International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C, ch. 7.3, pp. 644-648

## Section 7.3.3. Neutron detection processes

P. Converta and P. Chieuxa

aInstitut Laue–Langevin, Avenue des Martyrs, BP 156X, F-38042 Grenoble CEDEX, France

### 7.3.3. Neutron detection processes

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A detection process consists of a chain of events that begins with the neutron capture and ends with the macroscopic `visualization' of the neutron by a sensor (electronic or film). The quality of a detection process will depend on the efficiency of the conversion steps and on the characteristics of the emission steps, which alternate in the process (see Table 7.3.3.1). We present below typical detection processes.

 Table 7.3.3.1| top | pdf | Commonly used detection processes
1st conversion (neutron captures)2nd conversion3rd conversionSensor
Capture/solid → e     Film
n + 157Gd
Gas ionization Capture/gas Gas ionization → e   Electronics
n + 3He → p + t (3He + add. gas)
Capture/solid Gas ionization → e   Electronics
n + 10B, 6Li, 235U (e.g. Ar + CO2)
fission products
Scintillation Capture/solid: Fluorescence/solid:
+ ZnS(Ag) + ZnS(Ag) → ν   Film
n + 6Li → α + t
+ ZnS(Ag) + ZnS(Ag) → ν Photoelectric effect → e Electronics
n + 6Li → α + t
Ce3+ enriched glass enriched Li glass → ν Photoelectric effect → e Electronics
n + 6Li → α + t

υ = photon.

#### 7.3.3.1. Detection via gas converter and gas ionization: the gas detector

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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 NA, and the gas absorption cross section σa (barns) at λ0 = 1.8 Å, is

For 3He, with σa = 5333 barns at λ0 = 1.8 Å, b = 0.07417; for 10BF3, with σa = 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.

 Table 7.3.3.2| top | pdf | A few examples of gas-detector characteristics
Detection gasAdditional gasGas pressure (atm)Useful detection volume (mm × mm)MountingCapture efficiency
λ = 1 Åλ = 2 Å
10BF3   1 L = 200, Ø = 50 Axial 65.5% 88.1%
3He   5 L = 100, Ø = 50 Axial 97.5% 99.9%
3He   8 L = 250, Ø = 10 Radial 44.7% 69.5%
3He (monitor) C3H8 2 100 × 40 × 40   10−5 to 10−3
Value calculated for the diameter.

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:

 (i) Some gases have a tendency to form negative ions by the attachment of a free electron to a neutral gas molecule, giving a loss of detector current. This effect is negligible for 3He but it limits the use of 10BF3 to about 2 atmospheres pressure, although traces of gases such as O2 or H2O (e.g. detector materials and wall outgasing) are often the reason for loss by attachment. (ii) Pure 3He and 10BF3 gas detectors are practically insensitive to γ radiation. This is no longer the case when additional gases, which are necessary for 3He, are used, although the polyatomic additives C3H8 and CF4 are much better than the rare gases Kr, Xe, and Ar (Fischer, Radeka & Boie, 1983). (iii) For various reasons (the price of 3He and 10BF3 and the toxicity of BF3), neutron gas detectors are closed chambers, which must be leak-proof and insensitive to BF3 corrosion. The wall thickness must be adapted to the inside pressure, which sometimes implies a rather thick front aluminium window (e.g. a 10 mm window for a 16 bar 3He gas position-sensitive detector; aluminium is chosen for its very good transmission of neutrons, about 90% for 10 mm thickness).

#### 7.3.3.2. Detection via solid converter and gas ionization: the foil detector

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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, rα = 3.8 µm and rLi = 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.

#### 7.3.3.3. Detection via scintillation

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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:

• (i) The scintillation must be immediate after the neutron-capture triggering event.

 Figure 7.3.3.3 | top | pdf |(a) Schematic representation of the neutron capture, secondary α, and triton ionization volumes, and scintillator light emission in a cerium-doped lithium silicate glass. [Reproduced from Convert & Forsyth (1983).] (b) Schematic representation of a scintillation detector.
• (ii) The scintillation decay time must be short. It depends on materials, and is around 50–100 ns for lithium silicate glasses.

• (iii) A large fraction of the energy must be converted into light (rather than heat).

• (iv) The material must be transparent to its own radiation.

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.

#### 7.3.3.4. Films

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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.

### References

Baruchel, J., Malgrange, C. & Schlenker, M. (1983). Neutron diffraction topography: using position-sensitive photographic detection to investigate defects and domains in single crystals. Position-sensitive detection of thermal neutrons, edited by P. Convert & J. B. Forsyth, pp. 400–406. London: Academic Press.
Convert, P. & Forsyth, J. B. (1983). Position-sensitive detection of thermal neutrons: Part 1, Introduction. Position-sensitive detection of thermal neutrons, pp. 1–90. London: Academic Press.
Fischer, J., Radeka, V. & Boie, R. A. (1983). High position resolution and accuracy in 3He two-dimensional thermal neutron PSDs. Position-sensitive detection of thermal neutrons, edited by P. Convert & J. B. Forsyth, pp. 129–140. London: Academic Press.
Hohlwein, D. (1983). Photographic methods in neutron scattering. Position-sensitive detection of thermal neutrons, edited by P. Convert & J. B. Forsyth, pp. 379–390. London: Academic Press.
Strauss, M. G., Brenner, R., Chou, H. P., Schultz, A. J. & Roche, C. T. (1983). Spatial resolution of neutron position scintillation detectors. Position-sensitive detection of thermal neutrons, edited by P. Convert & J. B. Forsyth, pp. 175–187. London: Academic Press.
Thomas, P. (1972). Production of sensitive converter screens for thermal neutron diffraction patterns. J. Appl. Cryst. 5, 373–374.
Whaling, W. (1958). The energy loss of charged particles in matter. Handbuch der Physik, Vol. 34. Corpuscles and radiation in matter II, edited by S. Flugge, pp. 193–217. Berlin: Springer-Verlag.
Wolf, R. S. (1974). Measurement of the gas constants for various proportional counter gas mixtures. Nucl. Instrum. Methods, 115, 461–463.