Gaseous Detector Laboratory


Position Sensitive Neutron Detector Operating in Ionization Mode


Nearly all 3He based position sensitive neutron detectors (tubes, MWPC, MSGC...) operate in "proportional" mode with gas gain. The amplification in the gas boosts the signal to noise ratio necessary for position sensing in any global encoding scheme. With the dvances in modern microelectronics, specially in the area of low noise analog signal processing, it is now possible to resolve the primary ionization signal from a neutron-3He interaction in ionization mode without any gas amplification.


Compared with chambers operating in "proportional" mode, an ionization chamber is more robust and stable. It does not suffer from discharges and aging associated with the electron avalanche process. It can also be constructed into complex geometry to better conform to the need of a experiment.


Thermal neutron interacts with a 3He nucleus to create a proton and triton with a combined 764keV kinetic nergy.These two projectiles, if stopped in the gas, generates about 25,000 of electron-ion pairs. The fast moving electrons can induce up to 4fC of charge on a properly designed electrode.


The amount of charge induced on a given electrode by a point charge Q at (x,y) equals the charge Q times the value of an induction field U(x,y), which is calculated by applying a unit potential on the electrode of interest with all other electrodes grounded. In the case of one strip in a parallel plate geometry, the exact expression for this induction field is:
where a is the width of the strip and d is the distance between the two parallel plates. The figure above shows a contour plot of the function U(x,y) in the geometry of our detector.


The figures below show the induced current waveforms (left) and the simulated shaper output waveforms (right) from three adjacent strips by a single point charge starting from the origin, moving along the y axis towards the n=0 strip. Initially, the point charge induces small, nearly equal amount of current on all three strips. As the charge moves to a distance a few times the strip width from the strips, the induced current increases rapidly for the collecting strip, and reverses polarity for the non-collecting strips. Clearly, most of the signal current on the strip electrodes is induced when the point charge is very close to the strips.



A small test chamber was constructed to study the signal characteristics in an ionization chamber with strip electrodes. The figure below shows the pulse height spectra as a function of the drift field. The flat middle section of each spectrum is the results of charge sharing between two adjacent strips. Simulations have shown that a 95% detection efficiency can be achieved by setting a discriminator level between 20% to 40% of the full charge.

This study was aimed at developing a one dimensional position sensitive detector for a crystal backscattering spectrometer. Now we are also investigating the feasibility of building a 2D, pixelated ionization chamber with very high counting rate (108 cps) for a small angle neutron scattering experiment at SNS.


Reference:

B. Yu, G. J. Mahler, N. A. Schaknowski, and G. C. Smith, "A Position Sensitive Ionization Chamber for Thermal Neutrons," IEEE Trans. Nucl. Sci. NS-48, pp. 336-340 (2001)




Last Modified: Wednesday, 06-Feb-2013 22:33:56 EST