Chapter 7.3. Thermal neutron detection

The neutron capture and the trajectories of the secondary charged particles as well as the specific gas ionization along these trajectories are presented in Figs. 7.3.3.1(a) and (b) . Since the gas ionization energy is about 30 eV per electron (42 eV for ³He and 30 eV for CH₄), there are about 25 000 ion pairs (e⁻, He⁺ or e⁻, ) per captured neutron. Gases such as CH₄ or C₃H₈ are added to diminish the length of the trajectories, i.e. the wall effect [see Subsection 7.3.4.2(b)].

Figure 7.3.3.1| top | pdf | (a) Neutron capture by an 3He atom and random-direction trajectory (Ox) of the secondary charged particles in the gas mixture. (b) Calculated specific ionization along the proton and triton trajectory in a 65% 3He/35% CH4 mixture at 300 K and atmospheric pressure (Whaling, 1958). [Reproduced from Convert & Forsyth (1983).] (c) Range of a 0.57 MeV proton (from 3He neutron capture) as a function of the pressure of various gases. [Reproduced from Convert & Forsyth (1983).] (d) Schematic drawing of a gas monodetector. The arrows represent the incoming beam.

We give in Fig. 7.3.3.1(c) the proton range of an ³He neutron-capture reaction in various gases (Fischer, Radeka & Boie, 1983). A schematic drawing of a gas monodetector, which might be mounted either in axial or in radial orientation in the neutron beam, is given in Fig. 7.3.3.1(d).

For this type of detector, the efficiency as a function of the gas pressure, or gas-detector law, is written as with P(atm) = the detector-gas pressure at 293 K, t(cm) = the gas thickness, and λ(Å) = the detected neutron wavelength. The numerical coefficient b, obtained at 293 K from the ideal gas law, the Avogadro number N_A, and the gas absorption cross section σ_a (barns) at λ₀ = 1.8 Å, is

For ³He, with σ_a = 5333 barns at λ₀ = 1.8 Å, b = 0.07417; for ¹⁰BF₃, with σ_a = 3837 barns at λ₀= 1.8 Å, b = 0.0533. We give in Table 7.3.3.2 a few examples of gas-detector characteristics.

There are two modes of operation.

In the case of direct collection of charges, the 25000 electrons corresponding to one neutron capture (primary electrons) are collected by the anode in about 100–500 ns, and generate an input pulse in the charge preamplifier (see Section 7.3.4).

If the electrical field created by the high voltage applied to the anode exceeds a critical value, the electrons will be accelerated sufficiently to produce a cascade of ionizing collisions with the neutral molecules they encounter, the new electrons liberated in the process being called secondary electrons. This phenomenon, gas multiplication, occurs in the vicinity of the thin wire anode, since the field varies as 1/r. The avalanche stops when all the free electrons have been collected at the anode. With proper design, the number of secondary electrons is proportional to the number of primary electrons. For cylindrical geometries, the multiplication coefficient M can be calculated (Wolf, 1974). This type of detection mode is called the proportional mode. It is very commonly used because it gives a better signal-to-noise ratio (see Section 7.3.4).

A few critical remarks about gas detectors:

This mode of detection is generally used for monitors. In a typical design, a ¹⁰B deposit of controlled thickness, for example t = 0.04 µm giving a capture efficiency of 10⁻³ at λ = 1 Å, is made on a thin aluminium plate (see Fig. 7.3.3.2 ). One of the two particles (α, Li) produced in the solid by the capture reaction is absorbed by the plate; the other escapes and ionizes the gas. The electrons produced are collected by the aluminium plate, itself acting as the anode, or by a separate anode wire, allowing the use of the proportional mode. The detection efficiency is proportional to the deposit thickness t, but t must be kept less than the average range r of the secondary particles in the deposit (for ¹⁰B, r_α = 3.8 µm and r_Li = 1.7 µm), which limits the efficiency to a maximum value of 3–4% for λ = 1 Å. The fraction of the secondary particle energy that is lost in the deposit reduces the detector current, i.e. the signal-to-noise ratio, and worsens the amplitude spectrum (see Section 7.3.4).

Figure 7.3.3.2| top | pdf | Typical design of a 10B-foil detector.

In the detection process via scintillation (see Table 7.3.3.1), the secondary particles produced by the neutron capture ionize and excite a number of valence-band electrons of the solid scintillator to high-energy states, from which they tend to decay with the emission of a light flash of photons detected by a photomultiplier [see Fig. 7.3.3.3(a) ]. A number of conditions must be satisfied:

Most thermal neutron scintillation detectors are currently based on inorganic salt crystals or glasses doped with traces of an activating element (Eu, Ce, Ag, etc.) (extrinsic scintillators). (A plastic scintillator might be considered to be a solid organic solution with a neutron converter.)

