Thermal neutron fluxes produced in water by various isotope and accelerator neutron sources

N U C L E A R INSTRUMENTS AND METHODS 151 ( 1 9 7 8 )

175-182 ; ©

N O R T H - H O L L A N D PUBLISHING CO.

THERMAL NEUTRON FLUXES PRODUCED IN WATER BY VARIOUS ISOTOPE AND ACCELERATOR NEUTRON SOURCES L. HOLLAND* and J. WALKER

Department of Physics and Radiation Centre, University of Birmingham, Birmingham B15 2TT, England and W. J. OOSTERKAMP

lnstituto de Energia Atomica, Cx Postal 11049, Sao Paulo, Brazil Received 19 September 1977 A series of absolute thermal neutron flux measurements and calculations is described in which Am-Be and Sb-Be radioactive sources and D-T and D-D accelerator neutron sources (150 keV deuterons) were used. The radioactive sources were of a known absolute intensity; the output of the accelerator sources was found by the associated particle technique. The work was undertaken as part of a study of the suitability of non-reactor sources for thermal neutron radiography. The absolute flux distribution associated with an A m - B e source in a 0.9 m cubical water moderator was measured with gold foil detectors and calibrated counting equipment. Other flux distributions f o r different sources and different sized systems were measured with manganese foils and scintillation detectors, and normalized against the Am-Be flux. The measured distributions are compared with calculations based on transport and age-diffusion theory.

1. Introduction

An interest in portable sources of thermal neutrons for radiographic purposes (Hawkesworth and Walker~), Spowart2)) has led us to measure the thermal neutron fluxes produced in a water moderator by various isotope and accelerator sources of fast neutrons. The isotope sources were 24~Am-Be and ~24Sb-Be; the accelerator sources used the well known D(d, n)3He and T(d, n)4He reactions with an unanalysed ion beam from a Cockcroft-Walton 150 kV accelerator. It is clear that both the thermal neutron flux and the fast neutron yield have to be measured in order to compare the actual and calculated thermal fluxes. For the A m - B e sources, earlier flux measurements have been relative only, with the aim of providing spatial distributions, although some absolute measurements have been made more recently with 2s2Cf fission n e u t r o n s o u r c e s 3 ) . The long half-life, 458 y, and the low energy and intensity of the associated y-rays have maintained the value of A m - B e sources; with Sb-Be, the high associated gamma flux is frequently a disadvantage but there is still an interest because of the comparatively low energy of about 25 keV of the emitted neutrons. As far as accelerator sources are concerned, earlier measurements have usually obtained the yield of fast neutrons from the measured current of * Presently at Instituto de Energia Atomica, Cx Postal 11049, Sao Pauio, Brazil.

charged particles; this leaves uncertainties in the efficiencies of targets for neutron production. We have used the associated particle technique to obtain the fast neutron yield more directly. A preliminary report on our accelerator measurements was given at the Georgia meeting on Neutron Sources and Applications (Holland and Walker4)). 2. Measurement of fast neutron yields

The 24~Am-Be and 124Sb-Be isotope sources were supplied by the Radiochemical Centre, Amersham, England, and their details are given in table 1. Neutron yields of these sources were measured at Birmingham and at the National Physical Laboratory, London, by the manganese bath technique; the Birmingham apparatus has been described by ScottS). For the accelerator sources, the associated particle technique involves the detection of charged particles which are emitted from the same reactions as the neutrons. Fewel 6) has reviewed the method for the T(d, n)4He reaction in which the neutron yield is essentially isotropic in the centre of mass system at deuteron energies below 300 keV. The neutron yield per detected particle is given by: iv =

Jo d dE/dx

/dx--

'

de

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L. H O L L A N D

et al.

