1. ESS Detector Group Seminar Edoardo Rossi 14th August 2015
Characterization of the Spatial Resolution and the Gamma Discrimination of Helium-3 Tubes
ESS Detector Group Seminar
Edoardo Rossi
14th August 2015

2. Introduction Helium-3 Tubes Energy Resolution Spatial Resolution
Gamma Sensitivity
Pulse Shape Discrimination
Conclusion

3. Outline He
He-3 detectors are by far the most common choice for neutron detection because of their high efficiency.
Since 2001 the world is experiencing a severe He-3 shortage.
Detector research is focused on alternative isotopes as neutron converters (Boron-10, Lithium-6, Gadolinium etc.)
Some of the limits of the He-3 technology have never been quantified!
In this seminar I will quantify some of this limits. These can be used as a reference to study and characterize the performance of the new alternative technologies.

4. Helium-3 Tubes

5. He-3 PHS (1) He
1n + 3He H + 3H Q = 764 keV
573 keV keV

6. MONTE CARLO CALCULATION
He-3 PHS (2)
MONTE CARLO CALCULATION He

7. He-3 Detectors Reuter Stokes Length = 10’’ (25.4 cm)
Diameter = 1’’ (2.54 cm)
Pressure = 9.85 bar 3He bar
Reuter Stokes
Length = 1 m
Diameter = 8 mm
Pressure = 8.5 bar 3He bar
Position Sensitive Detector
Length = 1 m
Diameter = 8 mm
Pressure = 10 bar 3He + unknown
Position Sensitive Detector

8. Energy Resolution

9. Full Energy Peak Fitting
Only the ‘right’ side of the peak is fitted, in order not to take into account the proton- and tritium-escape shoulders

10. Energy Resolution vs HV (1)

11. Energy Resolution vs HV (2)

12. Spatial Resolution

13. Experimental Set-up

14. Sp. Res. as a Function of HV (1)

15. Sp. Res. as a Function of HV (2)

16. Spatial Resolution along X Axis (1)
Beam size = 1 mm x 1 cm
X axis
Beam

17. Spatial Resolution along X Axis (2)
Spatial resolution = (6.7 ± 0.5) mm
Spatial resolution = (5.6 ± 0.6) mm
SPATIAL RESOLUTION IS UNIFORM ALONG THE X AXIS OF THE TUBE

18. Spatial Resolution along Y Axis
Beam size = 2.5cm x 0.3mm
SPATIAL RESOLUTION IS UNIFORM ALONG THE Y AXIS OF THE TUBE
Y Axis

19. Gamma Sensitivity

20. Definition (1)
The Gamma Sensitivity (GS) is the efficiency for recording a gamma-ray from a given source:
This definition does not take into account the energy required for a gamma-ray pulse to be recorded.

21. Definition (2)
We define the GS as the efficiency for a gamma-ray to release more than 191 keV (proton-escape peak) in the gas.
The proton escape peak is easier to identify than the valley

22. Gamma-ray sources Isotope Half life Energy Branching ratio (%)
Intensity
60Co
5.3 y
1173 keV
1332 keV
99.85
99.98
2.39 MBq
137Cs
30 y
662 keV
85.1
7.11 MBq

23. Gamma-ray Conversion (1)
The gamma-rays are converted in photo-electrons mainly in the steel walls of the detector:
pinteraction(steel) ≈ pinteraction(3He) ≈ 10-4

24. Gamma-ray Conversion (2)
The gamma-rays are converted in photo-electrons mainly in the steel walls of the detector:
pinteraction(steel) ≈ pinteraction(3He) ≈ 10-4
We divide the GS in 3 main contributions:
Probability of interaction of a gamma-ray in steel (pinteraction)

25. Gamma-ray Conversion (3)
The gamma-rays are converted in photo-electrons mainly in the steel walls of the detector:
pinteraction(steel) ≈ pinteraction(3He) ≈ 10-4
We divide the GS in 3 main contributions:
Probability of interaction of a gamma-ray in steel (pinteraction)
Probability that a photo-electron is transmitted into the gas (ptransmission)

