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Calibration of neutron monitors for radiation protection at 24.5 keV and 2 keV
NUCLEAR
INSTRUMENTS
AND
METHODS
155 (1978)
307-308 : O
NORTH-HOLLAND
PUBLISHING
CO.
C A L I B R A T I O N O F N E U T R O N M O N I T O R S FOR RADIATION P R O T E C T I O N AT 24.5 keV AND 2 keV W O L F G A N G G. A L B E R T S
Physikalisth-Technische Bundesanstalt, D-3300 Braunschweig, Fed. Rep. ol Germany Received 30 March 1978 Two n e u t r o n dose rate meters frequently used for radiation proteclion m o n i t o r i n g were calibrated at the filtered n e u t r o n b e a m s of the PTB reactor at 24.5 keV and 2 keV.
In the working areas around nuclear installations, neutrons in the lower keV energy region may essentially contribute to the total neutron-induced dose equivalentS). It is therefore important to calibrate instruments for radiation protection purposes with neutrons of known energies in this region. There are essentially two approaches for obtaining suitable sources for monoenergetic neutrons below 30 keV which have only recently been used for neutron monitor calibrations. One involves the use of a Sb-Be (),, n) source emitting monoenergetic neutrons of 23 keV and a small contribution of 375 keV neutrons, or a wide neutron spectrum centered at 0.5 keV if the source is surrounded by a special moderating sphere"3). The other m e t h o d is to extract monoenergetic neutron beams through transmission filters from a nuclear reactor, taking advantage of the fact that the total cross sections of certain materials have deep relative minima acting as " n e u t r o n wind o w s " . Filters of this kind were first installed at the Idaho MTR reactor 4) in the late Sixties, but only recently have results been published 5) of neutron monitor calibrations using the 2 keV, 25 keV and 144 keV filtered beams at the NBS reactor in Gaithersburg 6). At the 1 MW research and measuring reactor Braunschweig (FMRB) we have installed two filtered neutron beam facilities for calibration purposes. One, for 24.5 keV neutrons, consists of 3 5 2 m m Armco iron, 2 3 0 m m aluminium and 7 5 m m sulphur with diameters o f 4 0 m m to 43 mm, built into a radial beam tube and looking directly at the reactor core. The other, for 2 keV neutrons, consists of 7 0 8 r a m scandium and 15 m m titanium with diameters of 57 m m built into a tangential beam tube. It transmits neutrons preferentially scattered from a manganese scatterer
which is mounted near the reactor core (scattering resonance at 2.38 keV) similar to the scandium filter at the NBS7). A detailed description of the facility is given elsewhereS). In order to reduce the unwanted contribution of neutrons of higher energies as well as the background due to scattered neutrons and the g a m m a radiation in the beams, a filter difference method ~) was invariably used: two measurements are made with the same detector, one in the direct beam and one in the beam passing through an additional filter (referred to as "difference filter"; 5 mm Ti for 24.5 keV and 25 m m Mn powder for 2 keV) which preferentially absorbs neutrons of the desired energy, leaving the high energy background almost unchanged. The difference between the two detector readings obtained can be assigned to a "difference b e a m " with reduced background. All figures given in this communication will therefore refer to the respective difference beams. Measurements of the beam profiles, together with the neutron current densities as determined by means of hydrogen-filled proton recoil proportional counters~°), yielded total neutron currents of 1.3× 105 s ~ (for 24.5 keV neutrons, beam diameter - 8 . 5 cm) and 2.4× 105 s ~ (2 keV, beam diameter - 1 4 cm) at 1 MW reactor power. The accompanying g a m m a radiation gave dose rates in the order of 5 0 / ~ G y / h , which is very low compared to 0.1 G y / h observed at 1 m distance from the Sb-Be (7, n) source3). The two commercial neutron monitors used for routine and survey measurements at the FMRB represent two types of so-called rein counters: the R E M / N type, originally designed by Andersson and Braun~), of cylindrical shape, and the spherical type 95/0075 originally developed by Leake~). These rein counters are insensitive to y-radiation of the level present at the filtered beams. How-