TABLE 1 Details of isotope sources used in flux measurements Source

Source reference

Source Dimensions (ram) Length Diameter

Am-Be

X3/350

31

22.4

Sb--Be

X3/816 X4/1069 ABN 1/58

31 48.5 48

22.4 22.4 23

Source intensity (neutrons s - l)

Calibration centre

(2.17_+0.015)x 106 (2.19_ 0.022) x 106 (2.11 -- 0.013) × 106 (6.16+_0.06)× 106 (4.67 :t: 0.038) x 104

NPL Birt~ingham Birmingham Birmingham Birmingham

where ~r(E)

Values of R and (Y(d, n)/Y(d, p)) have been calis the reaction cross section at a deuteron culated by Ruby and CrawfordT); the differential energy E in the laboratory system. cross section, do(d, p)/dcoc, is for the centre of Af2 is the solid angle subtended at the target mass system. by the detector. In our case, the charged particles associated is the solid angle conversion factor from with either the D - D or D-T reaction were detectthe centre of mass to the laboratory sys- ed with a silicon surface-barrier detector mounted ~dCOL]= tem for detected a~-particles. in a 25 mm OD, 1.6 mm thick aluminium (Dural) dE/dx is the rate of energy loss by deuterons in tube at 90° to the deuteron beam. Aluminium the target material. stops positioned at 36 mm, 126 m m and 221 m m The integration is over all deuteron energies from the target defined the solid angle between from zero to that given by the accelerator, on the the active target area and the detector. Care was assumption that a thick target is used. In the ex- taken in the construction of the apparatus to minperiments, the measured spectrum of a~-particles imize the contribution of scattered a~-panicles, and showed a well-defined peak which assured unam- mLherefore no allowance was made for them in the biguous selection of a~-panicles in the counter sys- analysis. The stops were tapered to 0.25 m m thicktem. ness arid positioned so that particles not travelling In the case of the D(d, n)3He reaction, direct directly to the detector could be detected only afmeasurements of the 3He panicles were not made; ter about three scatters. Careful alignment ensured instead, the clearly resolved protons from the that the (ecoil particle detector viewed the whole competing D(d, p)T reaction were used. The neu- active target area. tron yield from the first reaction per detected proTitanium targets 1.1 × 10 -2 mg mm -2 thick, conton from the second can be calculated from the re- taining tritium or deuterium (Radiochemical Cenlationship: -

N ........

fo" or(d, p) dE dE/dx

foe' da(d' P (dt°c') ) + f At2 o dE + dcoc ~drOLlP

tr(d,p) dE = 4__~n At2 4•

dE/dx

Io~ ~r(d,n) dE dE/dx

IfS "

dE

dE]dx

lozs_ _ _ TARGET

II ]

_ __

f:,(d,n) d• dE/dx

fo': dtr(d, P)(dc°c')dE f ~ a(d,P' dE dcoc ~dCOL/P dE/dx dE/dx

4re (Y(d, n)'~ = At2 R ~,g(d, P),]Thick"

COI~'ER

,..+o. PE RSPEX

ff

_

~

.+.,°. !

~-I ! i

! ~

E5 76

I

! I I _ I

Fig. 1. Details of the neutron-producing target used for absolute intensity measurements. All the dimensions are in ram.

THERMAL

NEUTRON

FLUXES

tre, Amersham; codes TRT51 or DBT51) were mounted as shown in fig. 1; amounts of material were kept as small as possible to minimize perturbations of the thermal neutron flux. A 3.2 mm diameter beam stop made from a 0.025 mm thick disc of tantalum ensured precise definition of the tritium target area: for clarity this stop has been omitted from the figure. Deuterons bombarding the target could themselves form a deuterium target but this effect was minimized by using a thin tantalum stop which became hot under bombardment. This stop alone was sufficient for the T(d, n) measurements because the neutron yield from the main tritium target was much higher than from an incidental deuterium one; for 150 keV deuterons, tritium is approximately 100 times more effective as a neutron source then deuterium. Additional precautions were taken for the D(d, n) measurements. The aperture of the first stop was increased to 6.7 mm and two further stops (6.7 mm diameter) were placed 0.52 m and 0.89 m from the target. These stops, together with the collimation in the accelerator, ensured that the bombardment of the target stop was small. The two other stops were clear of the region covered by the thermal neutron measurements so that neutrons produced from any deuterium deposited on them had little or no effect on the measurements. Accurate alignment and support of the whole target-detector assembly were simplified by enclosing the accelerator drift tube (AISI 321 grade stainless steel, 25.4 mm diameter, 0.25 mm thick) and detector assembly in perspex tubes joined at right angles and mounted in two adjacent ports of the moderator tank. Both the drift tube and detector assembly were located centrally in the perspex by 6.4 mm thick perspex discs, and the annular space was flooded with water.

