Improved methods for the generation of 24.5 keV neutron beams with possible application to boron neutron capture therapy

Nuclear Instruments and Methods in Physics Research A250 (1986) 565-572 North-Holland, Amsterdam

565

IMPROVED METHODS FOR THE GENERATION OF 24 .5 keV NEUTRON BEAMS WITH POSSIBLE APPLICATION TO BORON NEUTRON CAPTURE THERAPY G. CONSTANTINE, L.J . BAKER and N.P . TAYLOR

United Kingdom Atomic Energy Authority, Harwell Laboratory, Didcot, Oxfordshire, OXII ORA, UK

Received 7 April 1986 The production of epithermal neutron beams, filtered to provide a spectrum m which a small energy range predominates, is of importance for radiobiological research and in the development and calibration of instruments for monitoring intermediate energy neutrons. The penetration characteristics of intermediate energy neutrons m tissue lead to the possibility of application m the field of neutron capture therapy if beams of sufficient intensity and adequate spectral properties can be generated, In this paper methods of utilising the 24 .5 keV antiresonance in the iron neutron cross section are described, and the DENIS (depth enhanced neutron intense source) principle by which beam intensities may be optimised is explained . Calculations and experimental measurements in an in-core facility in the DIDO reactor at Harwell have indicated that a DENIS scatterer can achieve a 6-fold improvement in 24.5 keV beam intensity compared with a conventional titanium disc scatterer . 1. Introduction

tion :

Reactor-based neutron irradiation facilities are normally characterised by a broad spectrum of neutron energies in the range 14 MeV to thermal, the only common exception being the production of thermal neutron beams in some research reactors . The problem of producing neutron beams at intermediate energies resolves into two inter-related parts; that of optimising a scattering source within the reactor, and that of filtering the resultant beam to reduce the intensity of gamma rays and of neutrons at unwanted energies [1]. The investigations reported here have as their primary purpose the production of neutron beams at the 24 .5 keV iron window energy, using the facilities available in the cores and reflectors of the Harwell materials testing reactors (MTRs) . Beams of this energy have application in radiobiological research and in the development and calibration of the associated instrumentation, but in fact the original stimulus for this work was the possible application in the field of cancer therapy, where the penetrating characteristics of intermediate energy (but not necessarily monoenergetic) neutrons offer considerable advantages over thermal neutrons . Successful treatment of Glioma, a fatal form of brain tumor, by thermal neutron irradiation following selective perfusion by drugs containing i° B has been reported by Hatanaka in Japan [2]. Boron neutron capture therapy (BNCT) depends on the difference between the small dose received by healthy tissue and bone due to the neutron irradiation and the comparatively high local dose received in boron-doped tumor tissue due to the charged particles arising in the reac-

n + i° B _ 7 Li(0 .84 MeV) + 4 He(1 .47 MeV) .

0168-9002/86/$03 .50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Methods of optimising the ratio of these doses, while retaining a dose rate yielding reasonable irradiation times, are required in order to improve the quality of BNCT. As previously mentioned, the efficient extraction of thermal neutron beams from the reflectors of experimental reactors is readily achieved . However, the use of thermal neutrons for BNCT carries attendant problems which are not easily solved, due to the magnitude of the irradiation dose to the scalp and the poor penetration of the neutron beam through the healthy bone and tissue. These conditions result in the necessity for surgery and for protracted irradiation times. Epithermal neutron irradiation may offer the possibility of treatment without need for surgery since the lower neutron capture rate in hydrogen would result in a reduced scalp dose, and moderation of the neutrons within the head would give rise to a thermal neutron source at the tumor site. Neutron energies in the low-keV range are suitable for this purpose, the presence of y-rays and fast neutrons being inimical to the healthy tissue. In this paper we describe the optimisation of a beam of 24 .5 keV neutrons generated in a cylindrical iron scatterer, and filter combinations of iron, sulphur and aluminium which have been developed to yield a beam of adequate purity . Because the iron cylinder used as the neutron scatterer is extended in the beam (axial) direction, neutrons produced at the 24 .5 keV window energy are additive along its length, hence the principle

