1. BAND-GEM Istituto di Fisica del Plasma, IFP-CNR – Milano
Dipartimento di Fisica, Università di Milano-Bicocca
INFN, LNF e Sezione di Milano-Bicocca
ESS, Lund
Linköping University
Further help from
HZ Geesthacht
ISIS detector group
Beam time at LLB and IFE
Financial support from CNR; ESS pre-construction in-kind contract

2. SHOULD WE DETECT THERMAL NEUTRONS WITH GEMS?
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
GEMs offer the following advantages
High rate capability (up to MHz/mm2) suitable for high flux neutron beams like at ESS
Submillimetric space resolution (suited to experiment requirements)
Time resolution from 5 ns (gas mixture dependent)
Possibility to be realized in large areas and in different shapes
Radiation hardness
Low sensitivity to gamma rays (with appropriate gain)

3. 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

4. MEASUREMENTS OF GEM DETECTOR RATE CAPABILITY IN A HIGH FLUX NEUTRON BEAM

5. 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) n α
Detector Schematics
B4C coated aluminium cathode mounted on its support
B4C coated aluminium cathode assembled inside the bGEM chamber layout
Natural B: Low efficiency detector: 1%
Enriched 10B: 5% efficiency

7. *
*Borated cathode from HZ Geesthacht

10. GEM DETECTORS FOR NEUTRON DIFFRACTION MEASUREMENTS

11. Test of bGEM detector for neutron diffraction measurements
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
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

12. Efficiency comparison with corresponding 3He tubes
GEM
GEM
3He
3He
1st test (natural B)
He3 rate x25 GEM rate
2nd test (enriched 10B)
He3 rate x8 GEM rate

13. 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
79,43
3He
12673,94
77,43

14. BAND-GEM DETECTOR
Borated Array Neutron Detector = BAND

15. Scheme and Principle of operationθ
Al2O3
Triple GEM
Cathode
3D Lamella
System Ed α α
10B4C
10B4C e-
10B4C
10B4C
Al2O3
Padded Anode
Alumina Lamellas coated on both sides with 10B4C
Using low θ values (few degs) the path of the neutron inside the B4C is increased  Higher efficiency when detector is inclined

16. 10B coating: Chewbacca’s ancestor
coating machine Chewbacca being inaugurated in Linköping

17. Detector Assembly The full Lamella System
An aluminium cathode (few microns thick) has been mounted on top

18. Detector test with X-Rays
Test with X-Rays (in IFP-lab)
Detector completed

19. Detector test at IFE (JEEP II Reactor, RD2D beamline)

20. Beam Profile Ed = 230 V/cm Et1=Et2= 3 kV/cm; Ei = 5 kV/cm
Mixture Ar/CO2 70%/30%
Angle = 10 degrees
VGEM = 980 V
6 mm
12 mm

21. Efficiency c comes from
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

22. POSSIBLE APPLICATION @ ESS: LOKI (Small angle neutron scattering)
2 Å< λ < 12 Å
25 cm
Beam
Trapezoidal Triple GEM detector
Borated Lamellas of different lenghts
Minimise frame thickness especially near the n beam
At least 6 BAND-GEM modules
Detectors tilted by a few degrees

23. Conclusions
The 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-off
Trapezoidal geometry for LOKI
Explore lamella materials other than alumina (OK but fragile)