PRODUCED

IN W A T E R

177

purity of the tap water; the demineralized water had less than 10 ppm of Cu, Ag and Hg, less than 1 ppm of CI, and a conductivity of less than 5 x 10 -4 (~T1)-IThe thermal flux distribution due to a 1 Ci A m - B e source in a cubical tank of water, 0.9 m side, was measured by the simultaneous activation of eight gold foils (15 mm diameter, 0.05ram thick) placed at different distances from the source. The actual arrangement had the cylindrical source mounted with its axis vertical at the centre of a horizontal perspex disc, 7 mm thick, which also supported the vertical foils, These were sufficiently far apart t o avoid mutual shielding, and separate tests with additional perspex showed that the disc produced no measurable distortion of the neutron flux. The gamma activities of the foils were measured with a counter which had been calibrated with similar foils irradiated in the standard flux reactor GLEEP at AERE, Harwell (AxtonS)). This reactor has a graphite moderator, and the epithermal neutron spectrum will be different from that in water, but in both cases the epithermal contribution to the activation was measured by irradiating gold foils covered by 1.0 mm thick cadmium. The epithermal activity was subtracted from the activity of the bare foils to give the activity due to thermal neutrons only; this simple procedure is satisfactory 40

I

30

z E

zo

o

3. Measurements of thermal neutron fluxes 3.1. FLUXES PRODUCED BY THE ISOTOPE SOURCES Several different neutron detectors were used in this work because flux measurements were made over a wide range of neutron and gamma-ray intensities. Both tap water and demineralized water were used as moderators, and there were no measurable differences ( < 1%) in the thermal neutron fluxes. This result was in keeping with the known

10

+

, 5

i 10

Distance

~5

from

I 20

i 25

30

c e n t r e of s o u r c e ( r a m )

Fig. 2. C a d m i u m ratio distribution m e a s u r e d with gold foils for a 1 Ci A m - B e source in water. C a d m i u m of t h i c k n e s s 1.0 m m was used.

178

L. HOLLAND et al.

for gold because the epithermal activity results almost completely from the resonance at 5eV, well clear of the cadmium cut off energy. The cadmium ratios (ratios of foil activities or counting rates with or without cadmium covers) at different distances from the A m - B e source in water are given in fig. 2; in GLEEP the ratio was 4.6 at the irradiation site. When relating foil activities to neutron fluxes, the corrections due to self shielding of the foils for thermal neutrons were taken to be the same for the GLEEP and water irradiations; it is interesting to note that the calculated self-shielding factor was 0.93 although this actual value was not needed in the calibrations. The external flux depression produced in water by the foil was allowed for by the method of Skyrme, as described by Beckurts and W i r t z 9 ) ; in graphite the external flux depression was neglected. LOs

104

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I ZOO centre

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I 300

of source(ram)

Fig. 3. Reaction-rate distributions obtained in water with a bare NE-902 glass scintillation detector of thickness 0.5 ram, for different Am-Be source configurations.

