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A gas scintillation proportional counter for thermal neutron scattering measurements
• All anodes connected together• All cathodes connected together• Active area: 32x32 mm2 / 90x77 mm2
at 6 bar CF4
The PMTs signals• PMTs signals digitised at
400 MHz – 12 bit• Signals are filtered τ=150
ns• Amplitude (an) at the peak
of the signal is measured for each PMT
• The 2D position of the neutron is calculated using the Centre of Gravity (CoG) of the light
7 PMTs in hexagonal arrangementRaw signals
nn
nn,pmtn
a
xax
nn
nn,pmtn
a
yay
Position Resolution
• Four PMTs (Ø=38 mm) in a square array
• PMTs to MSGC 20 mm
• 1 bar 3He / 2 to 6 bar CF4
• Gain increased until not better position resolution
• Similar results with 7 PMT (Ø=29 mm) in hexagonal arrangement
0.6 mm FWHM
3H p
Electronics
ADC
ADC
FPGADSP
Peak Finderγ/n
Position
X,YPH
Position Reconstruction Algorithms • PMTs signals amplitudes as
input XY as output
• Centre of Gravity (CoG)• Maximum Likelihood (ML)• Least Square (LS)• Neural Network (NN)
Position Resolution FWHM (mm)
COG ML LS NN
0.8 0.83 0.83 0.82
Cd MaskHoles 2 mm Ø, 5 mm pitch
Gamma/neutron
Σ PMT signal
Ch
arg
e s
ign
al
nPMT signal
charge signal
γPMT signal
charge signal
Reason for slower gamma signal: Electron ionises much large volume of gas than proton + triton Takes longer for all charges to drift to MSGC Gamma signal is wider (and lower).
Full size detector • 40 x 40 cm Al vessel• fill pressure: 1 bar He + 7 bar CF4
• Entrance Al window (5 mm)• MSGC S8900 (9 cm x 7.3 cm)• 3.3 mm Borofloat glass window • 19 R5070A PMTs on 28.5 mm pitch
• Tested up to 400 kHz incident rate• Read out with the final electronic
Dx = 0.60 mm
Conclusions• The GSPC was developed in the NMI3-FP7 project• The obtained performance are the result of the precise
measurements of the detector’s physical parameters• The simulation tool (ANTS) has been crucial for the
development of the detector:– Position reconstruction– Position resolution– Rate Capability
• A real size detector is operative
Future• Try the detector on a reflectometer
Light Spectrum
A. Morozov, L.M.S. Margato, M.M.F.R. Fraga, L. Pereira, F.A.F. Fraga, Secondary scintillation in CF4; 2012 JINST 7 P02008.
Red PMTsS20 photocathodeFused Silica Window200-800 nmQE~10%
Blue PMTsBialkali photocathodeBorosilicate Window300-550 nmQE~20%