308
w . G . ALBER'I-S
ever, since they are designed to give a response proportional to dose equivalent independent of the energy of incident neutrons, they are much more sensitive to high-energy neutrons than to the neutrons considered here'3). The high-energy neutron background in the filtered beams was therefore investigated by evaluating the readings of several detectors with different response functions (3He proportional counter, de Pangher precision long counter, and the rein counters) when irradiated in the beams with and without difference filterS). For the difference beams, high energy portion to total neutron current ratios of 1.2% Fe) and 15% (Sc) were derived, contributing to about 5% and 50% of the count rate of the rein counters for the 24.5 keV and the 2 keV beam, respectively. These figures are based on the assumption of an essentially flat response of the long counter down to energies below 2 keV 2), which has not yet been satisfactorily confirmed 8.~4). The calibration had to be performed using a scanning procedure; the results are shown in table 1. The response in units of counts per dose equivalent was derived using fluence-to-dose equivalent conversion factors t3) of 1.85× 10-II Jkg-~ cm 2 for 24.5keV and 1×10 -I~ J k g ~cm 2 for 2keV. The uncertainties assigned to the response values are estimated to be about 20% (for 24.5 keV) and up to 50% (for 2 keV), mainly because of the aforementioned uncertainty in the response of the long counter influencing the estimation of the high energy neutron background in the beams. The results show an overresponse to the keV neutrons of a factor of ~ 4 (REM/N Type) and ~ 5 (Type 95/0075) as compared to neutrons of Pu-Be (~z, n) or Am-Be (m n) neutron sources. This confirms the results reported recently by HankinsS), who quotes a factor 4 for 25 keV and a factor 3 for 2 keV with an Andersson and Braun type remmeter. The value of 5.1 × 10 9 1/Jkg I determined at 24.5 keV for the Type 95/0075 rein counter reproduces the value of Harrison et al. 3) measured at 23 keV with the Sb-Be (y, n) source;
TABLE 1 Rein counter calibration results. Type REM/N 24.5 keV 2 keV Response in cm 2 (counts per neutron fluence)
0.069
0.034
Type 95/0075 24.5 keV 2 keV
0.095
0.045
Response in I/Jkg
I (s l/mremh 1) (counts per dose equivalent)
3.7×109 3.4xl09 (10.3) (9.5)
5.1×109 4.5×109 (14.3) (12.6)
a value at 2 keV for this type of rem counter is not available for comparison.
References ~) J. R. Harvey and S. Beynon, Proc. Ist Syrup. on Neutron dosirneoy in biology and medicine, Neuherberg, EUR 4896, Vol. 11 (1972) 955. 2) j. R. Harvey, A. Lavender. and 1. M. G. Thompson, Itealth Phys. 31 (1976) 363. 3) K. G. Harrison, J. R. Ilarvey and S. J. Boot, Nucl. Instr. and Meth. 148 (1978) 511. 4) O. D. Simpson, J. R. Smith and J. W. Rogers, Proc. Syrup. Neutron standards and flux normalization, Argonne, 111., CONF-701002 (1970) p. 362. 5) D. E. Hankins, Proc. IV. Int. Congr., Int. Rad. Prot. Association, Paris, Vol. 2 (1977) 553. 6) R. B. Schwartz, Proc. Syrup. Neutron standards and applications, NBS Spec. Publ. 493 (1977) 250. 7) I. G. Schr~Sder, R. B. Schwartz and E. D. McGarry, Proc. Conf. Neutron cross sections and technology, NBS Spec. Publ. 425 (1975) 89. 8) W. G. Alberts, Report PTB-FMRB-70 (1978). 9) E. N. Kuzin et al., Atomn. Energ. 35 (1973) 391. 10) K. Knauf, personal communication. II) I. O. Andersson and J. Braun, Nukleonik 6 (1964) 237. 12) j. W. Leake, Nucl. Instr. and Meth. 45 (1966) 151, and 63 (1968) 329. 13) Report ICRP publication 21 (1973). 1,t) D. R. Slaughter and D. W. Rueppel, Nucl. Instr. and Meth. 145 (1977) 315.
