IE42055B1 - Determining the dose equivalent or dose rate equivalent of neutrons - Google Patents

Abstract

1498969 Neutron responsive apparatus KERNFORSCHUNGSANLAGE JULICH GmbH 6 Oct 1975 [8 Oct 1974] 40866/75 Heading G6P Neutron responsive apparatus comprises: a neutron detector for emitting, in operation, energy-dependent voltage pulses having a maximum number at a level corresponding to the region of the energy liberated in the nuclear process occurring upon neutron absorption; a voltage discriminator set or adjustable to a level above that corresponding to at least 80% of the pulses; and means for counting the number or time-averaging the emission rate of pulses emitted by the discriminator. The detector I, Fig. 1, may comprise a 3He or BF 3 counter tube or a 6LiI scintillator. If a 3He counter tube is used, wherein nuclear reactions liberate an energy of 770 keV in response to thermal neutrons, the voltage discriminator may be set to a level corresponding to an energy of 800-850 keV. The detector 1 may be surrounded with an absorber layer 2 of cadmium of thickness 0À1- 2 mm. to absorb neutrons of energy lower than 0À5 eV. This will substantially reduce the number of voltage pulses produced by lowenergy neutrons in the detector, and enable the threshold of the discriminator to be set at a lower value. If neutrons of energy levels below 0À5 eV are to be measured, the cadmium layer may be perforated. A moderating casing 3 may be provided, of hydrogenous material, e.g. polyethylene, having a wall thickness of 3-5À5 cm. For use as a personal dosimeter, part or all of the casing 3 may be omitted at least in the region nearest the body of the wearer, since the body then functions as a moderator.

Description

The invention! relates’· to art apparatus and a method for determining Vita, dorta equivalent >>r the dupe vale equival&ht of neutrons.

Apparatus for determining the dose equivalent or the 5 dose rate equivalent of neutrons are employed for measurement, ' if - . especially 1 behind screens of neutron sources, vhere neutrons having energies of 1/40 up to about 10? eV are generally present. In such cases, the aim of the measurement is to indicate a value for the damage caused by the neutrons on the human body, which - in respect of an equal flux of neutrons of various energies - depends upon the energy of the neutrons. This dependence is set out in a dose equivalent curve which should be derived in accordance with the recommendations of the International Commission on Radiological Protection. Therdbre, in order to achieve the aforesaid aim, it is desirable that the shape of the curve of the response function of the apparatus, i.e. the counting rate per unit flux density as a function of the neutron energy in the aforesaid energy range, should agree as far as; possible -with the shape of the dose equivalent curve. For radiation protection purposes, the apparatus for the determination of the dose equivalent or of the dose rate equivalent is sufficiently independent of the energy if the measured value supplied by the apparatus does not deviate, for any energy, from the curve form aimed at by a factor of more than 2.

Apparatuses designed for the determination of the - dose equivalent are known as Rem-Counters. There exist known apparatuses having a detector for neutrons which is surrounded by a casing consisting of hydrogenous material such as paraffin, polyethylene or the like. The detectors' - 2 43055 employed are, for example, 3F^ counter tubes or He counter tubes which have high sensitivity to thermal neutrons, but lower sensitivity to neutrons of higher energy. A disadvantage of such apparatus is that the casing has a wall thickness of at least 12.5 cm., so that it is heavy and unwieldy.

In addition, in the smallest of the known apparatuses it is not possible to obtain an indication of the dose equivalent or of the dose rate equivalent for the neutron energies in the proposed neutron energy range which is completely independent of the energy. While it is known to improve the determination of the dose equivalent or of the dose rate equivalent by carrying out the measurement a number of times with one detector, which is surrounded by a casing of different thickness for each measurement, this method is complex and too costly for practical requirements.

An apparatus consisting of a number of detectors situated in one casing is also known. It is true that the desired measured value can be determined in a single measuring operation by this known apparatus, but it is too costly and also too complex and unwieldy for practical requirements.

