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The anomalous slowing down of neutrons in paraffin wax
Technical notes
199
International Journal of Applled Radiation and Isotopes, 1969, Vol. 20, 199-201. Pergamon Press. Printed in Northern Ireland
References
pp.
1. ~ x o N P. J. Iru. J. at~t. Radiat. Isotopes 4, 232 (1959). 2. URQvt~ata" D. F. Standardisation of Radionudides, Proceedings SM 79/27 I.A.E.A. Vienna (1967). 3. MEmuTr J. S. and TAYLORJ. G. V. Standardisation of Radionudides, Proceedings SM 79/66 I.A.E.A. Viemm (1967). 4. WXLLmUS A. and CAMPION P. J. Int. J. appL Radiat. Isotopes 14, 533 (1963). 5. RYTZ A. Report on the Int. Comparison of the 4~r/~(PC)- 7 method by means of 6°Co of March 1963, B.LP.M. (1965). 6. G~a~AVos C. E. and RYTZ A. Metrologia 2, 90 (1966). 7. SPEaNOLA., D~. ROOSTE. and L~RCH O. C.B.N.M. (Euratom), Report EUR.477.e (1964). 8. RYTz A. Compte Rendu des Resultats de la Comparison Internationale d'une Solution de ~aMn; April 1965, B.LP.M. (1965). 9. MULLER J. W. and RYTz A. Report on the International Comparisort of Dilution and Source Preparation Methods by means of 6°Co, M a y 1967, Pt. I, B.I.P.M. (1967).
,o4s /
/
~The A n o m a l o u s S l o w i n g D o w n Neutrons in Paraffin Wax
Introduction FoR MANY purposes, a small portable source of thermal neutrons is frequently used, which is usually obtained by moderating fast neutrons from a source such as Am/Be by means of paraffin wax ez}. I t is usually assumed that these moderated neutrons have an approximately thermal spectrum. I n the present work we have examined the spectrum of neutrons from an ~lAm/Be source moderated by commercial paragm wax using a mechanical velocity selector~ and find that the spectrum of neutrons is not thermal but has a peak at a considerably lower energy. I n addition we measure the spectrum of the fast neutrons moderated in water to be approximately thermal. Experixnental procedure A 3 c Am/Be neutron source, which produces 8 × 10 6 fast neutrons per second of average energy
zoc/s
[ 6c N°
OF"
NEUTRONS
' , , j D I F ' F E R E NCE CURVE
. 0
!
of
2 y¢~oclrv
3
~N m o o M/S
4
,~
Fzo. I. Calculated velocity distribution of neutrom passing through the mechanical veloclty selector for various rotor speeds.
200
Technical notes
4 MeV (s), was surrounded by up to 30 cm of paraffin wax moderator. These slow neutrons were then analysed by a mechanical velocity selector before being detected by a N.E. 905 6Li loaded glass scintillator 1.75 in. in dia. and 0.125 in. thick. The mechanical velocity selector consisted essentially of two circular discs 30 cm in dla., each disc being composed of 4 mm of aluminium and 2 mm of cadmium. The cadmium of each disc had a 4 ° sector removed which acted as a window for slow neutrons. The two discs were mounted, 50 cm apart, on a central axle connected to a d.c. motor capable ofrotatlng the two discs at up to 60 cycles/see. The disc nearest to the source led the other disc in phase by 1°. The calculated distributions of neutron velocities allowed through the selector at various speeds of revolution are shown in Fig. I for 10 and 20 cycles/see. At each speed the selector allows all the neutrons with a velocity above a well defined minimum value to reach the detector. The number of neutrons having velocities in the region between two of these minimum velocities is obtained by subtracting the responses of the detector for the two corresponding rotor speeds, the velocity resolution of the selector being determined by the rotor speed difference
considered. In the present case differences of 10 cycles/see between the respective rotor speeds were used in order to keep the differences in detector response to a statistically meaningful level.
