1 / 34

EXPERIMENTS WITH LARGE GAMMA DETECTOR ARRAYS Lecture I

EXPERIMENTS WITH LARGE GAMMA DETECTOR ARRAYS Lecture I. Ranjan Bhowmik Inter University Accelerator Centre New Delhi -110067. Why do we study nuclear spectroscopy ?. Nucleus is a many-body quantum mechanical system Effective n-n interaction short range, spin, isospin, density dependent

geona
Download Presentation

EXPERIMENTS WITH LARGE GAMMA DETECTOR ARRAYS Lecture I

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. EXPERIMENTS WITH LARGE GAMMA DETECTOR ARRAYSLecture I Ranjan Bhowmik Inter University Accelerator Centre New Delhi -110067

  2. Why do we study nuclear spectroscopy ? • Nucleus is a many-body quantum mechanical system • Effective n-n interaction • short range, spin, isospin, density dependent • nucleon motion described by a mean field • excited states strongly dependent on the nature of mean field • single particle vs collective excitation • Knowledge of excited states essential to understand the nature of mean field Lecture I SERC-06 School 13/2/06 - 2/3/006

  3. Mean Fields in nuclei • Closed shell nuclei : spherical • Mid-shell nuclei : • deformed • Mean field strongly affected by excitation and rotation • Need to study as a function of N,Z,I,E* Lecture I SERC-06 School 13/2/06 - 2/3/006

  4. Experimental Observables • Wave function overlap • Transition probability • Life time • Branching ratio • Quadrupole moment • Magnetic moment • Excitation energy , spin, parity Lecture I SERC-06 School 13/2/06 - 2/3/006

  5. Methods of Excitation • Radioactive decay : Limited in excitation energy • Direct Reactions: (p,p'), (p,n), (p,d), (p,t), (p,a), (a,a') • limited -transfer • Coulomb excitation: Collective 2+,3- states • Higher levels through multiple Coulomb Excitation • Fusion-evaporation: Non-selective; predominantly xn • high spin levels populated Neutron-deficient nuclei • Deep-inelastic: Wide-range of nuclei, limited E*,J Lecture I SERC-06 School 13/2/06 - 2/3/006

  6. Gamma Spectroscopy • Charge Particle spectroscopy: • E* obtained from Q-value • Resolution 10-100 keV • -transfer from angular distribution • Gamma spectroscopy: • Energy difference measured • Eg measured with 0.1-0.5 KeV • Change in I,p measured • Levels of similar energy but different decay path identified Lecture I SERC-06 School 13/2/06 - 2/3/006

  7. Progress in g-spectroscopy NIM25(1963)185 • 1950: NaI detectors • Limited resolution • 50 keV @ 661 keV • 1962 : • Li-drifted Ge detector • volume 1cc • 1% photopeak efficiency • resolution 6 keV @ 1332 keV • 1970: • g-g coincidence 2keV @ 1332 keV • 1990: • Large multi-detector array Ge(Li) NaI Lecture I SERC-06 School 13/2/06 - 2/3/006

  8. Progress in g-spectroscopy Nucl. Phys. 46 (1963) 210 Nucl. Phys. A219(1974)543 g-band -ve parity band gs-band 162Dy Lecture I SERC-06 School 13/2/06 - 2/3/006

  9. Progress in g-spectroscopy • Large increase in sensitivity due to high-fold coincidence data Nucl. Phys. A389(1982)218 Nucl. Phys. A764(2006)42 160Gd(a,2n)162Dy Lecture I SERC-06 School 13/2/06 - 2/3/006

  10. Complete Spectroscopy of 162Dy Nucl. Phys. A764(2006)42 Lecture I SERC-06 School 13/2/06 - 2/3/006

  11. Topics to be covered • Fusion Dynamics: g-Multiplicity • Large Gamma Arrays: Instrumentation • Data Acquisition & analysis • Assignment of spin & parity • Measurement of nuclear lifetimes • g-factor, Quadrupole moment, & other topics Lecture I SERC-06 School 13/2/06 - 2/3/006

