band-gem istituto di fisica del plasma, ifp-cnr – milano dipartimento di fisica, università di...

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

2

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

3

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

4

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)

5

B4C coated aluminium cathode

mounted on its support

B4C coated aluminium cathode assembled

inside the bGEM chamber layout

Detector Schematics

6

7*Borated cathode from HZ Geesthacht

*

8

9

GEM DETECTORS FOR NEUTRON DIFFRACTION MEASUREMENTS

10

11

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

12

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

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 79,433He 12673,94 77,43

BAND-GEM DETECTOR

14

Borated Array Neutron Detector = BAND

Scheme and Principle of operation

15

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

17

The full Lamella System An aluminium cathode (few microns thick) has been

mounted on top

Detector test with X-Rays

18

Detector completed

Test with X-Rays (in IFP-lab)

19

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

n

Beam Profile

20

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

21

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)

22

• 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)

23