Foil detectors provide complete discrimination against gamma rays but they are slow to use and are unsuitable for measuring very low fluxes. A scintillation detector system was therefore constructed for quicker measurements of neutron fluxes and currents; this was calibrated with the thermal neutron fluxes produced in water by the A m - B e source and measured by the foils. Two scintillators were used: a LiF-ZnS granular scintillator 12.7 m m diameter with 0.056 mg m m -2 of lithium fluoride enriched to 99.6% in 6Li and an NE-902 glass scintillator 12.7 m m diameter, 0.5 m m thick with 2.2% by weight of lithium enriched to 95% in 6Li. The granular detector was made by painting a 0.25 m m thick aluminium disc with cyclohexamine containing a mixture of LiF, ZnS (10-15 g m grain size) and polyvinyl chloride acetate copolymer (VYNS), in the proportion 1 : 2:1 by weight (Stedmanl°)). A perspex rod 0.6 m long and 12.7 m m in diameter, coupled the scintillator to an EMI 9524S photomultiplier. Conventional pulse-height analysis was used to distinguish between neutron and gamma-ray pulses. The LiF-ZnS had a sensitivity of about 0.04 counts per unit flux of thermal neutrons and good discrimination against gamma rays; typically the sensitivity for thermal neutrons to that for 6°Co gamma rays (en/~7) was 109. However, its granular nature resulted in a large spread in outputpulse intensity and the bias point was not precise. Periodic recalibration in a known flux was therefore necessary, but extensive stability tests showed that the neutron sensitivity remained constant to within 1% during measurements. The glass scintillation detector had a discriminator bias point which could be clearly selected and a neutron sensitivity which varied only slightly with pulse amplification; a 10% increase in amplification decreased the sensitivity by approximately 0.7%. However, because the discrimination against gamma rays was poor (8n/~7= 250), the detector was used only to measure the relative flux distributions from different configurations of A m - B e sources. Fig. 3 gives the results of these measurements and shows in the insert the different source arrangements. It is clear that the thermal flux per source neutron was the same for all the different arrangements at distances greater than 0.1 m from the source centre. For smaller distances, the flux perturbations due to the larger (or multiple) sources produce small reductions in the flux per source neutron.

THERMAL NEUTRON

FLUXES P R O D U C E D

A comparison of the Am-Be/water flux distributions measured with the different detectors showed that the LiF-ZnS and glass detectors may be used to within approximately 10 m m of the source surface. At the surface of the source the glass detector indicated an increase in the flux relative to the foil measurements owing to inadequate gamma discrimination under these unfavourable conditions. The thermal flux distribution from the 124Sb-Be source in water was measured along a line passing through its centre and perpendicular to its axis. A low intensity source was used to reduce health hazards, and fluxes were measured with the LiF-ZnS detector. In addition fluxes at two points near the source were measured with manganese-nickel foils (25.4 mm diameter, 0.076 mm thick, 12% Ni, 88% Mn). The high cadmium ratio meant that counting rates with the detector covered with cadmium were low, and it was only possible to extend the measurements to a radius of 0.24 m by covering the source with 25 mm of lead to shield against the 124Sb gamma rays. Fig. 4 shows the thermal flux distributions for the A m - B e (type X3) and the Sb-Be sources in the full water tank. Included in the diagram are the relative flux measurements made by Reier and de Juren ~]) for the Sb-Be case; their results have been normalized to the present measurements over the range 0.12 to 0.25 m. The standard deviations of the present results are shown in the figure with the exception of the A m - B e / L i F - Z n S measurements for which the standard deviation, essentially the statistical counting uncertainty, is less than 1%. Uncertainties in gold foil measurements were calculated using tr(~)Au=(a~+Cr]s) ½ where aN is the standard error arising from activation measurements and the uncertainty in the GLEEP reference flux, and as is the standard error on the source intensity. Overall errors on the measurements vary from 1.9% near the source to 6.2% at the largest distance. Uncertainties in the LiF-ZnS scintillation measurements were calculated using O'(¢scint.)=(O'2-'[ + tr~ + a~u)½, where aN is the standard deviation of the counts, and irA, is the estimated error in the standardisation against the gold foils. 3.2. FLUXES PRODUCED BY THE ACCELERATOR SOURCES

Thermal neutron fluxes produced by the accelerator sources were usually measured with the

IN W A T E R

179

LiF-ZnS detector. The flux distributions per neutron per second from the D - T source were obtained at 0°, 90° and 135° to the incident deuteron beam; for the D - D source the measurements were at 0° only. Some measurements with the manganese-nickel foils were also made, and in this case 0° information at one radius was included for the D - D source. As before, the foil activities were compared with those produced by activation in the thermal flux from the 1 Ci A m - B e source in water. For the foil measurements, a small fission chamber in the moderator monitored the neutron output of the accelerator to avoid radiation damage to the surface barrier detector during the long experimental runs; the counting rates of the fission and surface barrier detectors were recorded simultaneously at the beginning and end of an activation run. I