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G. Constantine et al. /Generation of 24 .5 keV neutron beams

is named depth enhanced neutron intense source (DENIS) . The DENIS principle is elaborated in the next section, and the following three sections describe, respectively, the theoretical approach to the optimisation of a reactor facility, the experimental development of an iron scatterer m the Harwell MTRs DIDO and PLUTO, and the investigation of the performance of beam filters. 2. The DENIS principle In principle a beam of intermediate energy neutrons can be extracted from a research reactor by placing a suitable scattering material in a high flux region accessible by a beam tube . In practice the broad range of neutron and gamma photon energies in the beam engenders a difficult filtering problem if a particular energy range is to be selected . The DENIS principle offers the possibility of enhancing the production of neutrons within a small energy range and reducing the filtering problem. The principle depends on the existence of a suitable large and sharply defined scattering resonance and accompanying antiresonance (or "window") . In the low-keV energy region scattering is predominantly elastic, and the average logarithmic energy decrement ~ is such that neutrons interacting in the resonance have a high probability of reaching the window, at which energy their transmission through the scatterer and hence to the irradiation target is probable . Thus for iron, ~ = 0.0354, and neutrons scattered in the 27 keV resonance have an average energy decrement of - 0.9 keV. The steep low-energy edge of this resonance can be seen in fig. 1 . If the lower edge of the resonance is wide compared with the energy decrement, scattered neutrons have a higher probability of diffusing out of the material before reaching the window, hence the importance of the steep slope. This resonance in iron is the

Û 10

200

250 300 Energy (keV)

350

400

Fig. 1 . Multigroup representation of the elastic scattering section for iron in the vicinity of the 27 keV resonance.

cross

prime example of the effect, although other materials are possible candidates, most notably scandium, which has a scattering resonance at 3.5 keV which, although less sharp than the iron resonance, produces neutrons in a 2 keV window . This would be an ideal energy for BNCT . Neutrons which reach the iron window energy have a long mean free path in that material (- 180 mm at full iron density), and this fact is used to enhance the output of 24 .5 keV neutrons . The experimental facilities within the hollow fuel elements of the Harwell materials testing reactors allow the use of vertical cylinders of iron, the size of which may be limited either by the space available or by the reactivity absorbed . Thus scatterers may be loaded which are extended in the axial direction (the reactor core height is 610 mm), the useful limit being of the order of the mean free path The difference between the scattering cross section at 24 .5 keV and at other energies results m a filtering effect, although neutrons at other window energies (73, 82, 128, 137 keV, . . .) will also be transmitted . The use of multicomponent filters to further attenuate the unwanted higher energy neutron current is discussed in sect . 5 3. Theoretical estimates In order to investigate the DENIS principle theoretically, a model is required which has fine enough detail in the energy domain to show the spectrum around the iron window . The multigroup weighted-tracking Monte Carlo code MORSE [3,41 was chosen for this purpose. The required energy resolution has been achieved by the generation of a special set of multigroup neutron cross sections in a fifty-five group structure with very narrow groups around the iron resonance energy . The narrow group structure was also extended upwards in energy to cover the 35 keV resonance in aluminium (see below). The library was derived from the ENDF/B4 files [51 using the AMPX-1 modular suite of codes [6]. The lethargy interval of the chosen group structure is 0.0143 per group from 41 keV down to the window energy . Fig. 1, the elastic scattering cross section for iron in this multigroup scheme, shows how the resonance and antiresonance are adequately represented . The window is represented by group 47 . At higher energies, up to 10 MeV, a lethargy interval of 0.5 per group is used, with 0.063 per group from the window down to 15 keV. Anisotropic scattering is represented by a Pi Legendre expansion of the scattering function ; within MORSE this is converted into a set of mean scattering angles for every possible energy group-to-group transfer. Initial Monte Carlo calculations investigated an iron scatterer located within an experimental hole in the

