1. Background Reduction for Quantitative Gamma-ray Imaging with the Electron-Tracking Compton Camera in High Dose Areas
May 26th, 2015
2-PS10A-10
T. Mizumoto, D. Tomono, A. Takada, T. Tanimori, S. Komura, H. Kubo, Y. Matsuoka, Y. Mizumura, K. Nakamura, S. Nakamura, M. Oda, J. D. Parker, T. Sawano, N. Bando1, A. Nabetani2
( Kyoto University, 1HORIBA, Ltd, 2Canon Inc. )
For the decontamination of long-lived cesium (Cs) in soils, it is necessary to estimate quantitatively a boundary condition for the necessity of decontamination. The Electron Tracking Compton Camera (ETCC) is the detector suitable for the monitoring gamma rays from radioactively contaminated soil for the estimation because of its imaging ability. We have developed an compact, battery-powered ETCC for the measurement of such environmental gamma rays [1]. With the ETCC, we measured gamma rays from contaminated soils in Fukushima [2]. However, we found that we can not ignore accidental events which can not be separated from Compton scattering events. To reduce accidental events, we tried to use low-Z and high electron density gas for the Compton scatterer.
1. Introduction
4. Solution of the problem
-Ne and CF4 based gas for the TPC-
For the quantitative measurement for the dose rate of the contaminated soil, it is necessary to reduce accidental events. To reduce accidental events, it is important to reduce X-rays in the TPC or to reduce X-ray detection efficiency of the TPC. If we use low-Z and high electron density gas for TPC, for example, CF4 gas, we can achieve low and high probability of the photoabsorption and Compton scattering in the TPC, respectively. As the result, we can achieve low accidental rate and higher gamma ray detection efficiency.
Previous gas
Ar:C2H6 (90:10) 1.5 atm
New gas
Ne:CF4:isoC4H10
(50:48:2) 1.5 atm
Energy Deposit in the TPC (Ke) [keV]
ETCC spectrum (Ke + Eg)
32 keV
662 keV
32 keV peak and low energy component is small
2D histogram (Eg vs Ke)
2. About ETCC
TPC
vessel
An ETCC consists of two parts
・Time Projection Chamber (TPC)
- Compton scatterer
- recoil electron tracker
・Pixel Scintillator Arrays (PSAs)
- absorber of the scattered gamma ray
3x3 PSAs
Schematic of an ETCC
Photograph of the ETCC
TPC : measuring 3D track and energy deposit of the recoil electron
PSAs : measuring the absorption position and energy of the scattered gamma ray
・We can get all information of Compton scattering kinematics
- Compton scattering position
- energy (Ke) and recoil vector of the recoil electron
- energy (Eg) and scattering vector of the scattered gamma ray
・An ETCC has powerful abilities of background rejection and gamma ray imaging.
Eg+Ke = 662 keV
Eg+Ke = 662 keV
Specification of the TPC
Gas species
Ar : C2H6
(90% : 10%)
1.5 atm
Ne : CF4 : iC4H10
(50% : 48% : 2%)
Drift electricity
192 V/cm
ΔGEM voltage
480 V
630 V
µ-PIC voltage
+400 V
+568 V
Gas gain
～ 2 × 104
～ 1.4 x 104
Electron drift velocity
4 cm / µsec
～ 3.3 cm/μsec
Typical Energy Resolution
31 keV)
～ 25 %
～ 40 %
Experimental results (137Cs 0.7 MBq, center of FoV, 50 cm away from the drift bottom of the TPC)
Photo-absorber: PSA
3D Electron Tracker: TPC
An electron track data
(Ar:C2H6(90:10) 1.5 atm gas)
Drift region
Drift plane
anode strip (y axis)
clock (z axis)
10 cm
15.5 cm
cathode strip (x axis)
GSO ×8 pixels
Multi Anode PMT H8500
48mm
drift top
drift top
drift top
15.5 cm
drift bottom
10 cm
drift bottom
10 cm
Demonstration gamma ray images of 0.7 MBq Cs-137 point source (0 deg., 12.5 deg.)
Components of a PSA
Cu-Kα
8 keV
4 channels readout with resistor chain in each PMT
Hit pixel is determined by center of gravity map of ADCs
Ba-Kα
31 keV
Performances of the ETCC
Ne+CF4+iC4H10
1.5 atm
GSO 26 mm thickness
Gas species
Ar : C2H6 (90 : 10) 1.5 atm
Ne : CF4 : iC4H10
(50 : 48 : 2)
1.5 atm
ARM
center of FoV,
FWHM)
6 degrees
8 degrees
Energy Resolution
keV, FWHM)
11.6 %
12.4 %
Gamma-ray detection efficiency
(662 keV, center of FoV)
0.93 x 10-4
1.34 x 10-4
Ar+C2H6 1.5 atm
GSO 26 mm thickness [1]
Energy [keV]
TPC energy spectrum of 133Ba (Ar:C2H6(90:10) 1.5 atm gas)
Ar+C2H6 1 atm
GSO 13 mm thickness [1]
Scintillator
GSO:Ce (6.71 g/cm3)
Pixel size
6 x 6 x 26 mm3
# of pixels
576 (3x3 PSAs)
Energy resolution
(FWHM)
10 % 662 keV)
Ionized electron cloud made by charged particle is transported in drift field
2D track (x,y) of the charged particle is measured by μ-PIC, which is one kind of Micro Pattern Gas Detector
z-axis information is obtained by clock count
scattered ɤ ray
TPC
GSO-PSAs
GEM
μ-PIC
137Cs
662 keV gamma ray
Compton scattering
Compton scattering event
photoabsorption
µ-PIC
photo-absorption
accidental event
32 keV (Ba-Kα)
4. What is problem?
Measured data (dots) and simulated data (lines) of detection efficiency of 662 keV gamma rays from the center of FoV
6. Summary
・For the environmental gamma ray measurement, we have developed an ETCC which uses a TPC with Ar:C2H6 (90:10) 1.5 atm gas. However, it has a problem about accidental events which can not be separated from Compton scattering events.
・To reduce accidental events, we tried to use Ne and CF4 based new gas (Ne : CF4 : iC4H10 (50 : 48 : 2) 1.5 atm), and compare the performance of the ETCC with new gas and that with previous one.
・From the measured data, accidental event rate for the ETCC with the new gas looks lower than that with the previous one, and the ETCC with new gas has higher gamma ray detection efficiency. These results are convenient to the quantitative environmental gamma ray measurement.
scattered gamma rays
We want to detect the Compton recoil electron in the TPC and the photoabsorption of the Compton scattered gamma ray in a GSO pixel. However, if the TPC detects an environmental X-ray and a GSO pixel detects gamma ray which did not scattered by an electron in the TPC, there is some possibility of mistaking it as a Compton scattering event.
7. References
[1] T. Mizumoto et al., PSD10 proceeding, Journal of Instrumentation accepted.
[2] D. Tomono, ICRR2015 poster presentation 2-PS10A-05.
15th International Congress of Radiation Research (ICRR2015), May , 2015, Kyoto International Conference Center, Kyoto, Japan