I

I

I

l

S b - Be

I

'

source

Reier and i Juren(1961) • L i F - Z n S : S o w c e Pb covered ® • bare 0 Mn foil Z~

lo-t Am - Be el c

o

xd3L a

io-~

l L~F-~.~

~-

I Au

~

A i n - Be source J

=

foil

i

lO-S

lO-i I

I

I 100 Dletena:o

, from

I

I 200

I

I 300

centre of source (lain)

Fig. 4. Measured and calculated thermal neutron flux distributions for Am-Be and Sb-Be sources in water. The transport (P3-S16) calculations are for a 400 m m radius sphere and used a mesh spacing of 20 ram.

180

L. HOLLAND et al.

In addition to these flux measurements based on the associated particle technique, others were made for the 90 ° direction with a D - T source and for 0 ° with a D - D source in which the scintillation counter recorded the fluxes and was normalized against the fixed fission monitor. A simpler and more compact target assembly was used to allow measurements to be made closer to the source; it had no beam stops and, in consequence, there was ~in uncertainty of ± 11 mm in the position of the beam on the target. Fig. 5 shows the measured thermal neutron flux distributions obtained for the two accelerator

sources in the full water tank. Absolute flux distributions for the D - T source, obtained at 0°, 90 ° and 135° to the incident deuteron beam, were found to be identical within the uncertainty of the measurements. Although the relative flux measurements were normalized to the absolute flux distributions for eight source-detector separations in the range 0.06 to 0.42 m, only the two points closest to the source are shown to avoid confusion. The cadmium ratio of the scintillator measurements increased to a constant value of 22 at a radius of 180 mm for the D - T source and a valRE"

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I

200 from

centre

I

:500 of

target

400 (mm

/ / /

t,/--

......

/

........

}

5OO

)

Fig. 5. Measured and calculated thermal neutron flux distributions for D-D and D-T sources in water. The transport (/'3" Sl6) calculations are for a 400 mm radius sphere and used a mesh spacing of 20 ram. The D - T water plus copper distribution assumed the central 20 mm radius sphere surrounding the source was filled with 0.15 kg of copper.

I I

l

i

I

100

200

300

400

Distance

from

centre

of

target

(mm

500

)

Fig. 6. Thermal neutron flux distributions for the D - T source with different depths of water.

THERMAL NEUTRON FLUXES PRODUCED IN WATER

ue of 30 for the D - D source; for both sources it decreased rapidly with decreasing radius at distances closer than 50 mm from the source. 3.3. FLUXES PRODUCED IN SMALLER VOLUMES OF WATER

The effective moderator size was altered by successively placing right cylinders of cadmium (1 mm thick) of different radii around the central neutron source (1 Ci Am-Be). The thermal neutron fluxes were measured along the axes of the cylinders. This arrangement meant that each cylindrical moderator had a reflector for epi-cadmium neutrons; the effect of this was investigated on the upper surface of two different cylinders (diameters 0.31 m and 0.47m) by measuring the thermal fluxes inside the cylinders with and without water above them. The measured change in flux was approximately 4% at 20 mm inside the cylinder, 2% at 40 mm and 1% at 60 ram. Near the centre of the moderator, the region of main interest in the present work, the effects of the reflector were less than the errors on the flux measurements. Thermal neutron fluxes were also measured for the Sb-Be source in different right cylinders, the smallest of which had a diameter of 0.3 m, and the same changes in flux were observed. These results showed that the reflected epithermal neutrons do not influence the thermal flux beyond about 80 mm from the boundary. It was therefore considered that the moderator size for the D - T / w a t e r system could be effectively altered by reducing the depth of water above the source and measuring the thermal flux along the vertical line between the source and water surface. Fig. 6 shows the flux distributions for three depths. It is clear from fig. 6 that even with the highest energy of source neutrons (14 MeV from D-T) there is no advantage for the central (peak) thermal flux from increasing the moderator thickness around the source beyond 150 mm. 4. Calculated fluxes Neutron transport calculations based on the XSDRN code (Greene and Crawford12)) were made to solve the transport equations in spherical geometry for the neutron flux defined by: io.6 ov ~(_e) d e