There is also known apparatus in which a perforated absorber layer is embedded in the casing consisting of hydrogenous material. While a reduction in the wall thickness of the casing is thereby achieved, the apparatus still has a diameter of about 20 cm. In addition, the required adaptation to the dose equivalent curve is not fully achieved with this apparatus. one aspect of According to/this invention, there is provided apparatus for determining the dose equivalent or dose rate equivalent of neutrons having - 3 5 (a) a neutron-absorbing detector which in operation emits voltage pulses the levels of which, at least for neutrons up to 10® eV» are dependent upon the sum of the energy liberated in the nuclear processes initiated upon absorption of neutrons and the kinetic energy of the absorbed neutrons, while the number of voltage pulses is a maximum at a voltage level in the region of the level corresponding to the said energy liberated in the nuclear processes occurring upon neutron absorption (i.e. it is a characteristic of the detector that it emits the maximum number of pulses in response to low energy neutrons). (b) a voltage discriminator for voltage signals derived from the detector, of which thd voltage threshold is set at a level above the level of signals corresponding! to at least 80% of voltage pulses emitted by the detector and distributed around the said maximum, and (c) means for counting the number of signals emitted by the discriminator or time averaging the rate of their emission.

It is essential that in use the detector is.surrounded by. moderating material. Such moderating material can be provided by a moderating casing preferably of a hydrogenous material in which there is optionally an absorber layer, which may be perforated. Alternatively, if a. person is wearing the apparatus as a personnel dosimeter, his body can function as moderating material in place of part or all of such a moderating casing.

According to a second, aspect of this invention there is provided apparatus for determining the dose equivalent or dose rate equivalent of neutrons having (a) a neutron absorbing detector which is at least partly surrounded by a casing at least the innermost part of which is of cadmium of thickness 0.1 to 2 mm, the:detector in operation emitting voltage pulses the levels of which, at least for neutrons up to 10® eV, are dependent upon the sum of the energy liberated in the nuclear processes indicated upon absorption of neutrons and the kinetic energy ofthe absorbed neutrons, while the number of voltage pulses is a maximum at a voltage level.in the region of the Tevel corresponding to the said energy liberated in the nuclear process Occurring upon neutron absorption, - 4 43 0 S 5 (b) a voltage discriminator for voltage signals derived from the detector, of which the voltage threshold is set, or is adjustable so as to be able to be set, at a level above the level of signals corresponding to at least 80% of voltage pulses emitted by the detector and distributed around the said maximum, and (c) means for counting the number of signals emitted by the discriminator or time-averaging the rate of their emission.

The layer of cadmium may be perforated.

The cadmium may be surrounded by a layer of moderating material, e.g. a polyethylene layer 3 to 5.5 cm thick. Alternatively» if a person is wearing the apparatus as a personnel dosimeter his body can function as part or all of the moderating material outside the cadmium layer.

In the operation of apparatus according to either of the above aspects of the invention voltage signals which have a lower level than the threshold value are not passed on by the discriminator to the means for counting or time averaging. These means are thus responsive only to voltage signals emitted by the discriminator; these originate as voltage signals whose level exceeds the threshold value.

A portion of the neutrons is retarded by the moderating material, For the retarded neutrons the chance of reaction in the detector is much higher than for the primary neutrons (e.g. the effective cross-section for ο 3 (n,pj reaction of He falls from a value of 1.3 X 10 barns for 0.5 eV neutron energy to 0.7 barn for 1 MeV neutron energy). Thus the few retarded neutrons, of low kinetic energy, create a large number of voltage pulses. Because of this the voltage pulses in the detector output are distributed about a maximum located in the region of the voltage level which corresponds to the energy of the nuclear reaction in the detector, independently of the neutron energy spectrum to which the apparatus is exposed. The total energy of the voltage pulses is of course dependent on the energy of the neutrons being detected, Thus the effect of the moderating material is that the energy level corresponding to the said maximum number of voltage pulses is independent of the energy spectrum of the neutrons being measured. Thus it is possible to select a level for the voltage threshold of the discriminator which is such that by far - 5 42055 the greatest number i.e. at least 80% preferably at least 90% more preferably at least 95% of the voltage signals are suppressed, the level selected being independent of the neutron energy spectrum.