INSTRUMENTS
AND
METHODS
155 (1978)
307-308 : O
NORTH-HOLLAND
PUBLISHING
CO.
C A L I B R A T I O N O F N E U T R O N M O N I T O R S FOR RADIATION P R O T E C T I O N AT 24.5 keV AND 2 keV W O L F G A N G G. A L B E R T S
Physikalisth-Technische Bundesanstalt, D-3300 Braunschweig, Fed. Rep. ol Germany Received 30 March 1978 Two n e u t r o n dose rate meters frequently used for radiation proteclion m o n i t o r i n g were calibrated at the filtered n e u t r o n b e a m s of the PTB reactor at 24.5 keV and 2 keV.
In the working areas around nuclear installations, neutrons in the lower keV energy region may essentially contribute to the total neutron-induced dose equivalentS). It is therefore important to calibrate instruments for radiation protection purposes with neutrons of known energies in this region. There are essentially two approaches for obtaining suitable sources for monoenergetic neutrons below 30 keV which have only recently been used for neutron monitor calibrations. One involves the use of a Sb-Be (),, n) source emitting monoenergetic neutrons of 23 keV and a small contribution of 375 keV neutrons, or a wide neutron spectrum centered at 0.5 keV if the source is surrounded by a special moderating sphere"3). The other m e t h o d is to extract monoenergetic neutron beams through transmission filters from a nuclear reactor, taking advantage of the fact that the total cross sections of certain materials have deep relative minima acting as " n e u t r o n wind o w s " . Filters of this kind were first installed at the Idaho MTR reactor 4) in the late Sixties, but only recently have results been published 5) of neutron monitor calibrations using the 2 keV, 25 keV and 144 keV filtered beams at the NBS reactor in Gaithersburg 6). At the 1 MW research and measuring reactor Braunschweig (FMRB) we have installed two filtered neutron beam facilities for calibration purposes. One, for 24.5 keV neutrons, consists of 3 5 2 m m Armco iron, 2 3 0 m m aluminium and 7 5 m m sulphur with diameters o f 4 0 m m to 43 mm, built into a radial beam tube and looking directly at the reactor core. The other, for 2 keV neutrons, consists of 7 0 8 r a m scandium and 15 m m titanium with diameters of 57 m m built into a tangential beam tube. It transmits neutrons preferentially scattered from a manganese scatterer
which is mounted near the reactor core (scattering resonance at 2.38 keV) similar to the scandium filter at the NBS7). A detailed description of the facility is given elsewhereS). In order to reduce the unwanted contribution of neutrons of higher energies as well as the background due to scattered neutrons and the g a m m a radiation in the beams, a filter difference method ~) was invariably used: two measurements are made with the same detector, one in the direct beam and one in the beam passing through an additional filter (referred to as "difference filter"; 5 mm Ti for 24.5 keV and 25 m m Mn powder for 2 keV) which preferentially absorbs neutrons of the desired energy, leaving the high energy background almost unchanged. The difference between the two detector readings obtained can be assigned to a "difference b e a m " with reduced background. All figures given in this communication will therefore refer to the respective difference beams. Measurements of the beam profiles, together with the neutron current densities as determined by means of hydrogen-filled proton recoil proportional counters~°), yielded total neutron currents of 1.3× 105 s ~ (for 24.5 keV neutrons, beam diameter - 8 . 5 cm) and 2.4× 105 s ~ (2 keV, beam diameter - 1 4 cm) at 1 MW reactor power. The accompanying g a m m a radiation gave dose rates in the order of 5 0 / ~ G y / h , which is very low compared to 0.1 G y / h observed at 1 m distance from the Sb-Be (7, n) source3). The two commercial neutron monitors used for routine and survey measurements at the FMRB represent two types of so-called rein counters: the R E M / N type, originally designed by Andersson and Braun~), of cylindrical shape, and the spherical type 95/0075 originally developed by Leake~). These rein counters are insensitive to y-radiation of the level present at the filtered beams. How-
