive to the coupling potential; in fact using V02 ~ 0.1 MeV (viz. fl = 0.001) and the usual 5gin-flip probabilit/

Predictions or collective modle

~ 1 ,~

=-

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2

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Igo

Fig. 3. Spin-flip p r o b a b i l i t y for t h e t h r e e l o w e s t 2 + s t a t e s o f ~ F e .

V02 ~ 16 MeV (viz. fl = 0.16), the calculated angular distribution of the spin-flip probability remained practically unchanged. Thus we performed the D W B A calculations. The optical potentials employed for 54Fe were those of Perey 9) for 54Fe at 11 MeV namely, Vr~.l

=

50.6 MeV,

grea!

Wim~g =

7.8 MeV,

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=

MeV,

= 1.25 fro,

areaj

= 65 fm,

Rim~g = 1.25 fro,

aimag

= 47 fm,

Rso

aso

= 65 fm.

= 1.25 fro,

566

R. BAkLIN1e t

al.

a n d for 52Cr, the i n t e r p o l a t e d values were o b t a i n e d f r o m ref. lo), n a m e l y Vreal =

50 MeV,

Wimag =

12 MeV,

Vso = 7 MeV,

a n d the same geometrical p a r a m e t e r s as for 54Fe. The e x p e r i m e n t a l p r o b a b i l i t i e s are given in a r b i t r a r y units (the same for the three levels o f 54Fe). To have the absolute probabilities, we need the a b s o l u t e efficiency o f o u r V-spectrometer. This calculation is being done at present with the Y O G I code 11 ). T h e e x p e r i m e n t a l points for the three different levels in 54Fe have been n o r m a l i z e d using an i n t e r p o l a t i o n o f the p u b l i s h e d efficiencies o f N a I crystals ~2). I n the calculated results o f 54Fe, a unity represents 20 O//oo f p r o b a b i l i t y . The fit for the 2.96 MeV level in 54Fe is the only satisfactory one, for the other levels, the fits are unacceptable. Recently ~3) Satchler tried to fit the inelastic scattering a s y m m e t r y S~Fe (Q = - 1.41 a n d - 2 . 9 6 M e V ) (i) In the f r a m e w o r k o f the collective m o d e l where, however, the real, i m a g i n a r y a n d s p i n - o r b i t terms o f the optical p o t e n t i a l u n d e r g o shape changes, the fit for 54Fe is better but still unsatisfactory. (ii) In the f r a m e w o r k o f the shell model, with a local Y u k a w a interaction with a range o f a p p r o x i m a t e l y 1 fm a n d strength a r o u n d 200 MeV, the results o f the calculations c o u l d n o t a c c o u n t for the experiments. The a d m i x t u r e o f (f~-3 p ~ ) c o n figurations a n d the use o f different interaction ranges did n o t i m p r o v e the fit. i t has been suggested t h a t shell-model calculations using c o m p l e x interaction and a m o r e general spin-orbit c o u p l i n g m i g h t i m p r o v e the s i t u a t i o n 13). Calculations are in progress in Saclay to calculate the spin-flip p r o b a b i l i t y with the D W B A code JUL1E ( O R N L ) which allows a r b i t r a r y form factors t h a t can be calculated i n d e p e n d e n t l y using different m o d e l s o f nuclear structure and interaction. W e also p l a n to use different f o r m factors for the s p i n - o r b i t p o t e n t i a l (which is always t a k e n o f the T h o m a s form). We are i n d e b t e d to the O a k Ridge g r o u p for their suggestions a n d use o f their codes. The efficient help o f B. D e l a u n a y a n d the accelerator g r o u p is gratefully a c k n o w l e d g e d . I) 2) 3) 4) 5) 6l 7) 8) 9) 10) 11) 12) 13)

References A. Garin e t al., Phys. Lett. 21 (1966) 73 D. J. Baugh et al., Proc. Gatlinburg Conf. (Sept. 1966) to be published J. Delaunay, C. Glashausser, R. de Swiniarski and J. Thirion, Saclay 1966. NoteCEAN 621,p.l 10 J. Delaunay, J. P. Passerieux and F. G. Perey, to be published; F. G. Perey, Proc. 2rid Int. Symp. on polarization, Karlsruhe, Sept. 1965, (Birkh~.user Verlag, Basel, 1966)p. 181 G. R. Satchler, Phys. Lett. 19 (1965) 312; F. G. Perey, private communication F. H. Schmidt, R. E. Brown, J. B. Gerhart and W. A. Kolasinski, Nuclear Physics 52 (1964) 353 B. Detaunay e t al., No. 91 (1965) - B.I.S.T. and NIM 35 (1965) 245 R. Ballini, D. Daronian and A. Pag6s, Compte Rendu d'Activit6 note CEA N 621, p. 64 ORNL 3714, part 1, p. 1 D. R. Winner and R. M. Drisko, unpublished (June 1965) S. Kellmann, UCRL 12005 C. E. Crouthamel, Applied gamma ray spectrometry (Pergamon Press, London, 1960) G. R. Satchler, Nuclear Physics A95 (1967) 1