The cross sections for reaction channels with different configurations are of key interest for the understanding of the photonuclear giant resonance process

Brajnik, D.

Jamnik, D.

Kernel, G.

Miklavžič, U.

Stanovnik, A.

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FIZIKA, 4 (1972) 65-68 LETTERS TO THE EDITOR THE PHOTODISINTEGRATION OF 40Ca THROUGH PROTON CHANNELS D. BRAJNIK, D. JAMNIK, G. KERNEL, U. MIKLAV:tIC and A. STANOVNIKInstitute Jof.ef Stefan, Ljubljana and University of Ljubljana, YugoslaviaReceived 25 June 1971 The cross sections for reaction channels with different configurations are of key interest for the undcrstanding of thc photonuclear giant resonance process. One way of obtaining such information is the study of cross sections leading to different states ot the residual nuclci. It was the aim of the pre­sent work to get expcrimental data for calcium by measuring photoproton spectra. The protons rcsulted from bombardment of a 6.6 mg/cm2 4°Ca target by betatron bremsstrahlung gamma rays with endpoint energies ranging from 15.2 to 24.6 MeV. They were detccled by silicon solid stale spcctro­meters. The 90° differential photoproton cross sections corresponding to dif­ferent excited states of the residual nucleus (Fig. 1) bave been extracted from thc photoproton spectra by means of a least-squares procedure. The lower diagram of Fig. 1 shows thc differential cross scction for the reaction (y, p0) in which the residual nuclcus 39K is left in its ground state. The structure and thc absolute value are in good agreemcnt wilh an earlier measurement of thc (y, p0) cross section1> and with the data from thc inverse rcaction21. lt is believed that the ground stale of 39K is a relativcly pure 1 d . , hole state31. In thc 1 p - 1 h coupled-channel shell-model calculation of Ma­rangoni and Saruis4> it should therefore be compared to the cross section for the 1 da/2 hole channel. As seen from Fig. J a  the position of the main peakseems to be well reproduced. For the (y, p1) channel, involving the 2.53 MeV !+ first excited state of39K, the cross section appcars to be conccntratcd to energies above li.S MeV. 66 i:: 12 3 vr • ;• 111' tl: 2 \I.I " v1• \ii rn� 1.. ... "' 5 THR. ::. o � BRAJNIK et al. 16 18 20 22 c. ) '1T. hT . C: :: .. ..... &n ::�.. ""' 2:"'.. .... i----.----..-.--..-.--......... 81T w l1T 2 8n IZ 1' 16 18 20 Fig. 1. Experimcntal points with error bars correspond to the 90° differencial cross section of the present experiment, while solid curves show calculatcd cross �cctions integrated over ang)es41. The absolute values of the cross sections are given by the scales on the Ieft hand side for the experiment, and on the right hand side for the theory. · · , a) The photopr:oton cross section (y, Pa) to the ground state of 39K compared tothe calculated cross scction for the 1 dJ/z bole proton channel;PHOTODISINTEGRATION . • . 67 b) The photoproton cross section to Lhe first cxcited 2.53 McV stale compared to the calculatcd valuc for the 2 s,/i hole channel. At energics bclow 15.5 MeV thccxpcrimental cross section could not bc resolved from thc trans1tions to thestates between 2.82 and 3.95 MeV;c) Expcrimental photoproton cross section to the statcs ol' J9K bctween 2.82 and 3.95 MeV;d) The photoproton cross section to thc l9K states in the cncrgy regional betwecn4.93 and 6.35 McV compared to thc calculatcd cross section for the I d,.,, proton hole channel ;c) The cross section for the (y, p) reaction, in which the rcsidual nucleus 39K isassumed to be Jeft in the continuum states between 6.4 and S.S MeV. Thc insertshows schematically the population of diffcrent excitcd statcs of the residualnucleus 39K in thc photoproton reaction. It has been obtaincd independcntelyby measuring thc dcexcitation gamma-rays resulting from bombarcimcnt of a"°Ca target with brcmsstrahlung gamma-rays of an cnd-cncrgy of 30 McV.The above stale is considered to bc prcdominantly a 2 sa/2 hole statc3> and the cross section is thercfore to bc comparcd to the 1 p - 1 Iz prediction for