Measurements of high energy g-rays from collective states

Transcription

1 Measurements of hih enery -rays from collective states Franco Camera University of Milano and INFN sect. of Milano Outline: - ELI-NP and the excitation of collective states - General Physics cases for IVGDR, PDR, IVGQR - Measurement of NRF radiation - Detector required performances - Backround - Electromanetic - Secondary NRF

2 ELI-NP A intense source of almost monocromatic (0.3%) polarized -rays NRF (Nuclear Resonance Fluorescence) reactions The NRF scan will not be limited by particle bindin enery It will be possible to measure the -decay in competition with particle decays

3 Typical NRF experiment IVGDR Excitation Decay

4 Typical NRF experiment IVGQR Excitation Decay

5 Typical NRF experiment PDR Excitation Decay

6 Typical NRF experiment n Excitation Decays

7 IV quadrupole strenth [fm 4 /MeV] IVGDR, PDR cross sections and IVGQR strenth function IVGDR IVGQR PDR Pb SLy5 SGII SkP SkI Enery [MeV] PRL 95, (2005) G.Colo private comunications

8 Physics Cases : NRF reactions Hih intensity monocromatic -rays on stable taret ~ rays per shot GDR-PDR excitation cross section ~ mb - Thin liht Taret (Ni 0.1 /cm 2 d=0.11 mm) 2 excited nuclei per shot - Thick liht Taret (Ni 1 /cm 2 d=1.1 mm) 20 excited nuclei per shot - Thin heavy Taret (Pb 0.1 /cm 2 d=0.09 mm) 0.6 excited nuclei per shot - Thick heavy Taret (Pb 1 /cm 2 d=0.9 mm) 6 excited nuclei per shot Thick heavy Taret (Pb 1 /cm 2 ) 6 excited nuclei per shot Events per shot Events per seconds ELI-NP hih intensity will provide several radiative GDR-PDR decays per second ELI-NP hih intensity will provide a lare amount of neutrons in coincidence with low enery -rays

9 Measurement of GDR strenth functions (5-20 MeV) Excitation Decay - Polarization and anular distribution selects the collective mode - The 0.3% BW will permit an accurate scan of the IVGDR strenth function - The accurate measurement of IVGDR full cross section - Measurement of the polarizability neutron skin thickness - Tamii et al PRL 107(2011) Measurement of the GS -decay branchin ratio - Measurement of the level density at E* hiher than particle bindin enery - Beene et al PLB-164(1985)19) In a similar way one can scan the IVGQR strenth function (i.e. PRL 107, (2011))

10 Measurement of PDR strenth functions (7-13 MeV) Excitation Decay - Polarization and anular distribution selects the collective mode - The 0.3% BW will permit an accurate scan of the PDR - The accurate measurement of PDR cross section - Measurement of the % of EWSR exhausted by PDR - Measurement of Symmetry Enery and neutron skin - Wieland et al PRL 102, (2009) and Carbone et al PRC-81(2010) Measurement of PDR structure and width

11 Measurement of PDR-GDR two step decay Excitation Decay -The direct measurement of the PDR-GDR decay to low lyin states allows the extraction of the component of the PDR-GDR wave function. - Direct check of the collective nature of PDR No chane in the two step decay pattern in nearby nuclei Collective Nature of PDR Sinificative chanes in the two step decay pattern structure relates states Non collective - Microscospical measurement of the PDR wavefunction test for nuclear forces

12 Typical NRF experiment Stable Taret ELI-NP Polarized -rays beam A Time resolution better than 3 ns will allow an unambiuos identification of the sinle shot and consequently the a priori knowlede of the enery of the primary -ray A ood enery resolution is not mandatory in the case of direct decay to the round state (the resolution is iven by ELI-NP bandwidth). A ood enery resolution is mandatory in the case of two step decay A ood enery resolution is important for backround rejection

13 Cpunts BaF 2 (lo) Time Response Time Spectra of BaF 2 e HPGe Counts HPGe (lin) t (ns) ELI-NP shot structure 15 ns Fast scintillator detectors easily provides an unambiuos identification of the primary -rays sinle shot. This is not the case for HPGe unless one uses sementation and complex PSA alorithms (NIM-A A620 (2010) e NIMA 80 (1970) 233)