G. Constantine et al. / Generation of 24.5 keV neutron beams

reactor core. The purpose was to examine the neutron spectra that might be achieved in such a facility and to study the effect of changes in the iron density on the neutron flux at the window energy . The model represents a cell of the reactor cores of DIDO or PLUTO at Harwell, comprising one lattice position, containing a standard hollow fuel element. Its four concentric fuel tubes are modelled, each as a homogeneous mixture of its fuel and cladding . Within the innermost fuel tube is a hole for the irradiation of rigs enclosed by a closed aluminium tube (or " thimble"), of diameter 54 mm, inside which the 50 mm diameter iron scatterer is located. Three densities of iron scatterer were used in separate calculations, corresponding to the normal density of iron and one half and one quarter of this value. Such reductions in iron density might be necessary to limit the reactivity absorption of the DENIS scatterer. Although lower densities were realised in practice by the use of a stack of appropriately spaced disks, in the MORSE model the iron was homogenized at each of the three average densities. Two further calculations with this model examined the effect of using a mixture of iron and aluminium as the scattering medium, to assess the enhancement obtained by the larger mean scattering energy decrement provided by the 35 keV resonance in aluminium (~ = 0.0723, mean energy decrement at 35 keV = 2.5 keV) . In the first calculation a homogeneous mixture of equal volumes of iron and aluminium was used for the entire 50 mm diameter scattering region, while in the second calculation alternate 10 mm thick disks of full-density iron and aluminium were modelled, to assess the effects of this practical heterogeneity . In all calculations, a specular reflective boundary condition was imposed at the upper and lower surfaces of the geometry, so that the modelling is essentially to two dimensions, the axial dimension being effectively infinite . On the four boundaries comprising the cell walls, a reflective condition is also imposed, so that the model becomes a two-dimensional infinite lattice cell . There is thus no escape of neutrons, and since MORSE operates using a weighting tracking technique in which neutron absorption is represented by a reduction of particle weight at each interaction rather than termina tion of the tracking, each history proceeds until the neutron energy is reduced to the lower bound of energy group fifty-five, 9.118 keV. In thus way, contributions to the flux in the region of the window energy could be rapidly accumulated, and statistical uncertainties reduced to small values in relatively short computer running times. The multigroup spectrum of the scalar neutron flux within the iron scatterer is shown in fig. 2, illustrating the enhancement of the flux at the window energy . Fig. 3 shows the energy range around the cross section resonance and antiresonance in more detail, and dis-

567

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2 101 1 0'

2

4

6

8105

2

4

Energy ev

6 8 06

2

4

6

810 7

Fig. 2. Calculated neutron spectrum in a full density iron scatterer within a hollow fuel element.

Fig. 3. Calculated neutron spectra m DENIS scatterers at three iron densities.

T ô v

6 4

20 0

22 0

24 0

26 0

28 0 30 0 32 0 Energy keV

34 0

36 0

38 0

40 0

Fig. 4. Calculated neutron spectrum in mixed iron/aluminium scatterer.

568

G. Constantine et al / Generation of 24 5 ke lr neutron beam s

plays the change in the spectrum as the iron density is varied . Fig. 4 is the flux spectrum in the homogeneous iron/aluminium mixture, which has a similar shape to the volume-averaged flux in the model using alternate iron and aluminium discs. The scalar flux in group 47 in the spectrum shown in fig. 2 cannot be used to derive the beam current emerging from a long iron cylinder since the angular distribution is highly anisotropic. Rather the quantity of interest is the axially directed component of the flux . At any axial position in the scatterer there are two effects governing the upward current of neutrons in the window energy. The first is the source term, i .e . their production by the scattering of higher energy neutrons into the useful energy range and into the beam, and the competing effect of removing them from the beam by scattering to a lower energy or into the wrong direction. The production of neutrons to group 47 by scattering (assumed isotropic) from higher energies has been obtained from the calculations at each of the three iron densities (full, one-half and one-quarter) . These show that this source has an essentially linear dependence on iron density. This group 47 scattering source m the mixed iron and aluminium scatterer is 27% lower than the result for pure iron, at full density. Conversely the addition of aluminium to iron at half density provides a worthwhile enhancement. This enhancement is highest (at 49%) when the iron and aluminium are homogeneously mixed. The reduction brought about by using alternate 10 m discs is 7% . In order to optimize the composition of the scatterer, the relative importance of the production and removal effects for 24 .5 keV neutrons was assessed by an algorithm which, starting at the base of the scatterer and working upwards in discrete steps, chose the optimum iron density at each stage to provide the maximum upward neutron current in the window energy range. The assumptions made in this process were that the scattering source into energy group 47 would be linearly dependent on the iron density, and that the production rate of these group 47 neutrons has an axial variation which follows that of the fast flux . The results of this process yield an optimum scatterer which is of full natural iron density to a height of 510 mm from the base (i .e . 100 mm short of the top of the fuelled region of the element), and void thereafter . If a limit is placed on the iron density at something less than this 100% value, the optimum height increases (e.g . to 580 mm at 509o' density) . A limiting feature of the type of scatterer described, located within a fuel element experimental hole, is the diameter which can be accommodated (54 mm including an aluminium thimble), whereas a larger diameter would be able to "collect" a higher proportion of the available neutrons and so produce a more intense source