308
w . G . ALBER'I-S
ever, since they are designed to give a response proportional to dose equivalent independent of the energy of incident neutrons, they are much more sensitive to high-energy neutrons than to the neutrons considered here'3). The high-energy neutron background in the filtered beams was therefore investigated by evaluating the readings of several detectors with different response functions (3He proportional counter, de Pangher precision long counter, and the rein counters) when irradiated in the beams with and without difference filterS). For the difference beams, high energy portion to total neutron current ratios of 1.2% Fe) and 15% (Sc) were derived, contributing to about 5% and 50% of the count rate of the rein counters for the 24.5 keV and the 2 keV beam, respectively. These figures are based on the assumption of an essentially flat response of the long counter down to energies below 2 keV 2), which has not yet been satisfactorily confirmed 8.~4). The calibration had to be performed using a scanning procedure; the results are shown in table 1. The response in units of counts per dose equivalent was derived using fluence-to-dose equivalent conversion factors t3) of 1.85× 10-II Jkg-~ cm 2 for 24.5keV and 1×10 -I~ J k g ~cm 2 for 2keV. The uncertainties assigned to the response values are estimated to be about 20% (for 24.5 keV) and up to 50% (for 2 keV), mainly because of the aforementioned uncertainty in the response of the long counter influencing the estimation of the high energy neutron background in the beams. The results show an overresponse to the keV neutrons of a factor of ~ 4 (REM/N Type) and ~ 5 (Type 95/0075) as compared to neutrons of Pu-Be (~z, n) or Am-Be (m n) neutron sources. This confirms the results reported recently by HankinsS), who quotes a factor 4 for 25 keV and a factor 3 for 2 keV with an Andersson and Braun type remmeter. The value of 5.1 × 10 9 1/Jkg I determined at 24.5 keV for the Type 95/0075 rein counter reproduces the value of Harrison et al. 3) measured at 23 keV with the Sb-Be (y, n) source;
TABLE 1 Rein counter calibration results. Type REM/N 24.5 keV 2 keV Response in cm 2 (counts per neutron fluence)
0.069
0.034
Type 95/0075 24.5 keV 2 keV
0.095
0.045
Response in I/Jkg
I (s l/mremh 1) (counts per dose equivalent)
3.7×109 3.4xl09 (10.3) (9.5)
5.1×109 4.5×109 (14.3) (12.6)
a value at 2 keV for this type of rem counter is not available for comparison.
References ~) J. R. Harvey and S. Beynon, Proc. Ist Syrup. on Neutron dosirneoy in biology and medicine, Neuherberg, EUR 4896, Vol. 11 (1972) 955. 2) j. R. Harvey, A. Lavender. and 1. M. G. Thompson, Itealth Phys. 31 (1976) 363. 3) K. G. Harrison, J. R. Ilarvey and S. J. Boot, Nucl. Instr. and Meth. 148 (1978) 511. 4) O. D. Simpson, J. R. Smith and J. W. Rogers, Proc. Syrup. Neutron standards and flux normalization, Argonne, 111., CONF-701002 (1970) p. 362. 5) D. E. Hankins, Proc. IV. Int. Congr., Int. Rad. Prot. Association, Paris, Vol. 2 (1977) 553. 6) R. B. Schwartz, Proc. Syrup. Neutron standards and applications, NBS Spec. Publ. 493 (1977) 250. 7) I. G. Schr~Sder, R. B. Schwartz and E. D. McGarry, Proc. Conf. Neutron cross sections and technology, NBS Spec. Publ. 425 (1975) 89. 8) W. G. Alberts, Report PTB-FMRB-70 (1978). 9) E. N. Kuzin et al., Atomn. Energ. 35 (1973) 391. 10) K. Knauf, personal communication. II) I. O. Andersson and J. Braun, Nukleonik 6 (1964) 237. 12) j. W. Leake, Nucl. Instr. and Meth. 45 (1966) 151, and 63 (1968) 329. 13) Report ICRP publication 21 (1973). 1,t) D. R. Slaughter and D. W. Rueppel, Nucl. Instr. and Meth. 145 (1977) 315.