December 3th, 2020 - The History of Neutron Detection

The discovery of the neutron was paramount to the development of atomic physics in the first half of the twentieth century. The essential nature of the atomic nucleus was established with the discovery of the neutron by James Chadwick  in 1932.  

Detection of the neutron requires a special environment because neutrons do not typically cause ionization since they are not charged particles.  However, if the interaction of the neutron is with a nuclide of high neutron cross-section¹ then a response to neutrons becomes probable.  Nuclides commonly used as neutron detector materials are helium-3 (³He), lithium-6 (⁶Li), boron-10 (¹⁰B) and uranium-235 (²³⁵U).  The detector material is surrounded by a moderator which reduces the kinetic energy of the neutron (slowing down the neutron).  Moderated neutrons are commonly called thermal neutrons providing a high probability of interaction with the target material. 

The following neutron detectors are discussed as the most common types used today.            

¹ The neutron cross-section expresses the likelihood of interaction between an incident neutron and a target nucleus.

Helium-3 (³He) Gas-Filled Proportional Detectors

³He was first proposed as a neutron detector in 1939.  It was first thought to be a radioactive isotope until samples of natural helium were found (which is mostly helium-4).  Helium is found just below the earth’s crust in a ratio of 300 atoms of ³He per million atoms of ⁴He.

With no electrical charge, neutron interaction with atomic electrons is not possible (as in X-ray, gamma, electron or beta detectors).  Therefore, we rely on interaction with an atomic nucleus.

The n + ³ He reaction is shown below with conservation of energy as:

                                   n + ³He → ³H + p + 764 keV

The energy of 764 keV is the sum of the proton kinetic energy of 573 keV plus the triton (tritium ion) kinetic energy of 191 keV.  This energy (charge) is collected as daughter products yielding an output pulse which is proportional to the 764 keV energy for thermal neutrons.

³He provides an efficient neutron detector when ³He reacts by absorbing thermal neutrons, producing a ¹H (proton) and a ³H ion (tritium). Its sensitivity to gamma rays is negligible, providing a very useful neutron detector. Unfortunately, the supply of ³He is limited to production as a byproduct from the decay of tritium (which has a 12.3-year half-life); tritium is produced either as part of weapons programs as a booster for nuclear weapons or as a byproduct of reactor operation.  Therefore, the price of these detectors is somewhat high due to limited availability.

Lithium-6 (⁶Li) Neutron Detectors

Scintillating ⁶Li glass for neutron detection was first reported in 1957.  However, it was not until the early 1990s that major advances were made by Pacific Northwest National Laboratory.  These new techniques were first classified and later declassified in 1994.

The scintillating glass fibers operate by combining ⁶Li and Cerium ions into the glass composition.  Since ⁶Li has a high cross-section for thermal neutron absorption this reaction will produce a tritium ion, an alpha particle, and kinetic energy.  The ionization produced is transferred to the cerium ions which results in an emission of photons with wavelength 390 to 600 nm.  This event results in a flash of light of several thousand photons for each neutron absorbed.  The glass fiber acts as a waveguide for the scintillation light which is coupled to a PMT.  Pulse shape discrimination (PSD) is then used to separate gamma and neutron events.

More recently a similar reaction with ⁶Li is used for neutron detection in CLYC detectors. The CLYC crystal uses 95% enriched ⁶Li and doped with cerium ions. This detector produces monoenergetic pulses above  3 MeV for thermal neutrons. This gamma/neutron separation is even further enhanced by PSD and algorithm improvements.  Other detectors like CLLBC are available and offer good gamma resolution and neutron separation up to gamma fields of 20 mR/h and higher.  Some detectors offer fast timing (~60 ns) so that a large dynamic range in counting rates is possible.

The above is just a brief introduction to effective neutron detectors. If you would like more information about this topic, please contact Allan Gonzalez at 415-453-9955 email at agonzalez@berkeleynucleonics.com .