  12. Reading Material • IN-BEAM GAMMA-RAY SPECTROSCOPY, H. Morinaga & T. Yamazaki, North Holland Publishing Company, 1976 • Large Arrays of Escape-suppressed Gamma Ray Detectors,P.J. Nolan et al, Ann. Rev. Nucl. Part. Sci. 45(1994)561 • Experimental methods of Nuclear Spectroscopy: High Spin States, R.K. Bhowmik in Structure of Atomic Nuclei, ed. L. Satpathy, Narosa Publishing House, 1999 • Nuclear Structure Experiments I, Pragya Das in Mean Field Description of nuclei, ed. Y.K. Gambhir, Narosa Publishing House, 2006 • http://www.ph.surrey.ac.uk/~phs1pr/lecture_notes/nuc_expt_phr03.pdf Lecture I SERC-06 School 13/2/06 - 2/3/006

  13. FUSION REACTION • AP(NP,ZP) + AT(NT,ZT)  AC*(NC,ZC) • NC = NP + NT ; ZC = ZP + ZT • Ecm =Elab *AT/(AP+AT) • EC* = Elab - (MC - MP -MT)c2 Lecture I SERC-06 School 13/2/06 - 2/3/006

  14. Fusion Dynamics V(r) = Vnucl(r) + Vc(r) + V(r) • Barrier described by RB, VB • Max.  given by maxħ= • [2m(Ecm -VB)]½.RB • <>  2/3 max • Fusion xsec: • f = 2 (2+1)T • = RB2 [1-VB/Ecm] Lecture I SERC-06 School 13/2/06 - 2/3/006

  15. DECAY OF COMPOUND NUCLEUS • Decay of thecompound nucleus depends on level density distribution w(E*) ~exp(E*/T) • Effective temperature T ~ (E*/a)½ • The evaporated particles have an energy distribution N(E)  (E-C) exp(-E/T) • Mean energy carried away by particles B+C+2T • For heavy nuclei, charged particle emission is suppressed due to Coulomb Barrier C; neutron emission preferred • Cooling of a nucleus at high spin brings it closer to Yrast line Lecture I SERC-06 School 13/2/06 - 2/3/006

  16. Decay of compound nucleus • 36S+124Sn at 150 MeV • Ecm = 116.2 MeV • E*(160Dy) = 67.0 MeV • RB = 10.9 F • VB = 94.3 MeV • max = 58 • T* ~ 2 MeV • f = 700 mb Reduced level density at high spin w(E*, J) ~ (2J+1) w(E*-Erot, J=0) Lecture I SERC-06 School 13/2/06 - 2/3/006

  17. Angular Momentum Balance • Particles carry away very little angular momentum • n ~ 15% of <> • Nuclei close to Yrast line would preferably decay by g-emission • Statistical gamma emission • Ns ~ 4 • s ~ 0.6 ħ /photon • Nc transitions along and parallel to Yrast line • c ~ 2ħ /photon for deformed nuclei • Mg = Ns +Nc PR157(1967)814 Lecture I SERC-06 School 13/2/06 - 2/3/006

  18. Fusion Gamma Multiplicity • Entrance Channel Spin • Ig = f (Mg - d) • Large spread of f, d in different mass regions • f ~ 1.5 -2 • d ~ 1.5 - 3.3 • Need to be calibrated from g multiplicity data taken well above barrier PRC21(1980)230 Yb nuclei Ig = 2(Mg -2) Lecture I SERC-06 School 13/2/06 - 2/3/006

  19. Channel Selection • High J nuclei decay by 3n • Low J nuclei decay by 5n • 3n,4n,5n events have different g-multiplicity • Subsequent g-decay along the band • Higher Mg selection would enhance the contribution from high spin states Number of particles emitted strongly dependent on initial spin Lecture I SERC-06 School 13/2/06 - 2/3/006

  20. Spin Distribution in Fusion • Intensity distribution within a band enhanced to higher J at higher bombarding energy and with heavier projectile • s (J) described by Fermi Function [1+exp(J-J0)/a]-1 Lecture I SERC-06 School 13/2/06 - 2/3/006

  21. Dependence on g-multiplicity distribution • Spin distribution parameters J0,a sensitive to the reaction mechanism • Distribution shifts to lower spin with more particles evaporated • Can be used to study reaction mechanism i.e. incomplete fusion at high bombarding energies Lecture I SERC-06 School 13/2/06 - 2/3/006