14 Enery Response One shot from ELI-NP consist of 10 5 amma rays within a bandwidth of 0.3% NIMA to be submitted Independently on the detector enery resolution the measured Full Enery Peak events will correspond to -rays within the ELI-NP bandwidth (s = 0.3 %) The critical point is a clean Full Enery Peak

15 Backround in ELI-NP Startin point : A clean 100% monocromatic (s = 0.3%) ray beam Detectors will not measure NRF amma radiation only, but in addition: - Continuous, beam correlated backround comin from the electromanetic interaction of the ELI-NP amma beam in the taret: - Compton Scatterin - Pair production - Discrete, beam correlated backround enerated by NRF radiation induced by the electromanetic. backround

16 Backround in ELI-NP 1) Continuous, beam correlated backround comin from the electromanetic interaction of the ELI-NP beam in the taret: An ELI-NP hih enery ray will interact with the taret throuh Compton and Pair Production interaction - It is beam correlated (no TOF technique to reject it) - The cross section is extremely hih - We expect a continuous spectrum toether with a lare 511 kev component - We expect an anular anisotropy of such backround (Direct Compton is forward peaked) - We expect spatial correlation of 511 kev -rays (from annihilation of positrons) - They will arrive in a detector within few ns in a time window of some ps, consequently one will measure their sum-enery - They will blind the detector Geant Simulation Simulations from O.Wieland

17 Backround in ELI-NP 1) Continuous, beam correlated backround comin from the electromanetic interaction of the ELI-NP beam in the taret: x 8 LaBr 3 :Ce at 30 cm rays Taret Density Anle -rays in the detector in one shot 511 kev Ni 3 /cm Ni 3 /cm Pb 3 /cm Pb 3 /cm Pb 2 /cm Pb 1.5 /cm Simulations from O.Wieland

18 Backround in ELI-NP 1) Continuous, beam correlated backround comin from the electromanetic interaction of the ELI-NP beam in the taret: x 8 LaBr 3 :Ce at 30 cm rays Physical pile up: Is stroner at forward anles Scales with the taret Z Scales with taret thickness Physical 511 kev pile up: Rejected with PET conditions Does not depend on anles Scales with taret thickness Taret should not be too thick! Simulations from O.Wieland If this backround does not pile up it could be partially identified and rejected

19 Backround in ELI-NP 2) Discrete, beam correlated backround enerated by NRF radiation induced by the previous e.m. backround Such kind of event mimic a NRF reaction with a different primary beam enery. It is much weaker if compared with the previous type of backround but it will produce discrete -rays. Compton Nuclear photoabsorption It should not contribute to the physical pile-up but it is a contribution which should be taken into account. A ood detector time resolution can sinificantly reduce the importance of such type of backround. Enery spectra of the -rays produced by rays of 10 MeV on a 3/cm 2 Pb taret Example: There are rays in the enery interval kev Simulations from O.Wieland

20 Summary: ELI-NP will be a intense source of quasi monocromatic and polarized -rays. With ELI-NP it will be possible to perform several experiments at the same time: Elastic scatterin experiments (IVGDR, IVGQR, PDR) Two step decay experiments (IVGDR, PDR) With ELI-NP it will be possible to study several key physics cases: PDR collectivity Neutron skin thickness one or two step round state decay fine structure of collective states Beam correlated backround in ELI-NP could interfere in NRF measurements Beam correlated n backround in ELI-NP could be rejected choosin the proper detector TOF measurements PSA techniques

21 Thanks for the attention

22 Spare transparenciees

23 Elastic Scatterin in ELI-NP Hih enery -rays elastic scatterin is a known technique to measure IVGDR and IVGQR i.e. Morsh works i.e. PRL 107, (2011) and PRL 68, (1992)3507 Cross sections is a coherent sum of amplitudes from 4 different mechanisms Nuclear Resonance Delbruck scatterin Nuclear Thomson Rayleih scatterin Only Nuclear Resonance amplitude is sensitive to IVGDR and IVGQR (with polarized -rays) ELI-NP hih intensity, small bandwidth and > 95% polarized -rays Detector hih FEP efficiency for hih enery -rays excellent time resolution ood enery resolution ( ELI-NP bandwidth) lare coverae in anles in the two polarization planes ( and )