The use of extrinsic scintillators (Convert & Forsyth, 1983), although less efficient energetically, permits better decoupling of the energy of the photon-emitting transition (occurring now in the activator centres) from that of the valence-band electron excitation or ionization energy. The crystal or glass is then transparent to its own emission, and the light emitted is shifted to a wavelength better adapted to the following optical treatment.

In order to maximize the light collected by the photomultiplier [Fig. 7.3.3.3(b)], a light reflector is added in front of the scintillator, and a light coupler adapts the dimensions of the scintillator to that of the photomultiplier (PM). The area of the scintillator might be very large (up to 1 m²). The optimum thickness of a glass scintillator is about 1 to 2 mm, corresponding to a neutron detection efficiency of 40 to 97% for λ = 1.8 Å, depending on the ⁶Li concentration (Strauss, Brenner, Chou, Schultz & Roche, 1983). In a Ce-doped Li silicate glass, the number of photons emitted per captured neutron is about 9000, giving finally about 1500 electrons at the photocathode in the optimum light-coupling configuration. The number of photons emitted per captured neutron, the number of those reaching the photocathode, and the scintillator decay time are parameters that might differ by an order of magnitude, depending on the scintillator material. However, the glass scintillator remains for the time being the best choice, since the possible gains given by other materials, in decay time or in the number of photons emitted, are always very severely offset by poor light output (e.g. the plastic scintillator). It is very important to maximize the number of photons per neutron reaching the photocathode, since this will help to discriminate between neutrons and γ rays [see Fig. 7.3.4.2(e) ]. The optical coupling between the different parts of the detection system must be of very good quality.

Films are classed as position-sensitive detectors. Two types of neutron converter are used in neutron film-detection processes. In the case of the scintillation film system, a light-sensitive film is pressed close to one or between two plastic ⁶LiF/ZnS(Ag) scintillator screens (Thomas, 1972). In the case of the Gd-foil converter, the conversion electrons are emitted isotropically, with a main energy peak at 72 keV, and collected by an X-ray film in close contact with the converter (Baruchel, Malgrange & Schlenker, 1983).

In addition to the advantages given by the film technique in itself (simplicity, low price, direct picture, etc.), neutron photographic methods give the best spatial resolution. However, the resolution is inversely related to the detector efficiency and thickness. A good compromise appears to be a thickness of 0.25 mm for a plastic scintillator [i.e. a capture efficiency of about 12% and a resolution of 0.1 mm for a one-screen converter at λ =1 Å; see the size of the ionization volumes in a scintillator, Fig. 7.3.3.3(a)]. Here, however, as in light scattering, the optical density depends on the exposure time as well as on the incoming flux (Schwartzschild effect), which necessitates a calibration (Hohlwein, 1983). For a natural Gd-foil converter, an optimum thickness is 0.025 mm, giving a resolution of 0.020 mm. The Gd-foil film detector is one order of magnitude less efficient than the scintillator, but, as in electron microscopy, the optical density is nearly proportional to the exposure. This explains the use of the Gd foil in neutron-diffraction topography.

If we take into account the possible inhomogeneity of the converter and the difficulties related to the film (homogeneity, development, and photodensitometry), an accuracy of 5 to 10% is achievable in the intensity measurements under good conditions.

Owing to the differences in the processes, neutron photographic techniques are much more efficient than those for X-rays. In the case of the plastic scintillator, the gain is about 10³, which compensates for the much lower neutron fluxes.

Each collected burst of electrons, corresponding to one captured neutron, will be successively amplified, identified (discriminated), and transformed to a well defined signal, by a chain of electronic devices that are represented in Fig. 7.3.4.1 . In the case of the gas detector, the complete electronic chain is generally contained in a grounded metallic box acting as an electrical shield. The detector is connected to this box via a coaxial cable as short as possible to avoid noise and parasitic capacitance. A high-voltage power supply feeds the detector anode through a filter. The charge-sensitive preamplifier contains a field-effect transistor (FET) to minimize the background noise, since, from the detector up to this stage, the electronic level is very low. At the output of the FET, the pulse corresponding to one neutron has an amplitude of about 20 mV. This pulse enters an operational amplifier with adjustable gain G, which delivers a signal of about 2 V, the analogue signal (ANA). The electronic pulse-rise time (0.5 to a few µs) is adapted to the detector electron-collecting time, i.e. its amplitude is roughly proportional to the number of electrons collected at the anode. The last part of the electronic chain is a discriminator with an adjustable threshold, followed by a trigger delivering a calibrated signal (e.g. +5/0 V), called the logic signal (LOG), which is sent to a scaler.

Figure 7.3.4.1| top | pdf | Electronic chain following (a) an 3He gas detector and (b) a scintillation detector.

