Neutron Detection Probe for Radiation Monitoring Systems for Use in Cyclotrons and Accelerators

Introduction: Neutrons do not interact directly with electrons in matter; therefore, the mechanisms for detecting neutrons are based on indirect methods through nuclear reactions in converter materials. Desirable converter characteristics include high neutron absorption cross-section, minimal gamma-ray interactions and high Q-value. This value determines the energy liberated in the reaction following neutron capture. Lithium-6 is considered among one of the most effective converters for thermal neutrons, this isotope offers a 940-barn cross section via the reaction 6Li(n,α)3H + 4.78 MeV, this capture reaction releases all the Q-value in charged particles. The most widely used thermal neutron scintillators are Li-glass and 6LiF:ZnS, the lithium content and its enrichment determine the neutron conversion efficiency. For thermal neutrons, intrinsic efficiencies higher than 90% are achieved by 2 mm thick Li-glass scintillators that content 6.6 wt.% lithium enriched to 95% 6Li. Li-glass is generally transparent to its emitted light; however, the light output is rather low (only ~6000 photons are emitted per absorbed neutron); thus, the light collection efficiency is crucial and is determined by proper coupling to a suitable light sensor.

Methods: In this work we are developing a neutron detection probe based on Li-glass scintillator coupled to an array of Silicon Photomultiplier (SiPM) light sensors. This detector is intended to broaden the scope of probes of the Rotem’s industry MediSmarts monitoring system. The MediSmarts is a radiation monitoring system for cyclotron facilities, accelerators and proton therapy centers. The detector sensor is composed of a 25-mm thick polypropylene moderator, Li-glass scintillator, an array of 64 SiPM’s, an electronic card and a reflector in the backside of the detector.

Results: Preliminary evaluations were performed using a 50x50 mm Gamma Scintillator coupled to an array of 64 SiPM’s. We connected all SiPM’s outputs signal to a single amplifier; the measurements show a resolution similar or better than using a photo-multiplier tube (PMT). To enable neutron flux measurements, we intend to replace the gamma scintillator with a Ce-activated lithium glass scintillator, such as Scintacor GS20®. Using MCNP code simulations, we calculated the intrinsic efficiency and response for 2 mm thick, 50 mm x 50 mm scintillators behind a 25 mm polypropylene moderator. The simulations predict the response of the detector to 252Cf source and show a sensitivity of nearly 6 cps/nv.

Conclusions: The simulated sensitivity of this type of detectors enable the development of a probe with neutron detection performances that were previously achieved only by 3He based tubes of similar dimensions. Since 3He tube-based detectors are no longer available at affordable prices, the technology presented in this work represents a potential alternative for neutron radiation monitoring in synchrotrons, cyclotrons and other accelerators facilities.