1. EXPERIMENTS WITH LARGE GAMMA DETECTOR ARRAYS Lecture II Ranjan Bhowmik Inter University Accelerator Centre New Delhi -110067

2. Lecture II SERC-6 School March 13 - April 2,2006 2 INSTRUMENTATION FOR LARGE GAMMA ARRAYS

3. Lecture II SERC-6 School March 13 - April 2,2006 3 High Resolution Photon Detectors High purity Ge detectors Resolution < 2 keV at 1 MeV Large volume > 100 cc Neutron-damage resistant : can be annealed Can be warmed up to room temperature for storage High e-h mobility : short collection time ~ 100 ns

4. Lecture II SERC-6 School March 13 - April 2,2006 4 Photon interactions in Ge Attenuation length ~ 3 cm @ 1 MeV Only ~ 3% of the interactions photo-electric Part of photon energy absorbed in Ge after each scattering Total number of interactions in the crystal depends on crystal volume Larger fraction of E  deposited with bigger detectors Monte-Carlo Simulation of Scattering in Ge MCNPECS4GIANT

5. Lecture II SERC-6 School March 13 - April 2,2006 5 Peak to total in Ge P/T increases linearly with detector dimensions Photo-peak efficiency increases linearly with volume  in  %  ~ V( in cc)/4.3

6. Lecture II SERC-6 School March 13 - April 2,2006 6 Requirement of large P/T 100 cc detectors have P/T ~ 23% at 1.33 MeV Nearly 3/4 of events do not have correct energy information In  coincidence only 5% of events are useful In  coincidence only 1% of events are useful Making crystals of intrinsic P/T > 50% prohibitively expensive Electronic removal of bad events by detecting escaping Compton events viable option

7. Lecture II SERC-6 School March 13 - April 2,2006 7 TESSA-II - First Compton Suppressed Array 6 Detector Array at Daresbury NaI Shield Front NaI catcher Ge detectors inserted from top Nucl. Phys. A409(1983)343c

8. Lecture II SERC-6 School March 13 - April 2,2006 8 TESSA-II Performance Detectors 5 cm  x 5 cm P/T improved from.20 (unsuppressed) to.59 (suppressed) Front-catcher removes the 'rabbit ears' for back-scattered photons 30% of  events have correct energy information Nucl. Phys. A409(1983)343c

9. Lecture II SERC-6 School March 13 - April 2,2006 9 TESSA3 - BGO Shield Symmetric BGO shield NaI front catcher BGO back catcher 16 Ge-ACS Array P/T 20% bare detector 55% with shield 59% with shield + back catcher NIMA236(1985)95

10. Lecture II SERC-6 School March 13 - April 2,2006 10 ACS for GAMMASPHERE 110 Ge detectors 7 cm  x 7.5 cm 70% efficiency P/T ~ 0.27 bare Symmetric BGO shield P/T ~ 0.6 with ACS 10% improvement with back-plug NIMA317(1992)101 NIMA353(1994)234

11. Lecture II SERC-6 School March 13 - April 2,2006 11 COMPOSITE DETECTORS Detectors larger than 7 cm  difficult to fabricate Large charge collection time & Doppler broadening Increased neutron damage sensitivity Solution : Composite detectors More than one detector within common cryostat and ACS Less dead space due to common ACS Increased solid angle coverage & granularity Scattering from one detector to another increases photopeak efficiency Compton Polarimeter

12. Lecture II SERC-6 School March 13 - April 2,2006 12 CLOVER GEOMETRY Four 5 cm  x 7 cm long crystals within the same cryostat Tapered side to allow close packing with square x- section High probability of a Compton-scattered event in one crystal being absorbed in another crystal 50% 'Addback efficiency' at 2 MeV

13. Lecture II SERC-6 School March 13 - April 2,2006 13 CLOVER EFFICIENCY with ADDBACK NIMA432(1999)085 SINGLES NIMA491(1999)113

14. Lecture II SERC-6 School March 13 - April 2,2006 14 IMPROVED DOPPLER CORRECTION Single hit events corrected for centre angle Double hit events corrected for average angle ~2/3 improvement in resolution over a single detector of same efficiency Better resolution important for detecting weak peaks !