Results and discussion The measured neutron velocity spectra are shown in Fig. 2a for neutrons which were moderated respectively by 30 cm of paraffin wax and 30 cm of water. Here it is seen that the spectrum of water moderated neutrons has the expected thermal neutron peak at 2200 m/see whereas the paraffin wax moderated neutron spectrum has a maximum at a neutron velocity of only 1050 m/see. We have taken a transmission Laue X-ray photo-
graph shown in Fig. 2b which indicates that the paraffin wax has an unexpectedly well ordered structure, the largest atomic spacing of the carbon atoms being 3.98 A. However, because neutrons, unlike X-rays arc coherently scattered by both the hydrogen and the carbon atoms in the paraffin wax, the relevant atomic spacing for neutrons is 1.99 A. Our paraffin results may be explained, in the same way as the phenomenon of cold neutron production (4), by considering the wave properties of neutrons,
150
l '1OO' NEUTRON No. DIFFERENCEISEC.
A ANDB NORMALISED so, .ERE I
o
16oo
s~oo
2000
NEUTRON VELOCITY
M/S
•
Fio. 2a. Spectra of moderated neutrons. Curve (A) shows the spectrum of neutrons moderated by paraffin wax, Curve (B) shows the spectrum of neutrons moderated by water.
FIO. 2b. Transmission X-ray Laue photograph of paraffin wax.
200
Technical notes which when incident on a structure of atomic spacing d will be diffracted from their initial direction and the detector providing that n 2 = Y d s i n O . Hence for first order diffraction only wavelengths longer than 2d will reach the detector. Using the de Broglie relation we find that this corresponds to a neutron velocity of 1050 m/sec. Figure 2a shows that the numbers of neutrons reaching the detector with velocities greater than 1050 m/sec are greatly reduced in the case of paratFm which agrees with the diffraction theory.
t~onclusion The results of this work shows that paraffin wax cannot be considered to be without structure and that when used to moderate neutrons from an Am/Be source the spectrum of these neutrons is considerably modified from the expected thermal distribution. The neutrons moderated in water have as expected a near thermal spectrum.
Physics Department University of Aston in Birmingham
A . J . Cox S . A . NEw
References l. COX A. J., FRANCOIS P. E. and GATRELL R. P. Int. J. appl. Radiat. Isotopes 19, 541 (1968). 2. Du~INO J. R., PEOm~t G. B., Free G. A., MrrCH~LL D. P. and SE~R~ E. Phys. Rev. 48, 704 (1935). 3. THOMPSON M. N. and TAYLOR J. M. Nucl. Inst. Meth. 37, 306 (1965). 4. ANDERSON H. L., FEm~I E. and MARSHALL L. Phys. Rev. 70, 815 (1946).
InternationalJournalof AppliedRadiationand Isotopes,1969,VoL20, pp. 201-203. PergamonPress. Printedin NorthernIreland
Emission Spectra of Thermolnminescent Lithium Fluoride and Lithium Borate: Manganese IN THE study of thermoluminescent phosphors valuable information may be obtained by determining the emission spectrum of the light evolved on heating. A recent example is the work on the thermoluminescence process in rare-earth activated CaFy{1), at temperatures up to 300°K, where the emission spectra for different glow peaks were found to be line spectra characteristic of the rare-earth dopant. Some thermoluminescent phosphors have properties which render them particularly suitable as radiation dosemeters{ 2) and in these cases a knowledge of their emission spectra is of value not only as an aid
2 01
to the basic understanding of the thermoluminescence process, but also to enable the phosphor to be matched to the most suitable photomultiplier for greatest accuracy in the read-out of absorbed dose. This note reports the use of a system for the determination of the emission spectra of thermoluminescent phosphors above ambient temperature, based on a grating monochromator with photomultiplier detector and calibrated to correct for wavelength dependence of sensitivity. The system has been used in the first instance for the common radlothermoluminescent phosphors lithium fluoride (TLD-100)* and lithium borate: Mn. Similar work has recently been reported by GORBmS(a) who used a quartz-prism spectrograph with photographic detection and who determined the variation of response with wavelength by means of a standardised tungsten filament quartz-iodlne lamp. The same type of lamp has been used in this work in order to determine the wavelength response of our detecting assembly.