26. Gamma-ray Conversion (4)
The gamma-rays are converted in photo-electrons mainly in the steel walls of the detector:
pinteraction(steel) ≈ pinteraction(3He) ≈ 10-4
We divide the GS in 3 main contributions:
Probability of interaction of a gamma-ray in steel (pinteraction)
Probability that a photo-electron is transmitted into the gas (ptransmission)
Probability that the photo-electron releases more than 191 keV in the gas (penergy release)

27. Gamma-ray Conversion (5)
The gamma-rays are converted in photo-electrons mainly in the steel walls of the detector:
pinteraction(steel) ≈ pinteraction(3He) ≈ 10-4
We divide the GS in 3 main contributions:
Probability of interaction of a gamma-ray in steel (pinteraction)
Probability that a photo-electron is transmitted into the gas (ptransmission)
Probability that the photo-electron releases more than 191 keV in the gas (penergy release)
GS = pinteraction x ptransmission x penergy release ≈
10-2

28. Gamma-ray Conversion (6)
PENELOPE SIMULATIONS
Backscattered
Absorbed
Transmitted

29. Gamma-ray Conversion (7)
PENELOPE SIMULATIONS

30. Gamma-ray Conversion (8)
PENELOPE SIMULATIONS
GS = pinteraction x ptransmission x penergy release ≈ ≈
10-2
10-1

31. GS Measurements (1)

32. GS Measurements (2)
RS 25 cm
Toshiba 1 m

33. GS = pinteraction x ptransmission x penergy release
GS Measurements (3)
RS 25 cm
Toshiba 1 m
GS ≈
GS = pinteraction x ptransmission x penergy release ≈ ≈ ≈
10-2
10-1

34. GS Measurements (4) RS 25 cm
Intuitively, the GS should not depend on the HV.
The gamma-ray PHS and the neutron PHS ‘get closer’ as the HV increases because of the space charge effect.

35. Pulse Shape Discrimination

36. Neutron Signal

37. Neutron/Gamma-ray Signals

38. PSD (1)
RS 25 cm tube

39. PSD (2)
RS 25 cm tube
Setting an amplitude threshold in the valley one gamma-ray over 106 is counted

40. PSD (3)
RS 25 cm tube
Setting this threshold, more than 97% of the gamma-ray signals now are discriminated. Less than 0.6% of the neutron signals are lost.

41. PSD (4)
Setting this threshold, more than 96% of the gamma-ray signals are discriminated. Less than 0.3% of the neutron signals are lost.

42. Rise Time Definition
RS 25 cm tube

43. Conclusion (1)
Our 3He tubes reach a spatial resolution of 6-7 mm ( % of their active length). The spatial resolution improves as the HV increases.

44. Conclusion (2)
Our 3He tubes reach a spatial resolution of 6-7 mm ( % of their active length). The spatial resolution improves as the HV increases.
The GS is The gamma discrimination deteriorates as the HV increases.

45. Conclusion (3)
Our 3He tubes reach a spatial resolution of 6-7 mm ( % of their active length). The spatial resolution improves as the HV increases.
The GS is The gamma discrimination deteriorates as the HV increases.
The PSD technique can be implemented to reduce by two order of magnitude the GS.

46. Conclusion (4)
Our 3He tubes reach a spatial resolution of 6-7 mm ( % of their active length). The spatial resolution improves as the HV increases.
The GS is The gamma discrimination deteriorates as the HV increases.
The PSD technique can be implemented to reduce by two order of magnitude the GS.
Most of the procedures can be also used to study the performance of 10B detectors and of the other new alternative technologies to 3He

47. Thank for your attention
And thank to the Detector Group for these 6 months.

48. This in an effect of the gas
GS Measurements (6)
60Co
The GS does not depend on the gas mixture, but only on the mass density of the gas.
1 bar of Ar/CO2 contributes to the gas density as about 10 bar of 3He.
RS 1’’ (counter): 10 bar 3He bar Ar/CO2 = 11 bar 3He density
RS 8 mm (PSD): 9 bar 3He + 3 bar Ar/CO2
= 39 bar 3He density
Toshiba 8 mm (PSD): ?? (10 bar 3He + 6 bar Ar/CO2?)
The thicker tube does not have a larger GS, even if gamma-rays have more ‘room’ to release their energy! Furthermore, it has thicker walls!
This in an effect of the gas