³He proportional neutron counters (helium-3 gas filled tubes) are currently the most sensitive type of neutron detectors. These detectors owe their high sensitivity to thermal neutrons to the exceptionally high probability of thermal neutron capture by helium-3 atoms (technically speaking, ³He nuclei have very high thermal neutron capture cross-section). The corresponding neutron capture reaction is as follows:

³He + ⁰n → ³H + ¹H + 0.764 MeV.

E.i. a helium-3 nucleus splits into a tritium nucleus and a proton upon capturing a neutron while releasing a significant amount of energy.

Thermal Neutrons

I have emphasized the term thermal because in general neutrons come in different energies:

• Fast neutrons (i.e. neutrons with energies greater than 1 MeV) interact very weakly with matter and therefore pass through most substances without being absorbed (like x-rays or gamma rays); these neutrons are difficult to detect unless the flux is large enough to cause statistically meaningful absorption (e.g. one may need a fast neutron flux on the order of 10,000 to 100,000 neutrons/second/cm² in order to register an equivalent count rate of just a 1-2 counts per second);
• Thermal neutrons are produced when fast neutrons loose most of their energy due to scattering and thus attain thermal equilibrium with matter (thermal neutron energy is on the order of a few eVs); typically substances such as purified carbon, heavy water, or high-density polyethylene (HDPE) are used to slow down fast neutrons, such substances are called moderators; thermal neutrons are readily absorbed by ordinary matter, although some elements (such as ³He, ¹⁰B or ⁶Li) are particularly good at absorbing thermal neutrons and therefore they are frequently used as an active material in neutron detectors.
• Finally, cold neutrons are characterized by energies much lower than 1 eV; such neutrons are rapidly absorbed by everything and therefore they can travel only a few microns in matter before being pulled into the nearest nucleus; therefore cold neutrons can be detected only in situ, e.g. via neutron activation.

Clearly, each type of neutron energy requires a dedicated type of detector as researchers so far were unable to come up with a one-fits-all solution characterized by satisfactory sensitivity.

Proportional Counters

When counting neutrons, sensitivity is key: most nuclear science research projects typically wrestle with the detection of extremely weak neutron fluxes. For such projects ³He-filled proportional counters are indispensable as the ³He proportional counters easily outperform all other types of thermal neutron detectors in terms of sensitivity.

To maximize the detector efficiency (i.e. to increase the count rate) it is beneficial to use as large of a detector as possible because the count rate for an isotropic source is proportional to the solid angle covered by the detector. This also means that the detector needs to be located as closely to the source as possible since the flux will rapidly decline as R² over distance.

Lastly, the detector would need some moderator to thermalize the neutrons. Just how much moderator is necessary depends on the type of moderator used and on the energy of the source neutrons. When HDPE is used, generally 2-3″ around the detector works best for general purpose applications.

Detector fill pressure is less important and plays only a minor role in increasing efficiency (e.g. increasing the detector fill pressure from 1 to 20 atm may increase the detector efficiency by 20% or so).

Neutron detector sensitivity is typically expressed in cps/nv, which is the measure of of the resulting counts per second (cps) per neutron flux of 1 neutron per second per cm². For ³He-filled proportional counters this measure is chiefly determined by the detector cross-sectional area wrt the flux of thermal neutrons with detector volume / diameter and fill pressure playing only a minor role.

Sensitivity to X-Rays

Contrary to popular belief, all proportional neutron counters are in principle sensitive to x-rays. In practice, however, this sensitivity does not present a major problem since pulses produced by a detector in response to x-rays are much lower in amplitude than pulses produced in response to neutrons. Therefore such x-ray pulses can be easily discriminated on the basis of their low amplitude. The problem arises only when the intensity of x-rays is great: then the magnitude of multiple overlapping x-ray pulses may become significant and begin to encroach onto the thermal neutron spectrum thus requiring one to increase the lower level discriminator threshold and sacrifice some of the detector efficiency in order to filter out the x-ray noise.

Detector Bias

Most ³He proportional counters require high voltage (aka bias) to operate. Typically the detector outer shell is grounded and a high voltage (on the order of 1000-1500 volts) is applied to the center anode wire of the detector. When a helium-3 nucleus within the detector fill gas captures a neutron the resulting nuclear reaction causes a brief ionization event, which temporarily shorts the anode wire to the ground causing the detector to output a tiny current pulse. This current pulse is typically amplified by a charge-sensitive amplifier and further digitized and processed by a pulse processor and a multi-channel analyzer (MCA).

NEUTRON-LITE

A NEUTRON-LITE system is a turn-key solution for neutron detection. The NEUTRON-LITE contains a built-in bias power supply and a charge-sensitive preamplifier (which is similar to Ortec 142PC in terms of performance). NEUTRON-LITE is controlled by PulseCounter software, which allows selecting a desired bias voltage and specifying a signal-processing algorithm to recover a robust neutron pulse-height spectrum.

A benchmark neutron detection setup is shown on Fig. 1.

The setup consists of a NEUTRON-LITE system with the detector enclosed in a 12″ tall by 8″ diameter HDPE cylinder (moderator). The cylinder has a side pocket made to house a Po-Be neutron check source (to maximize the count rate one should place the source as close to the detector as possible and put moderator around both detector and the source).

The Preset

A PulseCounter software preset for thermal neutron counting is as follows:

• Bias: 1350V;
• Signal Range: 2V, DC coupling;
• Pulse shaping: trapezoidal filtering, shaping time 16 μs, shelf 8 μs;
• MCA: threshold 20 mV, absolute, zero baseline.

Thermal Neutron Spectrum

The resulting thermal neutron spectrum acquired using a 5 mCi Po-Be check source is shown on Fig. 2.

The thermal neutron spectrum has several notable features:

• X-ray peak: these are the pulses due to background gamma spectrum; the x-ray peak must be rejected either by raising the MCA threshold or by setting up a separate channel-based pulse rejection criterion (e.g. in this example counts in channels below keV must be rejected);
• Tritium shelf: tritium peak starts at 191 keV and forms a continuum (“shelf”) because some tritons manage to reach the detector wall before depositing all of their energy into the detector fill gas;
• Proton shelf: proton peak starts at 573 keV and forms a continuum (“shelf”) because some protons also manage to reach the detector wall rather than spending their energy on ionizing the detector fill gas (both the tritium and the proton shelf are a feature of the detector fill pressure and geometric size);
• Thermal neutron peak: is the main feature of a thermal neutron spectrum; generally this peak must be as narrow and as tall as possible wrt the tritium and the proton shelves;
• High-energy continuum: there will be a few neutron events past the thermal neutron peak as ³He will capture a few of not-so-thermal neutrons also, although the quantity of these events will be vanishingly small compared to the magnitude of the thermal neutron peak.

Conclusions

1. The ³He-filled proportional counters are the most sensitive detectors of thermal neutrons known.
2. NEUTRON-LITE system is an easy, turn-key solution for counting neutrons and capturing thermal neutron spectrum.
3. Unique shape of thermal neutron spectrum provides an excellent indicator of detector performance: by looking at the spectrum we can tell if what we are counting are indeed thermal neutrons. If the spectrum deviates from the expected thermal neutron shape we know that our signal is affected by sources of systematic errors such as x-rays or electromagnetic noise (e.g. ground loop interference). For additional information review the Reliable Neutron and Gamma Detection paper.