Thermal neutron detector with ZnS(Ag)/LiF

Motivation for the development of a solid-state scintillator thermal neutron counter.

Leak of ³He gas from traditional counters of thermal neutrons.
High cost of helium counters and associated electronics.
The difficulty of preparation of helium counters for measurements (High voltage supply, counting characteristics, thresholds).
Normal counters with ZnS(Ag)/LiF and WLS-fibers transmit only 5% of the light from the scintillator to the photomultiplier. The efficiency of such detectors achieves 75%. The disadvantages are the use of high-voltage sources and photomultipliers. The use of WLS-fibers increases the sensitivity of the counters to gamma rays.

The design of the prototype neutron counter.

We tried to make a solid-state scintillator with parameters similar to helium thermal neutron counters in this project. We create counters for thermal neutrons , containing a ZnS(Ag)/LiF, the optical light guide from an Plexiglas, and two micropixel avalanche photodiodes. Counter dimensions are 100 x 15 x 6 mm³. The own efficiency of the counter was 75%.

Fig. 1. The scheme of detector using a lightguide with diffuse reflection. 1 – light guide with 10×4 mm² cross section, 2 – scintillation sheet, 3 – optic compound, 4 – SiPM, 5 – Teflon tape as diffuse reflector, 6 – jacket

Parameters of the neutron counter.

Variable active area of the counter
Scintillator ZnS(Ag)/LiF with efficiency of 46%
Own efficiency of the counter was 75%.
The count rate of detector is better than 10⁵/s
Gamma sensitivity on the order of 10^-7

Fig 2.Photo of various detector options

Ringneutron scintillator detectorsbased on SiPMand optic lightglide.

Based on these detectors, a ring detector was also created. The detector consist 12 independence channel.

*Fig 3. Photo of ring scintillation detector. 1- shaper,2- detector,3- borated polyethylene*

*Fig 4. Photo of neutron counters and shaper*.

First experimental data.

Fig. 5. The charge spectra of a type 2 counter with its neutron emission: 1 – without cadmium foil, peak corresponds to 95 photoelectrons; 2 – in cadmium foil 1 mm thick; 3 – dark noises.

Fig 6. Time-of-flight spectra measured on synthetic diamond powder using helium (a) and new scintillation (b) counters

Conclusion.

1. The efficiency of 75% was obtained for designed counters, comparable to the efficiency of the helium counters.

2.The time resolution of the counters is better than 1 µs, that allows the use of detectors for time-of-flight measurements.

3.Low sensitivity to gamma quanta ~10^–⁷.

4.Absence of thermal noise at the threshold of 10 ph.e. and temperature of +30 ⁰C.

5.Low voltage supply: +33V, +6V, – 6V.

6.The same values of power voltage and thresholds for all counters. No needs for individual tunings. Simple and noise-immune electronics.

7.It is possible to manufacture the detectors with the length up to several tens of cm.