In the case of the scintillator, the photomultiplier ensures the conversion of light to electrons and produces a strongly amplified electronic signal that is processed through a discriminator and trigger as for the gas detector.

For the gas detector, there are basically three parameters to be adjusted, the gas-amplification coefficient M (a function of the detector high voltage), the electronic amplification gain G, and the discriminator threshold T. Since these adjustments are interactive, the high voltage is initially set at the value given by the manufacturer. The gain G is adjusted in order not to saturate the amplifier. When the electronic amplifier power supply voltage is 5 V, a typical setting of the pulse maximum amplitudes is about 3 V.

We present now the three types of operation necessary to adjust the electronics.

The electronic adjustments and controls of types of detector other than ¹⁰BF₃ gas detectors are basically the same once the changes in the amplitude spectrum have been taken into account. We present in Fig. 7.3.4.2(e) the amplitude spectra for an ³He gas detector with significant wall effects, for a ¹⁰B solid-deposit detector with very low efficiency, and for a scintillator. The energy of the secondary particles produced in an ³He gas detector is 765 keV, about three times less than in ¹⁰BF₃, reducing the signal-to-noise ratio; the relative importance of the wall effect is greater and extends to A₀/4. In the case of the ¹⁰B deposit detector, only one of the secondary particles escapes the foil, so that we do not detect an amplitude A₀ corresponding to the full capture-reaction energy, but only that corresponding either to an average α or Li trace. The quality of the valleys depends on the t/r (foil thickness/particle range) ratio in the ¹⁰B solid (see Fig. 7.3.3.2). The figure corresponds to a monitor where . For the scintillator, the valley in the amplitude spectrum is not very good, even for good glasses and without γ radiation. The discrimination is therefore always much inferior to that of a gas detector. Moreover, the gain of the photomultiplier is very sensitive to the high voltage and has long-term stability problems.

In order to measure the scattered intensities, the single detector is mounted in a shield equipped with a collimator between sample and detector. The collimator is adapted to the sample (5 to 20′ Soller collimator for a powder diffractometer, or a hole adapted to the size of the beam diffracted by a single crystal). The sensitive area of the detector matches the size of the collimated diffracted beam. This geometry allows one to localize the scattered beam with adequate resolution and to avoid parasitic neutrons. In a powder diffraction measurement, the detector is scanned with a goniometer, each step being monitored.

With the advantages of speed and simultaneity of data collection over broad angular ranges, position-sensitive detectors (PSDs) have seen considerable development (Convert & Chieux, 1986). They are based on the fundamental detection processes described in Section 7.3.3. Their dimensions, the introduction of various systems for position encoding and decoding, and the multiplication of the number of detection chains (with necessary adjustments and controls) have produced detection systems of a complexity that can no longer be grasped by a researcher only occasionally involved in neutron-scattering experiments. The following lines give only a very crude introduction to PSDs at a descriptive level.

A PSD is always mounted in a shield that has an opening limited by the cone defined by the sample size and the detection area. Given the large opening in the shield, the PSD is very sensitive to parasitic neutrons, especially those coming from materials in the main monochromatic beam. However, if sufficient care is taken with the sample environment and beam-stop design, the signal-to-noise ratio may be as good as or even better than in a single detector. The scattering angle is determined by the angular position of the origin of the PSD, the sample-to-detector distance, and the location of the neutron capture in the PSD.

In a PSD, each individual neutron-capture reaction in the continuous converter is localized via an internal encoding system followed by electronic decoding. There are three types of PSD: the gas resistive wire, the gas multielectrode, and the Anger camera (scintillation).

Table 7.3.5.1 gives the characteristics of some PSDs, each example being a good compromise of detector characteristics adapted to the instrument needs, powder diffraction, single-crystal diffraction, and small-angle neutron scattering (SANS). From this table, it seems that the characteristics of the various types of PSD that have been presented are nearly equivalent. The homogeneity of neutron detection over the PSD sensitive area is better than 5% in all cases. Among those PSDs, the gas PSDs, which have been installed since the early 1970s, are the most commonly used. The definite advantage of the gas multi-electrode PSD is to digitize the encoding, thus offering very good stability and reliability of the neutron localization. The resistance-encoding gas PSD has less complex electronics and permits a choice of pixel size (elementary detection area) and thereby definition. The Anger camera also offers the same flexibility in the choice of the pixel size and, because of the small thickness of the scintillator (1–2 mm), has very little parallax and is well adapted to TOF measurements. However, the γ sensitivity of the scintillators is much higher than that of the gas PSDs.