15. Lecture II SERC-6 School March 13 - April 2,2006 15 Electronic Segmentation Total Energy signal from central n-type contact Position signals from the individual p-type outer contacts segmented longitudinally, electrically isolated No dead layer between segments Common energy, No degradation due to addition of noise Negligible cross talk between segments (~ zero induced charge) Doppler correction between segments

16. Lecture II SERC-6 School March 13 - April 2,2006 16 Four-fold Segmented Clover Detector 4 coaxial n-type germanium crystals arranged like a four leaf clover. Outer p-type contact of each crystal segmented longitudinally, splitting each crystal into four quadrants. Energy readouts from 4 crystals Position readouts from 9 crystal zones. Improved segment-wise Doppler correction Similar performance with only 3 position readouts using hit-pattern

17. Lecture II SERC-6 School March 13 - April 2,2006 17 CLUSTER DETECTORS Seven encapsulated detectors inside the same cryostat Common ACS shield P/T 39% without shield P/T 61% with shield 15 CLUSTER detectors used in EUROBALL currently in use at GSI with RISING project NIMA369(1996)135

18. Lecture II SERC-6 School March 13 - April 2,2006 18 RESOLVING POWER Average level spacing SE depends on spectrum complexity Many nuclei populated A nucleus has many bands SINGLESSPECIFIC NUCLEUSONE BAND Resolving Power R = P T *SE/  E P T = peak to total  E = FWHM

19. Lecture II SERC-6 School March 13 - April 2,2006 19 SINGLES DETECTION LIMIT Peak intensity  per fusion Singles photopeak rate: N1 = 0.76   P T Background comes from all transitions of higher energy. A fraction  E/E of these appear within energy window  E Total background under photopeak B1 =  (1-P T ). ~  (1-P T ).  E/SE N1/B1 = 0.76  R/(1-P T ) =  r = ave no of  per fusion  = photopeak efficiency = ave photon energy SE ~ / R = P T SE/  E r = Reduced resolving power NIMA385(1997)501 Ann.Rev.Nucl.Part.Sci. 45(1994)561

20. Lecture II SERC-6 School March 13 - April 2,2006 20 DETECTION LIMIT FOR M-FOLD COINCIDENCE For two-fold coincidence N 2 =  (0.76  P T ) 2 B 2 = B 1 (1-P T )  E/ = B 1 2 N 2 /B 2 =  r 2 For M-fold coincidence N M =  (0.76  P T ) M and peak/background N M /N B =  r M Peak to background improves with higher fold coincidence ! Peak count   P M Counts in the peak increase with no of detectors  N M M C K   (K  P T ) M   [  P ] M  P = Total Photopeak Efficiency

21. Lecture II SERC-6 School March 13 - April 2,2006 21 Yrast SD band in 149 Gd NPA584(1995)373 BACKGROUND LIMIT A peak must stand out above background: N P /N B > 0.2 setting a limit on minimum value of  STATISTICAL LIMIT Rapid decrease in peak count with increasing fold Must have at least 100 counts in the peak for 10 10 events

22. Lecture II SERC-6 School March 13 - April 2,2006 22 OBSERVATION LIMIT WITH MULTI- DETECTOR ARRAY Background limit higher sensitivity with fold Statistical limit Peak area decreases with fold Crossing of two curves sets the minimum value of detectable  Higher sensitivity with 1.Higher total photopeak efficiency 2.Higher Resolving power R ~ (1000/30)/2*0.5 ~ 8  Ph = Total Photopeak Efficiency INGA GDA

