Transcript Gamma spectrometry beyond the BaF2 crystal ball

Gamma Spectrometry beyond Chateau Crystal
J. Gerl, GSI
SPIRAL 2 workshop October 5, 2005
Ideas and suggestions for a calorimeter with
spectroscopy capability
Reactions with Relativistic Radioactive Beams
R3B at NUSTA/FAIR
A versatile setup for kinematical complete measurements
Large-acceptance measurements
Protons
Fragments
Exotic beam
from Super-FRS
g rays
Neutrons
Neutrons
Br = mg v / Z
High-resolution momentum measurement
Goals and Requirements
Total excitation energy; angular momentum;
decay path; discrete line spectroscopy;
neutron/charged particle discrimination;
background discrimination
g-sum energy
sum > 80%; (Esum)/<Esum> < 10%
g multiplicity
(Ng)/<Ng> < 10%
g energies
Eg /Eg < 2%
g times
tg < 3 (1) ns
144 Elements / 2p
CsI(Na) g-detector
( LAND )
LAND Target Detector
• Intrinsic energy resolution (60Co): 7 (1) %
• Sum energy resolution (60Co) :
6.7 %
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fe at 1.8 MeV:
- single mode:
9.3 %
- addback mode: 16.3 %
- summing mode: 37.0 %
• GEANT simulation for moving source:
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250 MeV/u 600 MeV/u
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fe:
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Doppler broadening:
~ 45 %
~ 3%
~ 55 %
~ 5 %
Crystal Ball at LAND
Crystal Ball
162 NaI(Tl) crystals
spherical shell, 22 cm thick, diam. 50 cm
int = 93 %, fe = 69 % @ 500 keV
sum = 85 %, (Esum)/<Esum> = 8% @ 5 MeV
<Ng> = 1.2, (Ng)/<Ng> = 15% @ M=10
Eg = 8 % @ 500 keV, tg = 3.5 ns
Doppler broadening:  10 % @ 250 MeV/u
Spectroscopy set-up at LAND
RIBs: E ≈ 200 MeV/u; I = 10 ... 100 pps !
n-rich Mg to Ca nuclei
Multiplicity gated g spectra from Crystal Ball
28Mg
+C→
28Mg
27Mg
26Mg
Mg increases with
number of knocked
out nucleons
ΔEg ≈ 10 %
dominated by
Doppler broadening
Sum energy – Multiplicity correlation
27Mg
+ C → 26Mg
Reaction types at relativistic energies
2+  0+
135 MeV/u 136Nd on Au
Triaxiality
140 MeV/u 55Ni on 9Be
Mirror symmetry
2+ 2 
2+
2+
50Cr
4+
6+
(8+)
Applications of a calorimeter
Spectroscopy
- Weak channel detection
- Selectivity by higher folds
- Sum/multiplicity filter
Reaction mechanism
- Excitation energy
- Angular momentum
- Isomerism
Independent of beam energy
New Opportunities
Solid state Ge detectors
efficiency, timing resolution, prohibitive cost
Liquid Xenon scintillators
experience?, purity maintenance, high pressure container
New inorganic scintillator materials
LaBr3,....
Cooled standard inorganic scintillators
CsI(pure), NaI(pure),...
Properties of pure, cooled CsI
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Light yield: 1∙104 Photons/MeV
Light emission: 320 nm
Decay time: 16 ns
Density: 4.5 g/cm3
• Energy resolution: 1.6 % at 662 keV (LN2 temp.)
• Time resolution: 380 ps (against plastic)
Properties of scintillators
Energy resolution of LaBr3:Ce
E = 60 keV
Crystal at room temperature,
E = 668 keV
 FWHM < 2% @ 1.3 MeV
Read-out: APD at -23 °C
K.S. Shah et al., IEEE NS/MIC/RTSD 2003
Energy resolution limitations
Photon statistics and quantum efficieny: N·
Crystal inhomogeinity, non-proportionality, light losses: Rsci
Electronic noise: Rnoise
(E/E)2 = 5.6·(1/N·) + Rsci2 + Rnoise2
 Cooling to improve Rnoise?
Time resolution of LaBr3:Ce
Time resolution depends
on Ce dopant concentration
5% Ce
K.S. Shah et al., IEEE NS Vol. 50 (2003) 2410
Manufacturer's view
 limited crystal size (LaBr3: 1" x 2", LaCl3 3" x 3")
 very hygroscopic → sealed housing, glass window?
hard and brittle → cutting and polishing problematic
 Best suited for medical imaging → Attractive market
Crystal treatment is expensive
Lot of development is going on
Costs:
Raw LaBr3 crystal: ~ 30€
Scint. detector:
~ 1500€ + 300€/cm3
What to build....
Pencil detector: ≈ 0.5x0.5 cm2, 16...20 cm long,
≈ (1...3)x104 elements
Large position sensitive LaBr3:Ce detector
5 cm
C
A
B
B
6 cm
A
D
E =  (Ea...ED)
a
b
pos(x,y,z) = centroid of light distribution
A few 100 elements...
Requires cooling to reduce noise
APDs from Radiation Monitoring Devices Inc.
14x14 pixel APD
Features
•Gain above 1,000 at operating condition of best signal-to-noise ratio.
(Maximum gain of 10,000.)
•Large active area
•High quantum efficiency (QE) extends beyond visible spectrum
•High speed at 1064 nanometer wavelength of YAG lasers
•Pulse counting mode is the most-frequent style of use.
•Optical Photon Counting (2-3 photons) when cooled
Conclusion
• A 4p scintillator shell is highly beneficial for
most spectroscopy experiments with RIBs!
• Cooled inorganic scintillators (CsI, NaI) or La
halides seem appropriate
• A lot of R&D is needed including crystals,
light readout, electronics
• Can we find a common solution....