¢=40

v

O.6ev l d E /3

181

The neutron sources were assumed to be distributed uniformly in the first mesh interval used in the discrete ordinate method; the mesh spacing of 20 mm was well below the transport length of the source neutrons (50-100 mm). Although there is no intention to include computational details in this paper, we mention that a step approach to the interpolation of fluxes between mesh points avoided the negative angular fluxes and oscillatory behaviour with energy sometimes observed with the diamond-difference method in the P3 approximation, a behaviour which could cause predicted fluxes to differ by a factor as high as 2 from those of the step method. Results of calculations and measurements with Sb-Be and A m - B e sources have been given already in figs. 3 and 4. The fluxes were calculated using the detailed neutron spectra of Werle~3), who measured the A m - B e neutron spectrum down to 0.1 MeV. The differences near the source between calculation and experiment are most likely the result of geometrical effects because actual cylindrical sources were treated as spheres in the calculations. Calculations in which beryllium instead of water was placed around the source did not give better agreement. For the D - T source, a significant improvement in the agreement between calculated and measured fluxes near the source was obtained when a copper target zone was included in the calculations (see fig. 5), probably because of inelastic scattering in the target material and the accelerator tube. Despite the limitations of age theory for hydrogenous moderators, we felt that it would be useful, because of their low computational demands, to make age-diffusion calculations of the thermal neutron fluxes in water due to the four different sources used in the experiments. A point source and spherical geometry were assumed, according to the formulation of Wallace and LeCaine~4). For these calculations the source spectra were divided into ten groups and the age-to-thermal for each group was taken from Goldstein~5). For the A m - B e and D - D sources the transport and agediffusion calculations differed by up to 30%, although much better agreement for the case of Am-Be was obtained when a spectrum somewhat different from Werle's was used. For the Sb-Be source, the two calculations gave essentially identical results. Comparison of the age-diffusion flux distributions with the measured ones for 14 MeV source neutrons (D-T) showed the calculated val-

182

L. HOLLAND et al.

ue to be about 4096 of the measured one in the region near the source. We thank Dr. M. R. Hawkesworth for helpful comments, and AERE, Harwell and the Science Research Council, London, for financial support. References l) M. R. Hawkesworth and J. Walker, J. Mat. Sci. 4 (1969). 817. 2) A. R. Spowart, Nuci. Instr. and Meth. 92 (1971) 613. 3) Californium-252. Californium-252 Progress 5 (1959) 17. 4) L. Holland and J. Walker, ANS Conf. Neutron sources and applications I1 (1971) 154 (Conf. 710482). 5) M. C. Scott, Nucl. Instr. and Meth. g2 (1970) 237.

6) T. R. Feweil, Nucl. Instr. and Meth. 61 (1968) 61. 7) L. Ruby and R. B. Crawford, Lawrence Radiation Lab. Report UCRL-10752, Suppl. (1963) (Berkeley, Calif.). s) E. J. Axton, J. Nuci. Energy (Pt. A/B Reactor Sci. and Technol.) 17 (1963) 125. 9) K. H. Beckurts and K. Wirtz, Neutron physics 2nd ed. (Springer, Berlin, 1964) p. 39. 10) R. Stedman, Rev. Sci. Instr. 31 (1960) 1156. 11) M. Reier and J. A. de Juren, J. Nucl. Energy (Pt. A/B Reactor Sci: and Technol.) 14 (1961) 18. 12) N. M. Greene and C. W. Craven Jnr., Oak Ridge Nat. Lab. Report ORNL-TM 2500 (1969). 13) H. Werle, Karlsruhe Nucl. Research Centre Report IRN 4170-25 (1970). 14) p. R. Wallace and J. LeCaine, Chalk River Report AECL336 (1943). 15) H. Goldstein et al., Oak Ridge Nat. Lab. Report ORNL2639 (1961).