It is known, in apparatus for determining the dose equivalent or the dose rate equivalent, to suppress voltage pulses by means ct a voltage discriminator connected between the detector and a device for counting the voltage pulses. However, such discrimination has hitherto been only in respect of voltage pulses of low value which are set up, for example, by gamma radiation and would constitute a disturbing background for the measurement of the neutrons. In contrast thereto, in the present invention, the voltage threshold in the discriminator eliminates additionally pulses which are initiated by neutrons having predominantly low energy, With apparatus of the invention, it can be unnecessary in order to achieve good adaption of the curve of the counting rate per unit flux density as a function of the neutron energy to the dose equivalent curve, to moderate the neutrons by a casing having a large wall thickness. The apparatus of the invention can therefore have a casing of smaller wall thickness than has hitherto been possible. As already mentioned, such a casing preferably consists of a hydrogenous material in which there is optionally embedded a perforated absorber layer, e.g, the cadmium layer of the second aspect of this invention.

The low-energy neutrons set up a very large number of voltage pulses owing to their very large; effective cross-section for the nuclear process which takes place in the detector for the generation of ions. The distribution of the voltage levels .of th.ese pulses, extends over a larger energy range owing to the random nature of the processes in the detector in the production of the voltage pulses, the maximum of the distribution of these voltage pulses caused by low-energy neutrons corresponding to the energy liberated in the nuclear process. In order to suppress by far the greatest number - preferably at least 95% - of these voltage pulses, the threshold value is set to a level which is sufficiently above the maximum of the distribution as to be at the foot of the distribution curve. The level of the voltage pulses passing.through the discriminator corresponds to the sum of the energy liberated in the nuclear process and the kinetic energy of the neiitrons absorbed in the detector. - 6 42055 According to the invention in a further aspect, there is provided a method of determining the dose equivalent or dose rate equivalent of neutrons by means of a neutron absorbing detector which is at least partly surrounded by moderating material and which in operation emits voltage pulses the levels of which, at least for neutrons up to 106 eV, are dependent upon the sum of the energy liberated in the nuclear processes initiated upon absorption of neutrons and upon the kinetic energy of the absorbed neutrons, while the number of voltage pulses is a maximum at a voltage level in the region of the level corresponding to the said energy liberated in the nuclear process occurring upon neutron absorption in which method voltage signals derived from the detector are passed to a voltage discriminator having a voltage threshold level above the level of signals corresponding to at least 80% of voltage pulses emitted by the detector, and distributed around the said maximum, and the signals emitted by the discriminator are counted or their rate of emission is time arranged.

Preferably the detector is at least partly surrounded by neutronabsorbing material. This may be provided as part of a moderating casing. Advantageously the neutron absorbing material is at least the innermost part of a casing at least partly surrounding the detector and consists of a cadmium layer of a thickness of between about 0.1 and 2 mm. In this way, it can be ensured that all neutrons having energies lower than 0.5 eV are absorbed outside the detector and thus do not enter the detector at all. The number of voltage pulses produced by low-energy neutrons in the detector is therefore very much reduced, so that it is possible to set the threshold value of the voltage discriminator to a lower level and hence to effect a better adaptation of the curve of the counting rate per unit flux density as a function of the neutron energy to the dose equivalent curve.

Although the sensitivity of apparatus in which there is a cadmium layer closest to the detector is substantially zero for neutrons having energies below 0.5 eV, the adaptation of the apparatus to the dose equivalent curve is not thereby materially effected because the proportion of the dose rate equivalent which is caused by neutrons having energies below 0.5 eV behind shields of neutron sources is generally lower than 5% of the total - 7 42055 dose rate equivalent,. If neutrons having energies below 0.5 eV are also to be measured, it is possible to provide I I a perforated cadmium layer or to associate the apparatus with a detector for measuring neu'crons having energies below 0.5 eV.

Advantageously there is employed as the detector a counter tube filled with the isotope He, other possible detectors are a BF^ counter tube and a °LiI scintillator. Thermal neutrons produce in the ^He counter tube a nuclear io reaction in which an energy of 770 keV is liberated. If a ^He counter tube is employed as the detector, it is therefore desirable f or the level of the. voltage threshold of the voltage discriminator to correspond to an energy In the energy range between 800 and 850 keV. also 15 It is/advantageous for a casing around the detector to consist, of a polyethylene shell having a wall thickness of 3 to 5.5 cm, which surrounds the above-mentioned cadmium layer, the wall thickness being chosen in accordance with the desired, accuracy of the adaptation of the apparatus to 20 the dose equivalent curve and in accordance with the threshold value set in the discriminator.