23. Lecture II SERC-6 School March 13 - April 2,2006 23 LARGE DETECTOR ARRAY

24. Lecture II SERC-6 School March 13 - April 2,2006 24 GASP

25. Lecture II SERC-6 School March 13 - April 2,2006 25 GAMMASPHERE

26. Lecture II SERC-6 School March 13 - April 2,2006 26 EUROBALL EUROBALL

27. Lecture II SERC-6 School March 13 - April 2,2006 27 INDIAN NATIONAL GAMMA ARRAY

28. Lecture II SERC-6 School March 13 - April 2,2006 28 INGA STRUCTURE at NSC INGA Stand-alone mode Number of Clover detectors with shield = 24 5% photopeak efficiency Additional 6 detectors without shield can be accommodated LEPS detectors covering 4% of solid angle HYRA-INGA Coupled Mode 16 shielded Clover detectors + 3 LEPS detectors 3% photopeak efficiency

29. Lecture II SERC-6 School March 13 - April 2,2006 29 32° 57° LEPS 61° 90° 6 Rings at 32, 57, 90, 123, 148 deg

30. Lecture II SERC-6 School March 13 - April 2,2006 30 INGA STRUCTURE AT IUAC

31. Lecture II SERC-6 School March 13 - April 2,2006 31 INGA AT TIFR 7 Rings at 22.5, 45, 67.5, 90, 112.5, 135, 157.5 deg

32. Lecture II SERC-6 School March 13 - April 2,2006 32 CHANNEL SELECTION USING AUXILIARY DEVICES Identification of weak reaction channels reduces  - background from strong channels More efficient than high-fold  -gating Factor of 2-3 improvement in sensitivity Important for A 200 Measurement of entry channel energy and spin Measurement of charged particle multiplicity to identify (pxn), (  xn) Measurement of neutron-multiplicity for neutron-deficient channels Detection of recoiling nucleus for fissioning nuclei Identification of A & Z for weakly populated channels

33. Lecture II SERC-6 School March 13 - April 2,2006 33 CHARGED PARTICLE FILTER Large solid angle coverage Discrimination between p &  Compact size to fit inside Ge array Radiation-damage resistant High counting rate capability High granularity Energy information On-line Doppler correction DETECTORS Si wafer ISIS CsI-photodiode DIAMANT Microball Plastic-phoswich HYSTRIX CPDA

34. Lecture II SERC-6 School March 13 - April 2,2006 34 MICROBALL at GAMMASPHERE 98 detectors in 9 rings NIMA381(1996)418  2p Improvement in resolution due to Doppler correction

35. Lecture II SERC-6 School March 13 - April 2,2006 35 CPDA at IUAC 13 C + 100 Mo 65 MeV 108 Cd 40% (&) 109 Cd 45% (@) 108 Ag 10% ( #) 109 Ag 4% (*)  spectra (bottom) dominated by 108-109 Cd lines (4-5n) Cd lines suppressed in p-gated spectra (top)

36. Lecture II SERC-6 School March 13 - April 2,2006 36 NEUTRON GATING Spectroscopy of Ar and K isotopes in A=40 region 28 Si + 12 C 88 MeV SINP Group

37. Lecture II SERC-6 School March 13 - April 2,2006 37 28 Si + 58 Ni 95 MeV GATING BY HIRA

38. Lecture II SERC-6 School March 13 - April 2,2006 38 Dual Mode Operation of HYRA, NSC ● Gas-Filled Mode: ● For A > 200 amu ● Normal Kinematics ● Good Collection Efficiency (q, v focus) ● Z, A identification using recoil decay technique ● Vacuum Mode: ● For N ~ Z ( A< 100) ● Inverse Kinematics ● Good primary beam rejection (two stage) ● Z, A identification using X,  E and E

39. Lecture II SERC-6 School March 13 - April 2,2006 39

40. Lecture II SERC-6 School March 13 - April 2,2006 40