Light Collection Optimization for Large Area Plastic & ZnS(Ag) Scintillator Embedded in a Thin Detector Configuration

Abstract

The use of thin plastic scintillators coated with a thin layer of ZnS(Ag) has become very common in large area monitors for the detection of Alpha & Beta contamination. This work is focused on finding the best light collection configuration for a 100x100mm² detector. The light collection is done by using up to 9 Silicon Photo Multipliers (SiPM) with dimensions of 6x6mm. We will try to find the optimum positioning height and distribution pattern of the SiPM above the scintillator using measurements and simulations.

Introduction

The traditional gas filled proportional counters used in contamination monitors for Alpha & Beta particles are getting replaced by large area plastic coated with a thin layer of ZnS(Ag). Since proportional counters require the use of a carrier gas in order to detect Beta particles there is a need of constant gas flow making these systems hard to maintain. Using a Plastic scintillator as the detector would contribute greatly to the robustness of such systems. In order to collect the light from the plastic scintillator there is a need of a Photo Multiplier Tube which requires quite a distance between the scintillator plate and the PMT window. Therefor such detectors are also very thick. By using SiPM the thickness of the detector can be greatly reduced.

The problem is that SiPM have a lower quantum efficiency then a PMT and very small detection area. Using only one SiPM might not gather enough light photons to be considered as a detectable signal. Using too many SiPM will increase the noise level and give false signal. Finding the optimal SiPM configuration needed for a detectable signal from a 100x100 mm² scintillator by adjusting the number of SiPM units and the height above the scintillator.

Method

A "Monte Carlo" simulation of several detector configurations was made: 9 SiPM`s in several positions were placed above the scintillator. A Beta emitting source with a very low energy was placed 0.2 cm under the scintillator. Simulation data was collected and analyzed.

Results

When placing all SiPM stacked together in the middle of the detector we get a poor light collection distribution, although we get many events with high photon count, there are many events with very low photon count. As advancing with the SiPM position from the middle of the detector towards the edge we get a better signal but at a certain point the results are degrading yet still much better than the stacked SiPM configuration.

As for the height of the SiPM above the scintillator, when starting from placing the SiPM directly on the scintillator we get the worst results as some events get high readout and other get none. As we increase the height of the SiPM position the results are getting better and at a certain point are starting to degrade.

Discussion & future work

It appears that for getting the best light collection results we need to place the SiPM not too close to the scintillator but not too far, same for SiPM distribution, stacking all sensors together gave bad results and spreading them too far did so too.

Our next step would be to build such a detector in few configurations that were simulated and try to find a correlation between simulation and measurements. Later on an optimization will be made to find a better light collection configuration.