Table 7.3.5.1| top | pdf | Characteristics of some PSDs 1D = one dimensional; 2D = two dimensional; F/ = flat; C/ = 1D curved; X = single crystal.   Gas Gas Scintillation Resistance encoding Multi-electrode Anger camera 1D 2D 1D 2D 1D 2D Location F/ MURR (USA) F/ ORNL (USA) F/ ILL (France) F/ ILL (France) F/ Jülich (FRG) F/ ANL (USA)     C/ CENG (France) C/ ILL (France)     Instrument reference Powder (1) SANS (2) F/ Liquids (3) F/ SANS (3,5) Powder (6) X (7) C/ Powder (4) C/ X (3)     Detection area  (mm) l = 610 (Ø = 25.4) 650 × 650 F/ l = 162.5 (h = 70) F/ 640 × 640 l = 744 (h = 20) 300 × 300 C/ l = 2096 C/ 1300 × 80     or 80° (h = 70) or 64° × 4°     Cell size or pixel (mm)   10 × 10 F/ 2.54 F/ 5 × 5 0.7 0.3 × 0.3 C/ 2.62 C/ 2.54 × 5     or 0.1°       Resolution  (mm) 2.5 10 × 10 F/ 3.2 F/ 5 × 5 2.5 2.7 × 2.7 C/ 2.6 C/ 3 × 6.6     Efficiency  (%) 70 (1.3 Å) 90 (4.75 Å) F/ 90 (0.7 Å) F/ 75 (11 Å) 70 (1.2 Å) 80 (1.8 Å) C/ 52 (2.5 Å) C/ 85 (1.5 Å)     MURR: Missouri University Research Reactor, Columbia, Missouri, USA; ORNL: Oak Ridge National Laboratory, Tennessee, USA; ILL: Institut Laue–Langevin, Grenoble, France; ANL: Argonne National Laboratory, Argonne, Illinois, USA; CENG: Centre d'Etudes Nucléaires de Grenoble, Grenoble, France. References: (1) Berliner, Mildner, Sudol & Taub (1983); (2) Abele, Allin, Clay, Fowler & Kopp (1981); (3) Institut Laue–Langevin (1988); (4) Roudaut (1983); (5) Ibel (1976).; (6) Schaefer, Naday & Will (1983); (7) Strauss, Brenner, Chou, Schultz & Roche (1983).

PSDs permit a simultaneous measurement of intensities over relatively large areas. As compared with the scanned single detector, this opens the way to two possibilities, either shorter counting time (e.g. real-time experiments) or improved statistical precision. However, the high counting rates achieved, as well as the complexity of the detection, increase the effective dead time and the occurrence of defects. The efficient use of a PSD therefore requires a detailed understanding of its operation (detection process, encoding–decoding, data storage, etc.) and periodic tests and calibrations.

When it is useful to have a large detection area without requiring spatial continuity of the detection, the solution is in the juxtaposition of several detectors. The selection of the type of detectors, the way of regrouping them, and the design of the collimation system depend on the measuring instrument. An appropriate geometry will optimize the signal-to-noise ratio and the instrumental resolution.

Banks of single detectors, up to 64 covering up to 160° in the diffraction plane with Soller collimators in front of each detector, are used for powder diffractometers in reactors [D1A and D2B, Institut Laue–Langevin (1988)]. The relative position of the detectors plus collimators and their response to the neutron intensity have to be measured and calibrated. The bank of detectors is scanned in small steps over the interval between two successive detectors in order to obtain a complete diagram over the angular range of the bank.

In a similar way, a juxtaposition of small PSDs with individual collimators is also used [D4, Institut Laue–Langevin (1988)].

In the case of spallation sources, the time-of-flight powder diffractometers are made of arrays of detectors at selected diffraction angles, to increase the detection area (Isis, 1992). Whenever it is possible, the time focusing geometry is used (Windsor, 1981). This implies a particular alignment of the detector array in order for it to become equivalent to one detector at one angle.

On the same instrument [HRPD, SANDALS, LAD (Isis, 1992)], various types of detector are used (³He gas single detectors of small diameter, Li glass or Li+ZnS scintillators). For each selected diffraction angle, the choice of detector depends on the required resolution, which is better for scintillators because of their small thickness, and on other properties of the detectors (background, stability, γ discrimination), which are better for ³He detectors.

For up to 10% of dead time in the counting rate, the correction for the dead-time loss is generally considered as linear. If Δt is the electronic dead time for one neutron (1–10 µs for the gas detectors) and n the number of counts per second, the dead-time correction factor is 1/(1 − nΔt).

In the case of a bank of detectors used for a powder diffractometer in a reactor, one has to calibrate the relative positions of the detectors and their response to the neutron intensity by scanning the detectors through a Bragg pattern.

In the case of TOF measurements, the detector banks are installed at fixed angles. For each detector, the measured intensity depends on the detector type, size, and distance to the sample. The neutron and γ background depends moreover on the detection angle. After background corrections, the intensities measured by each detector bank are calibrated and matched using the overlaps between spectra.