Investigating Quasi-Continuum Decay to Discrete Levels

Transcription

1 Investgatng Quas-Contnuum Decay to Dscrete evels Maths Wedekng Themba ABS, Somerset West, South Afrca Ths work s based on research supported by the Natonal Research Foundaton of South Afrca and under the auspces of the U.S. Department of nergy awrence vermore Natonal aboratory under contract D-AC52-07NA27344 and Unversty of Rchmond under D-FG52-06NA26206 and D-FG02-05R For awrence Berkeley Natonal aboratory ths work was supported by the Drector, Offce of Scence, Offce of Nuclear Physcs, of the U.S. Department of nergy under Contract No. D-AC02-05CH Support from the research plan MSM of the Mnstry of ducaton of the Czech Republc and from the Czech Scence Foundaton under Grant No S s also acknowledged.

2 Outlne. Motvaton.. Revew of expermental approach to study the regon of hgh-level densty.. 95 Mo results and conclusons. v. 74 Ge campagn and prelmnary results. v. Spectrometer + Clovers at T. v. Summary.

3 ow-nergy nhancement Frst report of enhancement. Tavukcu, Ph.D. thess, North Carolna State Unversty, Contnued observatons of low-energy enhancement for lght- and medum-mass nucle. Much debate over the last decade f the enhancement s real or an artfact. Some experments do not provde conclusve evdence. Others show that the lowenergy enhancement does not exst.

4 Impact of PDR and -nhancement The low-energy part s partcularly mportant for understandng nuclear reactons n astrophyscal envronments. Predctons compared to GO estmates for T=10 9 K typcal of r-process synthess. Impact of PR on the r-abundance dstrbuton wth T=10 9 K and neutron densty cm -3. Abundance calculatons wth standard GDR component and the GDR+PR strength compared to the solar system. Use of GDR produces A~90-110, PR accelerates producton of A~130 S. Gorely, Phys. ett. B 436, Influence of the enhancement on n,g reacton rates becomes more mportant wth ncrease n neutron number. ffect s sgnfcant and can ncrease reacton rates by order of magntude A.C. arsen and S. Gorely, Phys. Rev. C 82,

5 Drect reactons Motvaton and xperment Applcaton: Impact of astrophyscal envronments on reacton rates? n-capture Guttormsen, PRC 71, Physcs: Does the low-energy enhancement exst? What s ts orgn? Do we all measure the same thng? Reacton dependence? Valdty of the Axel-Brnk Hypothess? Needed: New expermental approach Use drect or scatterng reactons 94 Mod,p 95 Mo. Clover and Slcon detectors STARS-IBRAC at BN AFRODIT + Slcon at T

6 Revew: xtractng nformaton 1 2, f f f N j j j, 3 j γ = - j xplotng ths equaton and assumng dpole transtons domnate: N j corrected for effcences event-by-event and branchng ratos.

7 95 Mo data /2+ γ = - j 13 states wth enough statstcs: 7 states of 3/2 + Two ways to present the results: 2 states of 1/2 + 2 states of 7/2 + 2 states of 9/2 + χ 2 method Rato method j / / / / / / / / / / / /2+ 95 Mo 3/2 + states

8 Strength functon MeV Mo 3/2 + states comparson 2-08 = 5 MeV data ponts M. Guttormsen et al., PRC71, γ MeV ach set of ntenstes, for a gven, feedng states of same spn and party orgnates from: 1 same level densty. 2 Same spn and energy dependence of d,p cross secton. χ 2 mnmzaton for each set of ponts at.

9 95 Mo: χ 2 mnmzaton 95 Mo 3/2 + states

10 = 204 3/2 + 2 = / = 821 3/2 + 2 = / Rato 1 = /2 + 2 = / Rato xctaton nergy MeV 1 = 204 3/2 + 2 = 821 3/2 + xperment, Guttormsen 3/2+ 95 Mo 3/2+ 95 Mo γ1 = 1 γ2 = N N f f R 24 ratos n total: Independent of level densty, d,p cross-secton, systematc uncertantes f N j j j, 3

11 Photon strength n 95 Mo d,p reactons to populate 95 Mo. New expermental approach studyng statstcal decay to ndvdual dscrete levels. proton-gamma-gamma concdences to extract prmary transtons. Strength extracted wthout any model dependences or assumptons eg level densty. For the frst tme ndependent confrmaton of the shape of photon strength. Now focus can shft to understandng the underlyng mechansm.