  22. Excitation Function • Statistical codesfor residual nucleus yield: • PACE2, CASCADE • Sensitive to transmission coeff and level density parameters • Charged particle emission important for • highly neutron-deficient nuclei (reduced Sp) • A < 100 (lower barrier) Lecture I SERC-06 School 13/2/06 - 2/3/006

  23. EXPERIMENT PLANNING • Select P,T for the required compound nucleus • Calculate theoretical excitation function for the required residual nucleus • Heavier P & higher Elab to enhance high spin states • g Multiplicity gate for high spin selection • Charged particle, neutron gate for channel selection • Thin target ( ~ 1mg/cm2) to minimize recoil energy spread in target • Backed target for minimizing Doppler shift • Stacked targets ( ~ 3x 300mg/cm2) to minimize Doppler broadening - Doppler correction in software Lecture I SERC-06 School 13/2/06 - 2/3/006

  24. Measurement of g-Multiplicity • Array of photon detectors: • NaI or BGO array  high detection efficiency for Eg < 2 MeV • BGO detectors compact & less sensitive to scattering & neutron events • small number of gating detectors for suppressing low multiplicity events • <Mg> = 1+ Ncoinc/(WNsingles) 3"x3" NaI PRC31(1985)1752 Lecture I SERC-06 School 13/2/06 - 2/3/006

  25. BGO Multiplicity array for GDA • 14 Element BGO array • Detection efficiency ~ 20% • Broad Multiplicity distribution Lecture I SERC-06 School 13/2/06 - 2/3/006

  26. High Efficiency Mg Array • N = total no of detectors • K = no of detectors hit • eW = total detection efficiency • M = Photon multiplicity • P(K) = K-fold hit probability TESSA Array of 62 BGO detectors W = 95% of 4p Nucl. Phys. A245(1975)166 Lecture I SERC-06 School 13/2/06 - 2/3/006

  27. Large g-Multiplicity Array • Hydelberg Crystal Ball • 4pg array of NaI detectors • 4p array of photon detectors • Number of hits per event recorded • Hit pattern sensitive to higher moments of Mg distribution • Can distinguish between xn and (x+1)n channels event by event Nucl. Phys. A409 (1983)331c Lecture I SERC-06 School 13/2/06 - 2/3/006

  28. Dependence of xn on (E*,J) • Hydelberg Crystal Ball • 4pg array of NaI detectors • 4n and 5n channels can be separatedby either • total photo energy ET • total no of detectors hit • improved cleanup with gates on both ET, K Nucl. Phys. A409(1983)331c Lecture I SERC-06 School 13/2/06 - 2/3/006

  29. Multiplicity Gate • Heidelberg Crystal ball • Good separation of 4n & 5n channels on the basis of double gate on Sum Energy & Fold • Back-bending at high spin ! Nucl. Phys. A409(1983)331c Lecture I SERC-06 School 13/2/06 - 2/3/006

  30. BGO Innerball at GASP • 80 BGO crystals covering 80% of 4p • Polyhedron with 122 faces • 40 opening for GE-ACS • Individual detector (65 mm) efficiency 95% @ 1 MeV • Total photopeak detection efficiency ~70% • Read-out of the crystal with PMT : individual E & T Lecture I SERC-06 School 13/2/06 - 2/3/006

  31. GASP BGO Ball • Active collimator in front of Ge-ACS detectors • 100% detection efficiency for fusion events • Suppression of unwanted channels R ~ 2-4 Lecture I SERC-06 School 13/2/06 - 2/3/006

  32. Inner BGO ball at G.A.S.P. ACS BGO BALL Lecture I SERC-06 School 13/2/06 - 2/3/006

  33. Summary • Improvement in detector technology has allowed very weakly populated channels to be investigated • Select projectile-target combination to optimize population of levels of interest • Heavier beam & higher bombarding energy help to populate high spin states • Channels with a small number of particles emitted would be preferable to channels with high particle multiplicity • Higher total photon energy & photon multiplicity can be used to select entry point spin • A high efficiency photon multiplicity filter is an effective tool for channel selection Lecture I SERC-06 School 13/2/06 - 2/3/006

  34. Lecture I SERC-06 School 13/2/06 - 2/3/006

More Related