of neutrons at 24 .5 keV, as well as providing the increased flux over a wider diameter beam . One possibility is to site the scatterer within the heavy water reflector close to the core boundary . Here a larger scatterer can be used and a 175 mm diameter cylinder has been envisaged, inserted into a hole (designated 10 V) in the PLUTO reactor. In order to be in a sufficiently high fast neutron flux, such a facility would have to be angled so that it is near to the outermost fuel elements, as close as would be allowed by considerations of absorbed reactivity . The intensity of the 24 .5 keV neutron beam that might be obtained in such an arrangement has been investigated by a further model again using the MORSE code . It is found from this model that the larger extent of iron, allowing more multiple scattering of neutrons from the resonance energy, shifts the peak in the flux spectrum a little lower in energy and, as shown in fig. 5, enhances the proportion of 24 .5 keV neutrons to the spectrum . Taking into account the likely available flux levels in this core-side position, as well as the geometrical factors, it is estimated that the 24 .5 keV flux in the emergent beam would be some 5 to 6 times higher than that from an in-core facility. In capture therapy terms the wider beam would result in a further increase by a factor of 3 in thermal neutron flux generated at the tumor site . In-core rigs in the materials testing reactors normally operate at considerably elevated temperatures unless cooling circuits are employed, and a possible consequence of operation at high temperatures is the degeneration of the DENIS effect due to Doppler broadening of the resonance. Nuclear heating in the iron scatterer is at its highest when placed in a central fuel element hole, and calculations using a coupled neutron-photon discrete ordinates model in lattice cell geometry (7], have yielded a value for total heating of 4.5 W/g for a fuel element power of 1 MW . However, the temperature

2

4

6

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6

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Fig. 5 . Calculated neutron spectrum in a solid iron scatterer in the 10 V facility m the reactor reflector

G. Constantine et al. / Generation of 24.5 keV neutron beams

effect on the shape of the 27 keV resonance and antiresonance by Doppler broadening is negligible, the total resonance width being 1 .6 keV while at a likely operating temperature of 400°C the Doppler width is only 11 eV. Hence it is possible to operate rigs at full reactor power without the necessity to cool the iron scatterer. 4. Experimental confirmation of the DENIS principle The Harwell research reactors DIDO and PLUTO normally operate at 25 .5 MW but are available for short runs at low power (up to 100 kW) during the monthly shutdown/maintenance period when the tests described here were carried out in a fuel element thimble the lower end of whose 3 m length lies within the core height . For beam production a scatterer is located at the core centre level, while filters are suspended within the top shield, the latter absorbing unwanted neutrons scattered out of the beam . In order to verify our calculations on the DENIS principle three scatterers were made, one a titanium disc 5 mm thick, 48 mm diameter which could be suspended at core centre level, the other two of pure Swedish iron

