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

© International Union of Crystallography 2006
International Tables for Crystallography (2006). Vol. C. ch. 7.3, pp. 644-645

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

P. Converta and P. Chieuxa

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

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[link](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−, [{\rm CH}_{4}^{+}]) 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)[link]].

[Figure 7.3.3.1]

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[link]). [Reproduced from Convert & Forsyth (1983[link]).] (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[link]).] (d) Schematic drawing of a gas monodetector. The arrows represent the incoming beam.

We give in Fig. 7.3.3.1(c)[link] the proton range of an 3He neutron-capture reaction in various gases (Fischer, Radeka & Boie, 1983[link]). 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)[link].

For this type of detector, the efficiency as a function of the gas pressure, or gas-detector law, is written as [\varepsilon (\lambda)=\xi [1-\exp (-bPt\lambda)], ]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 [ b= {273 \over 293}\times {N_{A}\over 22\ 414}\times {\sigma _{a} \over \lambda _{0}}. ]

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[link] 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 1L = 200, Ø = 50Axial65.5%88.1%
Radial†23.4%41.3%
3He 5L = 100, Ø = 50Axial97.5%99.9%
Radial†84.4%97.5%
3He 8L = 250, Ø = 10Radial†44.7%69.5%
3He (monitor)C3H82100 × 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[link]).

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[link]). 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[link]).

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[link]).

  • (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).

References

First citation 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.Google Scholar
 First citation 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.Google Scholar
 First citation 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.Google Scholar
 First citation Wolf, R. S. (1974). Measurement of the gas constants for various proportional counter gas mixtures. Nucl. Instrum. Methods, 115, 461–463.Google Scholar








































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