24 From Milano Habs talk

25 IVGQR Strenth Function in 208 PB - Strenth is present at low enery - Strenth is different for different forces From M.Brenna and G.Colo A scan in incident photon beam enery The identification of the E2 character of the -ray A direct ate on the 3 - low lyin 208 Pb state

26 GDR-PDR two step -decay over the threshold Example on ISGQR Brenna et al Phys. Rev C 85, (2012) Forces which are capable to correctly reproduce the ISGQR centroid and B(E3) have very different predictions for the decay strenth on the first 3- state

27 Physics case 3 GDR-PDR neutron decay Almost 100% efficient array Very compact confiuration ( sum-enery) Basic idea The transitions identify the reaction channel selection of the (,n) reaction En = E 0 SumE(measured ) En will be extracted with hih resolution Direct measurement of the neutron part of the GDR-PDR wavefunctions The comparison of the branchin ratio with the CN prediction will ive the direct measurement of the neutron escape width G Neutron emission is times stroner than one LaBr 3 :Ce at 3 m subtends a solid anle 100 smaller than that at 30 cm Basic idea measure neutron + coincident amma ray event check that E 0 = En + E En measured with TOF E measured with LaBr 3 :Ce Direct measurement of the neutron part of the GDR-PDR wavefunctions The comparison of the branchin ratio with the CN prediction will ive the direct measurement of the neutron escape width Bracco et al. PRL-60(1998)2603 R.Alacon et al PRC-R 43(1991)43

28 Physics case 4 GDR-PDR proton decay Almost 100% efficient array Very compact confiuration ( sum-enery) Basic idea The transitions identify the reaction channel selection of the (,p) reaction Ep = E 0 SumE(measured ) Ep will be extracted with hih resolution Direct measurement of the proton part of the GDR-PDR wavefunctions The comparison of the branchin ratio with the CN prediction will ive the direct measurement of the proton escape width G Hunyadi et al PLB 576(2003)253

29

30 Lare LaBr 3 :Ce detectors (3.5 x 8 9 x 20 cm) The presence of the first escape peak miht affect a clean, hih resolution, response of the detector for hih enery -rays - Lare Volume collimated LaBr 3 :Ce - lead cilinder with 1 cm hole

31 A super-intense source of monocromatic (0.3%) polarized -rays Experimental problem Measurement of hih enery -rays HPGe detectors Very Lare NaI detectors Lare LaBr 3 :Ce detectors Enery Resolution 0.2% at 662 kev Time Resolution > 10 ns Linearity 0.05% at 15 MeV Density 5 /cm 3 Z(Ge) 32 - Small Crystals (3 x 3 ) - Low efficiency - Lare 1EP - lare 2EP Very Sensitive to neutron damae Complex handlin - Coolin - FET failures Very hih costs Enery Resolution ~ 6 % at 662 kev Time Resolution ~ 2-3 ns Linearity bad Density 3.7 /cm 3 Z(I) 53 - Very Lare Crystals (10 x 10 ) - Very hih efficiency - small 1EP (with collimator - No 2EP No Sensitivity to neutron damae Easy Handlin PMT non idealities Low Costs Enery Resolution ~ 3 % at 662 kev Time Resolution ~ 0.5 ns Linearity ood Density 5.2 /cm 3 Z(I) 57 - Lare Crystals (3.5 x 8 ) - hih efficiency - small 1EP (with collimator) - No 2EP No Sensitivity to neutron damae Easy Handlin PMT non idealities Hih Costs Talk of C. Ur S.S.Henshaw et al PRL 107(2011)222501

32 Physics case 5 lowest states of 12 C with spin and parity quantum numbers 0 +, 2 +, and 1 Note: The 1 - dipole state in 12 C has approximately the same enery as the Λ(1s 1/2 1p j ) 1 ω sinle-particle excitations of 13 ΛC they can mix ELI-NP can excite the correspondent states in pure nucleonic 13 C N. Pietralla et al. PLB681 (2009) known levels of the hypernucleus 13 ΛC Gamma detector can measure -decay, -rays nature and level quantum number Is the splittin identical? The 3/2 +, 5/2 + doublet structure observed near 4.8 MeV has been interpreted as the Λ hyperon in the Λ(1s 1/2 ) orbital weakly coupled to the state of 12 C The 1/2, 3/2 doublet near 10.8 MeV is due to the Λ hyperon in the Λ(1p 1/2, 3/2 ) orbitals