The apparatus Of the invention may with advantage be employed as an albedo personnel dosimeter. In this case, a cadmium, layer of-a thickness of about OJ to 2 mm is preferably ’5 provided around the deteGtor, at least some moderating material outside this layer being formed by that part of the person which is situated closest to the detector. If a polyethylene wall is provided as that portion of the casing which serves as the moderator, this wall may be omitted in the direction of ’ - 8 42055 the body of the person wearing the personnel dosimeter, because that portion of the person to which the dosimeter is secured performs the function of the moderator. On the side further from the body of the person, it is sufficient to provide a wall of smaller wall thickness, depending upon the required adaptation of the dosimeter to the dose equivalent curve. The polyethylene wall could also be entirely omitted, depending upon the accuracy aimed at in the dosimeter.

Insensitivity of the apparatus when employed as an albedo dosimeter to neutrons having energies below 0.5 eV is immaterial, because neutrons of this energy can be detected by means of the cadmium absorbers of known film badges.

If the apparatus when employed as an albedo personnel dosimeter is also to be sensitive to neutrons having energies below 0.5 eV, a perforated cadmium layer is preferably provided. Then if the wall of a moderating casing of the apparatus is not of equal thickness on all sides, the ' .^apertures in the cadmium layer should be differently distributed.

The defector of an apparatus embodying the invention is diagrammatically illustrated in the accompanying drawings, with reference to which it will be described below. In the drawings :Figure 1 illustrates the detector and its casing in sectional view; ahd Figures 2 and 3 are sensitivity curves of two apparatuses embodying the invention of which the detectors f have casings of different thickness. - 9 43055 There is illustrated, in Figure la detector 1 with a casing consisting of an absorber layer 2 in contact with the detector 1 and an outer shell 3 of hydrogenous material.

For the determination of the sensitivity curves shown in Figures 2 and 3, there was employed as the detector 1 of Fig. 1 a He counter tube known under the designation SP9. The absorber layer 2 consisted of a layer of cadmium of a thiokness of about 1 mm. The detector 1 was connected to a high-voltage unit not shown in the drawing. In addition, there were connected to the output side of the detector 1, in a manner also not. shown in the drawing, a pre-amplifier and a main amplifier, succeeded by a voltage discriminator and a counter connected to the latter. The level of the threshold of the voltage discriminator corresponded to an energy of 825 keV. s In the curves of Figures 2 and 3, the sensitivity of the apparatus is plotted along the ordinates and the energy of the neutrons impinging upon the casing of the detector 1 is plotted along the abscissae.

The sensitivity is given by:Sensitivity .

The crosses in the curves represent the results of measurements with neutrons from a preparation-neutron source and mdnoenergetic neutrons of 14 MeV. The values . obtained with the preparation-neutron sources are plotted in the curves at an energy which corresponds to the mean energy of the neutrons of the preparation-neutron sources. The broken-line region of each curve is interpolated.

Over the energy range investigated, it can be seen that sensitivity of each detector deviates from the mean value - 10420 5 5 (about 0.5 in Fig. 2 and about 0.7 in Fig, 3) by a factor of less than two.

The apparatus of which the sensitivity curve is shown in Figure 2 had a shell 3 of polyethylene with a wall thickness of 5-08 cm. The polyethylene shell 3 of the apparatus of which the sensitivity curve is shown In Figure 3 had a wall thickness of 3.81 cm. As is apparent from Figures 2 and 3< the apparatus having the shell 3 of greater wall thickness was better adapted to the dose equivalent curve, but the apparatus having the smaller wall thickness also produoes a sufficiently accurate adaptation to the dose equivalent curve.

The invention thus renders possible a sufficiently accurate determination of the dose equivalent or of the dose rate equivalent of neutrons of a neutron spectrum, which corresponds to the spectrum behind the shields of neutron sources, with apparatus which is more convenient to handle than hitherto known apparatus.