the 2 s,/2 channel. As secn from Fig. 1 b thc corresponding thcoretical crosssection is conccntratcd at energies below 1 7.S MeV, in disagreement with the experiment. Therc does not scem to be much corrclation bctween thc mcasured and thc calculated results in this casc. The cross section to the rcsidual statcs in the cnergy rcgion bctween 2.82 and 3.95 MeV (Fig. l e) corresponds mainly to the reaction channels with negative parily holc states as i t  is sccn from the population schcme in thc insert of Fig. le. Thc existence of such reaction channcls is out of scope of thc model of Ref.4J. The remaining cross scction to residual bound states belongs to excita-tions bctween 4.93 Me V and 6.35 Me V in 39K. About 60 O/o of the deexcitation gamma-ray yicld in this encrgy region (scc thc population schcme in Fig. 1 )  i s  due to  the 5.27, 5.62 and 6.35 McV states which havc a predominant </;J� holc configuration3>. The study of thc '"'Ca (d, 3He) reactions1 indicate thatthe 1 ds/2 hole statcs in 39K may extcnd to excitation cncrgies as high as 8 McV or more. It yiclds for the lowcr limit of the centcr of gravity of the l ds/2 bole states an cnergy of 6.6 McV. I t  is thereforc probablc that a sub­stantial part of thc measured cross section corresponding to thc continuum rcsidual states (uppcrmost diagram of Fig. 1 )  also belongs to the ds/2 holechannel. If this is thc casc the absolutc value of thc cxpcrimcntal ds/2 con­tribution is of thc same order of magnitudc6l as the thcorctically predictcd one (the solid linc in Fig. ld)41 .From the mcasurcd differential cross scctions for thc reactions 40Ca (y, JJ) one can draw thc following main conclusions: - the rcactions cannot bc quantitativcly describcd in terms of the1 p - 1 h shell model coupled-channcl calculation of Ref.41. The moststriking discrcpancy betwccn thcory and experiment appears in the cross sections for thc reaction (y, p1) to the first cxcitcd state Qf 39K; 68 BRAJNIK et at. - the presence of an appreciable radiation width for the negative­·parity bole channels could be explained by thc impurities in theground statc of 40Ca7, s, which bave not been taken into account in thccalculation of Marangoni and Saruis;- the experimcnt suggests that the wavc functions of thc 40Cagiant-resonancc state contain more 1 d5/2 hole configuration relative tothe 1 d3/2 configuration, than prcdicted by the simple 1 p - 1 11 shell­·model calculalion4, 9);- it scems typical for the mcasured photoproton cross scctions tobave similar gross structure for ali channels. This may suport the assumption of an energy indcpcndent configuration composition of the giant resonancc wave function as already suggcstcd in Ref.hlJ. We thank F. Detter for his work on thc dcsign and construction or the target chamber. R e f e r c n c e s1) C. P. Wu, J. E. E. Baglin, F. W. K. Firk and T. W. Phillips, Phys. Letters 298 (1969) 359;2) J. C. Hafclc, F. W. Bingham and J. S. Allcn, Phys. Rev. 135 (1964) B365;3) S. Hinds and R. Middlcton, Nucl. Phys. 84 (1966) 651;4) M. Marangoni, privatc communication. In lhe paper M. Marangoni and A. M. Saruis,Nucl. Phys. A132 (1969) 649 details of calculation are given, but only summcd photo­proton speclra are shown. Here thcir rcsulls for the absorplivc polcnlial W (McV) == 0.06 E (Me V) - 0.5 are used;5) J. C. Hicbcrt, E. Ncwman and R. H. Basscl, Phys. Rev. 154 (1967) 898;6) Thc anisotropy of thc photoproton reactions is given by thc following avcragc valuesof thc ralio of Legendrc polynomial cocrricicnts A2/� : -0.4 (rcaction a of Fig, I),-O.S (rcactions b + c) and O.O (rc:ictions d + e);7) W. J. Gerace and A. M. Grccn, Nucl. Phys. 93 (1967) 1 10; 6) M. E. Cage, R. R. Johnson, P. D. Kunz and D. A. Lind, Nucl. Phys. Al62 (1971) 657; 9J V. Gillet and E. A. Sandcrson, Nucl. Phys. 54 (1964) 472; 10) R. A. Allas, S. S. Hanna, L. Meyer-Schiitzmcister, R. E. Scgcl, P. P. Singh aml Z.Vager, Phys. Rev. Lellcrs 13 (1964) 628.

The photodisintegration of 40Ca through proton channels

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