12 Collaboraton Goals Many open questons need to be answered: Statstcal gamma-ray spectra for applcaton purposes NIF. Is the shape of statstcal spectra for a gven nucleus reacton dependent? Do dfferent expermental methods yeld the same result? What s the orgn of the low-energy enhancement? Is the Brnk Hypothess vald? To tackle these and more questons a collaboraton was establshed n 2010: Collaboraton Whte Paper 2010:

13 74 Ge 74 Ge to be studed n a seres of measurements: Use dfferent beams on 74 Ge to populate states p, 4 He, 3 He, gamma, n questons to be answered: s the statstcal spectrum ndependent on ncdent beam? PSF to ndvdual dscrete states n two dfferent reactons usng p and 4 He beams uly 2011 total of 112 hours 4 Slcon annular partcle detectors wth a total of 128 channels and 6 hgh-purty Germanum detectors wth a total of 24 channels. populated n the reacton 74 Gep,p wth 18 MeV. Analyss by B.V. Kheswa Ph.D. student at Themba ABS n South Afrca. Beam tme n November hours 2 Slcon W1 partcle detectors wth 64 channels and 9 Clover detectors. populate 74 Ge nucle n the reacton 74 Ge 4 He, 4 He 47 MeV Analyss by D. Neg post-doc at Themba ABS n South Afrca Combnng these results followng ndvdual analyses wll be very nterestng and educatonal.

14 Rato Rato 74 Ge: Rato Method / xtracted quas-contnuum feedng to 7 states red. Other good canddates are shown n black. Total of fve 2+, three 3+, two 3-, fve 4+, one 6+ χ 2 mnmzaton for 2+ and 4+ levels and Rato Method: 10 for 2+ and 4+, three for 3+, one for 3- These ratos are ndependent of level densty, cross-secton, systematc errors. γ1 = γ2 = 2 N j R f f f 1 2 j N N 1 2, j Thanks to B.V. Kheswa / = = = = xctaton nergy MeV xctaton nergy MeV

15 Themba ABS The Themba aboratory for Accelerator-Based Scences s a group of mult-dscplnary research laboratores admnstered by the Natonal Research Foundaton of South Afrca. Isotope Producton, Medcal Irradaton, Research. 3 accelerators K200 cyclotron, 11 MeV cyclotron and 3MV Van de Graaff Materals Research. K200 Separated Sector Cyclotron Themba ABS Cape Town Themba ABS Gauteng Accelerator Mass Spectrometry

16 Separated-Sector Cyclotron Faclty Beam tme s shared

17 Nuclear Physcs Scatterng Chamber: reacton mechansm studed wth lght and heavy ons. Two movable arms makes t useful n measurng angular dstrbuton n dfferent nuclear reactons. Quas-monoenergetc neutrons of energy MeV produced n the 7 p,n 7 Be or 9 Bep,n 9 B reactons. Faclty used for physcs studes and applcatons eg. Metrology, nvronmental Radaton aboratory 9 Compton suppressed Clover detectors and 8 PS detectors can be coupled to CSI DIAMANT, recol detector, annular and square slcon detectors, solar cells, Cologne plunger. Dgtal electroncs XIA as of uly K600 used for hgh resoluton 25 kev measurements n the p energy regon to 200 MeV. Zero degree mode avalable. Flght path for a partcle from target to focal plane ~8m. Focal plane conssts of mult-wre drft chambers and a par of plastc scntllaton detectors. Dec 2012: coupled AFRODIT to K600 for the frst tme.

18 K600+AFRODIT 24 Mgα,α at 160 MeV 15 mn wth 0.8pnA on 2.1mg/cm 2

19 Concluson. Method to characterze feedng from the quascontnuum to dscrete states.. 95 Mo data and comparson to Oslo data.. Confrmaton of the low-energy enhancement. v. Frst theoretcal nterpretaton talk by tvnova. v. Investgatng reacton dependence of PSF to states of dfferent spn/party n 74 Ge. v. Couplng Spectrometer, slcon, and γ-ray detectors at Themba ABS South Afrca.

20 Collaborators.A. Bernsten D.. Bleuel.T. Burke A. Krtcher R. Hatark S.R. esher N.D. Scelzo D.H.G. Schneder A-C. arsen S. Sem A. Görgen. Sahn M. Guttormsen T. Renstrøm T. W. Hagen F. Gacoppo M. Klntefjord H-T. Nyhus F..B. Garrote M. Krtčka B.. Goldblum P.T. ake K. Van Bbber P.F. Davs D. Neg O. Shrnda. Brown B.V. Kheswa. aston P. ones. Ndayshmye S.N.T. Majola T.S. Dnoko S.P. Noncelela R.A. Bark P. Papka B.V. Kheswa R.T. Newman.A. awre M.A. Stankewcz.. awre.m. Allmond.N. Orce N. rasmus N. Kumalo M.S. Basuna P. Fallon R.B. Frestone I-Y. ee S. Paschals M. Petr. Phar A. Hurst S. Bvumb.P. Masteng