band-gem istituto di fisica del plasma, ifp-cnr – milano dipartimento di fisica, università di...
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BAND-GEM
Istituto di Fisica del Plasma, IFP-CNR – MilanoDipartimento di Fisica, Università di Milano-Bicocca INFN, LNF e Sezione di Milano-Bicocca
ESS, LundLinköping University
Further help fromHZ GeesthachtISIS detector groupBeam time at LLB and IFE
Financial support from CNR; ESS pre-construction in-kind contract
SHOULD WE DETECT THERMAL NEUTRONS WITH GEMS?
GEMs offer the following advantagesHigh rate capability (up to MHz/mm2) suitable for high flux neutron beams like at ESSSubmillimetric space resolution (suited to experiment requirements)Time resolution from 5 ns (gas mixture dependent)Possibility to be realized in large areas and in different shapesRadiation hardnessLow sensitivity to gamma rays (with appropriate gain)
GEM detectors born for tracking and triggering applications (detection of charged particles)....
...but if coupled to a solid state converter they can detect Thermal Neutrons 10Boron converter
Neutrons are detected using the productus (alpha,Li) from nuclear reaction 10B(n,alpha)7Li
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WORK IN PROGRESS (STARTED MAY 2011)
MEASUREMENT OF GEM RATE CAPABILITY IN A HIGH FLUX NEUTRON BEAM
GEM DETECTORS FOR NEUTRON DIFFRACTION MEASUREMENTS
HIGHER EFFICIENCY -> BANDGEM DETECTOR
A POSSIBLE APPLICATION IN ESS
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MEASUREMENTS OF GEM DETECTOR RATE CAPABILITY IN A HIGH FLUX NEUTRON BEAM
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n
α
Natural B: Low efficiency detector: 1%
Enriched 10B: 5% efficiency
GEM detector for thermal neutrons (bGEM)
• Triple GEM detector equipped with an aluminum cathode coated with 1μm of B4C
• Exploits the 10B(n,α)7Li reaction in order to detect thermal neutrons
• ΔV GEM = ΣVGEM = 870 V
• Gain ≈ 100
• Ar/CO2 70%/30% (5 l/h)
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B4C coated aluminium cathode
mounted on its support
B4C coated aluminium cathode assembled
inside the bGEM chamber layout
Detector Schematics
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7*Borated cathode from HZ Geesthacht
*
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GEM DETECTORS FOR NEUTRON DIFFRACTION MEASUREMENTS
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Test of bGEM detector for neutron diffraction measurements
Cd mask (not really visible, but it is there…..)
Cd slabs to cover the GEM sides
Cd collimator
Incident neutron beam
Transmitted neutron beam
INES (ISIS) sample pos.
GEM position
• bGEM with enriched borated cathode• Cd mask and rough collimator• The bronze sample is different • The same bGEM position (90°) was
no longer available FOCUSSING
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Efficiency comparison with corresponding 3He tubes
1st test (natural B)He3 rate x25 GEM rate
2nd test (enriched 10B)He3 rate x8 GEM rate
GEM GEM
3He 3He
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bGEM (time) resolution
Resolution OK by adding up data from pads that lie on the same Debey-Scherrer cone, i.e. constant Lsenϑ
Counting area ToF [us] FWHM (us)All GEM 12648,66 229,9
Focussed GEM 12714,22 79,433He 12673,94 77,43
BAND-GEM DETECTOR
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Borated Array Neutron Detector = BAND
Scheme and Principle of operation
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Padded Anode
n
α
e-
α
10B4C 10B4C
10B4C 10B4C
nAl2O3
Al2O3
Alumina Lamellas coated on both sides with 10B4C
θ
Using low θ values (few degs) the path of the neutron inside the B4C is increased Higher efficiency when detector is inclined
Triple GEM
Cathode
3D LamellaSystem
Ed
10B coating: Chewbacca’s ancestor
coating machine Chewbacca being inaugurated in Linköping 16
Detector Assembly
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The full Lamella System An aluminium cathode (few microns thick) has been
mounted on top
Detector test with X-Rays
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Detector completed
Test with X-Rays (in IFP-lab)
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Detector test at IFE (JEEP II Reactor, RD2D beamline)
n
Beam Profile
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Ed = 230 V/cmEt1=Et2= 3 kV/cm; Ei = 5 kV/cm
Mixture Ar/CO2 70%/30%Angle = 10 degrees
VGEM = 980 V
6 mm
12 mm
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Efficiency
c comes from
where if λ=λ0=1.54 A ε=ε0=0.15 for 10 degrees and ε=ε0=0.20 for 7 degrees
POSSIBLE APPLICATION @ ESS: LOKI (Small angle neutron scattering)
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• At least 6 BAND-GEM modules• Detectors tilted by a few degrees
• Trapezoidal Triple GEM detector• Borated Lamellas of different lenghts• Minimise frame thickness especially near
the n beam
25 cm
Beam
2 Å< λ < 12 Å
ConclusionsThe rate capability measured with GEM+electronics is
about 40 MHz/mm2
The GEM detector can be used as diffractometer and its time resolution matches those of 3He tubes
BAND-GEM detector tested in a thermal neutron beam.
Next: optimize efficiency/ gamma discrimination trade-offTrapezoidal geometry for LOKIExplore lamella materials other than alumina (OK but fragile)
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