4 lever operated coaxial tubes to swing titanium discs into beam

56 9

Table 1 Mean thermal flux for three scattering sources Scatterer 5 mm thick

titanium disc

600 mm long iron

at 20% density

360 mm long solid

iron

Thermal flux [n cm - ' s - '] 2.2 X 10 7 5 .6 X 10 7

12 .2 X 10 7

of the same diameter. The first comprised 40 discs 3 mm thick evenly spaced along a 600 mm threaded rod to give an effective 20% density iron scatterer. The other was of solid iron 360 mm long . The filters were made in unit lengths of 100 mm, from pure Swedish iron, pure aluminium and sulphur, the last being cast into a thin iron pot. The filters, which were a snug fit in the thimble could be lowered to to rest on the step halfway through the shield . On emerging from the thimble the beam crosses a 600 mm gap before passing through an access hole left open in the 350 mm thick steel reactor top plate. Fig. 6 shows a typical experimental configuration . Beam characteristics were monitored with several sensors. The first was a 5 1 polythene bottle filled with water, loaded with axial and radial arrays of manganese foils and irradiated above the top plate for 20 min at 20 kW reactor power. Integration of the thermal flux profiles over the water volume derived from the foil activities, multiplied by 2a for water of 0.022 cm -t gave the overall neutron absorption . Correcting for leakage, extrapolating to a full reactor power, and assuming a 60 mm beam diameter at top plate level gave the neutron current there. The mean thermal flux (at full reactor power) averaged over 0-6 cm depth on the axis of a water-filled 5 1 polythene bottle is given in table 1 for three scatterers, in each case using a 300 mm iron filter . 5. Filter optimisation

Fig . 6 . Experimental arrangements for Ti difference measure-

ments showing DENIS scatterer and filters in a DIDO/PLUTO fuel element facility .

For comparison of different filters a much more convenient measure of total neutron current was given by a similar polythene bottle filled with paraffin wax, through which a BF3 proportional counter was mounted horizontally 40 mm from the base in the region of peak thermal flux . A measure of the dose rate was given by a type 0949 total neutron dosimeter, a Harwell design [8] based on a 200 mm diameter polythene sphere with a central 3He chamber surrounded by a perforated

57 0

G. Constantine et al / Generation of 24.5 keV neutron beams

cadmium shell, developed for radiation protection monitoring . Assessment of a wide range of filter configurations has been made comparing beam intensity and quality using these sensors in conjunction with the titanium difference technique. Several different thicknesses of titanium, which has a high scattering cross section for 24 .5 keV neutrons, were placed in the beam well below the top plate to minimise multiple scattering into the sensors. The reactor was operated at up to 100 kW depending on filter configuration to give satisfactory count rates from the BF,/wax and 0949 monitors . Fig.

7 shows the count rate for the BF3/wax sensor as a function of titanium thickness for various filter assemblies, in each case with the 360 mm solid iron scatterer . The results for the dosimeter are essentially similar. The results have been analysed on the rudimentary basis that neutrons can be grouped into "24.5 keV window" and "non-window" categories ; uncertainties in our measuring instruments and the complexities of the real situation do not warrant further sophistication . Each set of count rate measurements with five titanium thicknesses (0, 5, 10, 25 and 50 mm) for every filter arrangement has been fitted to the sum of two exponen-

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300/0/0 -400/0/0

400/0/0 1 500/0/0

500/0/0

v

150/500/0

x 150/500/0

150/400/100

150/400/100 150/500/100

Fig 7. Response of BF3 chamber m wax container to a range of filter configurations as a function of thickness of titanium m the beam .

571

G. Constantine et al. / Generation of 24.5 keV neutron beams

10,9 -Other

Fi ( fer Cômpoiients mm) -Aluminium/Sulphur

8 7 6 5 4

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0 U

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A  = 24 keV component B  ='hon 24keV" component

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101

0

50

100

150

200

250

300

350

400 L

450

I

500 Aluminium thickness,mm

Iron thickness, mm

Fig. 8. Variation m amplitudes of A (24.5 keV) and B (non-24.5 keV) components of filtered beams with thickness of iron . tials, in the form : C =A . e -ax + B e- Bx

A  =24keV component B  = - non 24 keV component

(2)

where A and B are amplitudes of the window and non-window components for the nth set, a and ß the corresponding titanium macroscopic total cross sections and X the titanium thickness . Common values of a and ,ß were applied to a total of 15 sets of 5 data points from both the BF3/wax sensor and the 0949 dosimeter. For a given a and /3, A and B can be determined for each combination of filter and sensor by forcing a fit at X = 0 and X = 50 mm . The curves in fig. 7 are plots of eq. (2) for values of a and ß of 40 .6 and 2.47 b respectively . These values were obtained by a leastsquares fit to the measured count rates at the various titanium thicknesses for all filter combinations . The points in fig. 7 are the results of these measurements, and the plots show a good fit to these in most cases. The amplitudes of the components A  and B are also shown for each filter configuration . The values of A and B, derived from the BF3/wax sensor measurements, are plotted against thickness of the iron component of otherwise unvarying ranges of filter assemblies in fig. 8. This enables us to determine effective iron cross sections for window and non-window neutrons . It is clear from fig. 8 that they are very similar, indicating that the B component is mainly