Claims (16)

1. Apparatus for determining the dose equivalent or dose rate equivalent of neutrons having (a) a neutron-absorbing detector which in operation emits voltage pulses the levels of which, at least for neutrons up to 10 eV, are dependent upon the sum of the energy liberated in the nuclear processes initiated upon absorption of neutrons and the kinetic energy of the absorbed neutrons, while the number of voltage pulses is a maximum at a voltage level in the region of the level corresponding to the said energy liberated in the nuclear processes Occurring upon neutron absorption, (b) a voltage discriminator for voltage signals derived from the detector, of which the voltage threshold is set at a level above the level of signals corresponding to at least 80% of voltage pulses emitted by the detector and distributed around the said maximum, and (c) means for counting the number of signals emitted by the discriminator or time-averaging the rate of their emission.

2. Apparatus according to Claim 1 wherein the detector is at least partly surrounded by a moderating casing.

3. Apparatus for determining the dose equivalent or dose rate; equivalent of neutrons having (a) . a neutron absorbing detector which isat least partly surrounded by a casing at least the innermost part of which is a layer of cadmium of thickness 0.1 to 2 mm, the detector in operation emitting voltage pulses the levels of which, at least for neutrons up to 10® eV, are dependent upon the sum of the energy liberated in the nuclear processes indicated upon absorption of neutrons and the kinetic energy of the absorbed neutrons, while the number of voltage pulses is a maximum at a voltage level in the region of the level corresponding to the said energy liberated in the nuclear process occurring upon neutron absorption, (b) a voltage discriminator for voltage signals derived from the detector, of which the voltage threshold is set, or is adjustable so as to be able to be set, at a level above the level of signals corresponds to at least 80% of voltage pulses emitted by the detector and distributed around the said maximum, and -12 4 2 0 55 (c) means for counting the number of signals emitted by the discriminator or time-averaging the rate of their emission.

4. Apparatus according to Claim 3 wherein the casing has, outside the cadmium, a polyethylene moderating layer about 3 to 5.5 cm thick.

5. Apparatus according to any one of the preceding claims wherein the said level of the voltage threshold of the discriminator is above the level of signals corresponding to at least 90% of voltage pulses emitted by the detector.

6. Apparatus according to Claim 5 wherein the said level of the voltage threshold of the discriminator is above the level of signals corresponding to at least 95% of voltage pulses emitted by the detector.

7. Apparatus according to any one of the preceding claims having an amplifier to amplify the pulses emitted by the detector before they are fed to the discriminator.

8. Apparatus according to any one of the preceding claims wherein the detector is a He counter tube.

9. Apparatus according to Claim 8 wherein the said level of the voltage threshold of the discriminator corresponds to a neutron energy in the range 800 to 850 KeV.

10. Apparatus for determining the dose equivalent or dose rate equivalent of neutrons substantially as herein described with reference to and as shown in Fig, 1 in combination with Fig. 2 or Fig. 3 qf the accompanying drawings.

11. A method of determining the dose equivalent or dose rate equivalent of neutrons by means of a neutron-absorbing detector which is at least partly surrounded by moderating material and which in operation emits voltage pulses the levels of which, at least for neutrons up to 10 6 eV, are dependent upon . the sum of the energy liberated in the nuclear processes initiated upon absorption of neutrons and the kinetic energy of the absorbed neutrons, while the number of voltage pulses is a maximum at a voltage in the region of the level corresponding to the said energy liberated in the nuclear processes occurring 1 upon neutron absorption, in which method voltage, signals derived from the detector are passed to a voltage discriminator having a voltage threshold level above the level of signals corresponding to at least 80% of voltage pulses - 13 emitted by the detector and distributed around the said maximum, and the signals emitted by the discriminator are counted or their rate of emission is time averaged.

12. A method according to Claim 11 wherein the detector is at least 5 partly surrounded by a casing, at least the innermost part of which is a layer.

13. A methdd according to Claim ll or Claim 12 wherein at least some of the moderating material is provided by the body of a user of the detector.

14. A method according to Claim 11, Claim 12 or Claim 13 wherein the 1-- said level of the voltage threshold of the discriminator is above the level of 10 signals corresponding to at least 90% of voltage pulses emitted by the detector.

15. A method.according to Claim 14 wherein the said level of the voltage threshold of the discriminator is above the level of signals corresponding to at least 95% of voltage pulses emitted by the detector.

16. Methods according to Claim 11 substantially as herein described.