Neutron detector

Neutron detectors are used to detect, measure the flow and spectroscopy of free neutrons ( radiation protection monitoring , basic research in nuclear physics and solid state physics (including neutron scattering )). Since neutrons themselves do not have an ionizing effect, they have to be detected via scattering on atomic nuclei or via nuclear reactions that produce ionizing radiation or a radionuclide ( neutron activation ).

Fast neutrons

1. Fast neutrons can be decelerated in a moderator and subsequent detection of the thermal neutrons via a suitable nuclear reaction , e.g. B. ¹⁰ B (n, α) ⁷ Li can be detected. These detectors have a high detection probability. Disadvantages are (1) the necessary large volume of the moderator, which prevents finely localized measurements, and (2) the loss of information about the original energy of the neutrons. Example: Long Counter ; Application: e.g. B. for radiation protection tasks.
2. Neutrons with kinetic energies above about 50 keV can be detected by elastic scattering on hydrogen nuclei - i.e. protons - and registration of the signal generated by the protons ( recoil protons ) in an ionization chamber , a proportional counter or a scintillation counter . The energy of the recoil proton also provides information about the neutron energy. Proton recoil detectors also respond to gamma radiation , which is mostly present in fast neutron fields. Materials for such scintillators are, for example, special transparent plastics, stilbene or, more recently (2015), elpasolite . Organic liquid scintillators allow the gamma background to be distinguished from the neutrons by means of pulse shape discrimination.
3. Detection in photographic nuclear trace emulsions . Here the recoil protons cause traces that are visible after the film has been developed. Application: Personal dosimeter for radiation protection monitoring .
4. Proof of the measurement of the activity of material samples that have been activated by nuclear reactions with neutrons ( neutron activation ). With a suitable endothermic nuclear reaction with a known threshold energy, it is achieved that only fast neutrons above this threshold are detected.

Slow neutrons

Slow, especially thermal neutrons are detected via suitable nuclear reactions with large cross- sections, such as B. ¹⁰ B (n, α) ⁷ Li, ⁶ Li (n, α) ³ H or ³ He (n, p) ³ H. The reaction substance can be gaseous or, in the case of boron or lithium, as a wall layer in ionization chambers or counter tubes or used as part of a scintillator , e.g. B. Boron trifluoride counter tubes, boron ionization chambers, scintillators made of lithium iodide (LiI) or lithium-containing glass with lithium enriched with ⁶ Li .

The neutron-induced fission of ²³⁵ U in fission chambers (ionization chambers in which an electrode is coated with enriched uranium ) is also used to monitor the neutron flux in nuclear reactors .

Fission chambers that remain permanently in the reactor core (e.g. the power distribution detectors in boiling water reactors ) would become unusable within 3–4 years if they burned up. That is why a coating made of ²³⁴ U with a small proportion of ²³⁵ U is often used here. In this way, the fission losses are constantly compensated for by the incubation of new ²³⁵ U via neutron capture, and the detector can be used for 10 years without deteriorating the measurement accuracy.

Slow neutrons can also be detected by neutron activation of nuclides whose cross-section is large enough for a low neutron energy and where the isotope created by the activation emits radiation with typical energy and half-life . One example is the ball measurement system that is used in some nuclear reactors.

literature

Glenn F. Knoll: Radiation detection and measurement. 2nd ed. New York: Wiley, 1989. ISBN 0-471-81504-7

References

1. ^ DHS: Stilbene, an Organic Scintillator for Fast Neutron Detection
2. ^ DHS: Dual Gamma-Neutron Detector Materials
3. ↑ "Instrumentation" booklet of the Kraftwerkschule e. V. Essen, March 2001