Fig. 9. Variation in amplitudes of A  (24.5 keV) and B (non-24.5 keV) components of filtered beams with thickness of aluminium.

transmitted through windows in the iron cross section other than the one at 24.5 keV. Fig. 9 shows the equivalent dependence of A  and B on aluminium thickness. Here there is a very considerable difference in slope and hence in the derived window and non-window cross sections. In line with these findings the filter configurations tested were modified to include less iron,

10 09 08 07 É 06

w

~as

=04 03 â Zoz 01 00 00

02

04

06 08 10 Neutron energy (MeV)

12

14

16

Fig. 10 . Spectrum obtained by proton recoil spectrometry on a filtered beam in PLUTO. A 360 mm solid iron scatterer was used with a 200 mm Fe/200 mm Al/100 mm S filter .

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G. Constantine et al. / Generation of 24 5 keVneutron beams

Table 2 Spectrum obtained by proton recoil spectrometry on a filtered beam in PLUTO. A 360 mm solid iron scatterer was used with a 200 mm Fe/200 mm Al/100 mm S filter, and the reactor power was 7 kW .

between the two iron scatterers of different lengths and densities is in accordance with calculated values. In

Peak

Flux [CM-2 s-']

Kerma

treatment times m the application to boron neutron

[MeV] 0.024 0 .070 0.130 0 270 0 360 0 620 100 1 32

10330 213 107 222 167 111 76 44

61 .4 3.6 27

be controlled by including aluminium and sulphur in the filter configuration.

Totals

11270

Fluence

91 .7 1 9 09 20 1 .5 1 .0 0.7 0.4

100

ICRU muscle kerma rate [mrad h -1 ] 6.81

0.40 0.30 0.94

rate [%]

8.5 76 64 6.1 37

0.84 0.71 0 .68 0 .41 11 .09

100

more aluminium, and in addition in some a 100 mm length of sulphur was added.

These titanium difference method measurements on a wide range of filters were augmented by neutron

spectrometry using proton recoil chambers by Birch et al . [91 on a few filter arrangements, of which an exam-

ple, for (200 mm Fe, 200 mm Al, 100 mm S), is shown in fig. 10 . Their measured spectrum comprises a series of peaks at discrete energies, table 2 giving a breakdown into current and kerma rate, and demonstrating a relative window contribution A/(A + B) of 91 .7%. This

compares well with the value of 91 .8% obtained from

the titanium difference technique using the BF3/wax instrument for the same filter (see fig. 8) .

6. Conclusions We have

shown experimentally

that

the

DENIS

principle is well-founded ; the ratio of neutron currents

addition the technique produces a 6-fold increase in beam current over a conventional titanium scatterer, which would be highly significant in minimising the capture therapy. We have demonstrated that other windows to iron transmit neutrons of higher energies, but that these can

Finally, the calculations showing a factor 5-6 improvement m the beam current for a larger DENIS facility placed outside the reactor core illustrate a possible future development of the technique.

References [1] A J Mill and J.R . Harvey, CEGB Berkeley Nuclear Laboratories report RD/B/N4776 (1980) . [2] H. Hatanaka, 1st Int Symp. on Neutron Capture Therapy, Boston, MA (1983) . [3] N.P . Taylor and J. Needham, Harwell Laboratory report AERE-R 10432 (1982). [4] M.B . Emmett, Oak Ridge National Laboratory report ORNL-4972 (1975) . [5] M.K . Drake (ed), Brookhaven National Laboratory report BNL-50274 (1970) [6] N M. Greene et al ., Oak Ridge National Laboratory report ORNL-TM-3706 (1974) . [7] N P. Taylor, Harwell Laboratory report AERE-R 10558 (1983) . [8] K.G . Harrison, Nucl . Instr and Meth . 166 (1979) 197. [9] R. Birch, L.H .J. Peaple, H.J . Delafield and K.G . Harrison, 5th Symp . on Neutron Dosimetry, GSF Neuherberg (Sept. 1984).