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

EXPERIMENTS WITH LARGE EXPERIMENTS WITH LARGE GAMMA DETECTOR ARRAYSGAMMA DETECTOR ARRAYS

Lecture IILecture II

Ranjan BhowmikInter University Accelerator Centre

New Delhi -110067

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

INSTRUMENTATION FOR LARGE INSTRUMENTATION FOR LARGE GAMMA ARRAYSGAMMA ARRAYS


High Resolution Photon DetectorsHigh 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


Photon interactions in GePhoton 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 GeMCNP ECS4 GIANT


Peak to total in GePeak to total in Ge

P/T increases linearly with detector dimensions

Photo-peak efficiency increases linearly with volume

in%~ V( in cc)/4.3


Requirement of large P/TRequirement 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


TESSA-II - First Compton Suppressed ArrayTESSA-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


TESSA-II PerformanceTESSA-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


TESSA3 - BGO ShieldTESSA3 - BGO Shield

Symmetric BGO shieldNaI front catcherBGO back catcher

16 Ge-ACS Array

P/T 20% bare detector55% with shield59% with shield + back catcher

NIMA236(1985)95


ACS for GAMMASPHEREACS for GAMMASPHERE

110 Ge detectors7 cm x 7.5 cm 70% efficiency P/T ~ 0.27 bare Symmetric BGO shieldP/T ~ 0.6 with ACS10% improvement with back-plug

NIMA317(1992)101NIMA353(1994)234


COMPOSITE DETECTORSCOMPOSITE DETECTORSDetectors larger than 7 cm difficult to fabricateLarge charge collection time & Doppler broadeningIncreased 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


CLOVER GEOMETRYCLOVER 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


CLOVER CLOVER EFFICIENCYEFFICIENCY

with ADDBACK

NIMA432(1999)085

SINGLES

NIMA491(1999)113


IMPROVED DOPPLER CORRECTIONIMPROVED 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 !


Electronic SegmentationElectronic 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


Four-fold Segmented Four-fold Segmented Clover DetectorClover 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


CLUSTER DETECTORSCLUSTER 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


RESOLVING POWERRESOLVING POWER

Average level spacing SE depends on spectrum complexity Many nuclei populated A nucleus has many bands

SINGLES SPECIFIC NUCLEUS

ONE BAND

Resolving Power R = PT *SE/EPT = peak to totalE = FWHM


SINGLES DETECTION LIMIT SINGLES DETECTION LIMIT Peak intensity per fusion Singles photopeak rate:

N1 = 0.76 PT

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-PT). <M E/<E> ~ (1-PT). E/SE

N1/B1 = 0.76 R/(1-PT) = r

<M> = ave no of per fusion = photopeak efficiency<E> = ave photon energySE ~ <E> /<M>R = PT SE/Er = Reduced resolving power

NIMA385(1997)501Ann.Rev.Nucl.Part.Sci. 45(1994)561


DETECTION LIMIT FOR M-FOLD DETECTION LIMIT FOR M-FOLD COINCIDENCECOINCIDENCE

For two-fold coincidenceN2= (0.76 PT)2

B2 = B1 (1-PT)<M>E/<E> = B12

N2/B2 = r2

For M-fold coincidenceNM = (0.76 PT)M

and peak/background NM/NB = rM

Peak to background improves with higher fold coincidence !Peak count P

M

Counts in the peak increase with no of detectors

NM MCK

(KPT)M

[P]M

P = Total Photopeak Efficiency


Yrast SD band in Yrast SD band in 149149GdGdNPA584(1995)373BACKGROUND LIMITBACKGROUND LIMIT

A peak must stand out above background:

NP/NB > 0.2 setting a limit on minimum value of STATISTICAL LIMITSTATISTICAL LIMITRapid decrease in peak count with increasing foldMust have at least 100 counts in the peak for 1010 events


OBSERVATION LIMIT WITH MULTI-OBSERVATION LIMIT WITH MULTI-DETECTOR ARRAYDETECTOR 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 with1. Higher total photopeak

efficiency2. Higher Resolving power R ~ (1000/30)/2*0.5 ~ 8

Ph = Total Photopeak Efficiency

INGAINGA

GDAGDA


LARGE DETECTOR ARRAYLARGE DETECTOR ARRAY

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

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


EUROBALLEUROBALL

EUROBALLEUROBALL


INDIAN INDIAN NATIONAL NATIONAL

GAMMA GAMMA ARRAYARRAY


INGA STRUCTURE INGA STRUCTURE at NSCat NSC

INGA Stand-alone modeINGA Stand-alone modeNumber of Clover detectors with shield = 24 5% photopeak efficiency Additional 6 detectors without shield can be accommodatedLEPS detectors covering 4% of solid angleHYRA-INGA Coupled HYRA-INGA Coupled ModeMode16 shielded Clover detectors + 3 LEPS detectors 3% photopeak efficiency


32°32°

57°57°

LEPSLEPS 61° 61°

90°90°

6 Rings at 32, 57, 90, 123, 148 deg


INGA STRUCTURE AT IUACINGA STRUCTURE AT IUAC


INGA AT TIFRINGA AT TIFR

7 Rings at22.5, 45, 67.5, 90, 112.5, 135, 157.5 deg


CHANNEL SELECTION USING CHANNEL SELECTION USING AUXILIARY DEVICESAUXILIARY 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 < 100 and A > 200

Measurement of entry channel energy and spinMeasurement of charged particle multiplicity to identify (pxn), (xn)Measurement of neutron-multiplicity for neutron-deficient channelsDetection of recoiling nucleus for fissioning nucleiIdentification of A & Z for weakly populated channels


CHARGED PARTICLE FILTERCHARGED 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

DIAMANTMicroball

Plastic-phoswichHYSTRIXCPDA


MICROBALL at GAMMASPHEREMICROBALL at GAMMASPHERE

98 detectors in 9 rings

NIMA381(1996)418

2p

Improvement in resolution due to

Doppler correction


CPDA at IUACCPDA at IUAC

13 C + 100Mo 65 MeV• 108Cd 40% (&)• 109Cd 45% (@)• 108Ag 10% ( #) • 109Ag 4% (*)

spectra (bottom) dominated by 108-109Cd lines (4-5n)

Cd lines suppressed in p-gated spectra (top)


NEUTRON GATINGNEUTRON GATINGSpectroscopy of Ar and K isotopes in A=40 regionSpectroscopy of Ar and K isotopes in A=40 region

2828Si + Si + 1212C 88 MeVC 88 MeVSINP GroupSINP Group


2828Si + Si + 5858Ni 95 MeVNi 95 MeV

GATING BY HIRA


Dual Mode Operation of HYRA, NSCDual 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



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