Equipment and response calibration procedure The apparatus consisted of a small furnace and monochromator (Bausch and Lomb, model 33-8645-01) with photomultiplier housing and shutter at the exit slit. These items were contained in a light-tight box and leads passed from the photomultiplier to an E H T supply and d.c. amplifier (A.E.R.E., Type 1388A). The output of the d.c. amplifier was fed on to the Y-axis of an X/Y,T plotter (Bryans, model 22020). The furnace consisted of an aluminium rod on which the heater coils were wound directly. The flat end of the rod had a groove which held closely a small silica sample tube 0-4 cm in dia., but left a large portion of its surface exposed so that light from the heated phosphor could emerge. The tube, into which a fine thermocouple could be inserted, was positioned close to the inlet slit of the monochromator. The wavelength drum was driven by a small synchronous motor. Calibration of the variation in response with wavelength was achieved by the use of a quartziodine tungsten filament lamp operated at a known colour temperature. The entrance slit of the monochromator was uniformly illuminated by reflection from an opal screen. The current necessary to operate the lamp at a colour temperature of 3000°K and the variation of the reflection coefficient with wavelength of the opal screenft were determined by NPL Metrology Centre. Also supplied was a wavelength dependent factor f2 to correct for the deviation of the lamp from a true black-body radiator. The spectral distribution of radiant energy * Harshaw Chemical Co.
199
International Journal of Applled Radiation and Isotopes, 1969, Vol. 20, 199-201. Pergamon Press. Printed in Northern Ireland
References
pp.
1. ~ x o N P. J. Iru. J. at~t. Radiat. Isotopes 4, 232 (1959). 2. URQvt~ata" D. F. Standardisation of Radionudides, Proceedings SM 79/27 I.A.E.A. Vienna (1967). 3. MEmuTr J. S. and TAYLORJ. G. V. Standardisation of Radionudides, Proceedings SM 79/66 I.A.E.A. Viemm (1967). 4. WXLLmUS A. and CAMPION P. J. Int. J. appL Radiat. Isotopes 14, 533 (1963). 5. RYTZ A. Report on the Int. Comparison of the 4~r/~(PC)- 7 method by means of 6°Co of March 1963, B.LP.M. (1965). 6. G~a~AVos C. E. and RYTZ A. Metrologia 2, 90 (1966). 7. SPEaNOLA., D~. ROOSTE. and L~RCH O. C.B.N.M. (Euratom), Report EUR.477.e (1964). 8. RYTz A. Compte Rendu des Resultats de la Comparison Internationale d'une Solution de ~aMn; April 1965, B.LP.M. (1965). 9. MULLER J. W. and RYTz A. Report on the International Comparisort of Dilution and Source Preparation Methods by means of 6°Co, M a y 1967, Pt. I, B.I.P.M. (1967).
,o4s /
/
~The A n o m a l o u s S l o w i n g D o w n Neutrons in Paraffin Wax
Introduction FoR MANY purposes, a small portable source of thermal neutrons is frequently used, which is usually obtained by moderating fast neutrons from a source such as Am/Be by means of paraffin wax ez}. I t is usually assumed that these moderated neutrons have an approximately thermal spectrum. I n the present work we have examined the spectrum of neutrons from an ~lAm/Be source moderated by commercial paragm wax using a mechanical velocity selector~ and find that the spectrum of neutrons is not thermal but has a peak at a considerably lower energy. I n addition we measure the spectrum of the fast neutrons moderated in water to be approximately thermal. Experixnental procedure A 3 c Am/Be neutron source, which produces 8 × 10 6 fast neutrons per second of average energy
zoc/s
[ 6c N°
OF"
NEUTRONS
' , , j D I F ' F E R E NCE CURVE
. 0
!
of
2 y¢~oclrv
3
~N m o o M/S
4
,~
Fzo. I. Calculated velocity distribution of neutrom passing through the mechanical veloclty selector for various rotor speeds.
200
Technical notes
4 MeV (s), was surrounded by up to 30 cm of paraffin wax moderator. These slow neutrons were then analysed by a mechanical velocity selector before being detected by a N.E. 905 6Li loaded glass scintillator 1.75 in. in dia. and 0.125 in. thick. The mechanical velocity selector consisted essentially of two circular discs 30 cm in dla., each disc being composed of 4 mm of aluminium and 2 mm of cadmium. The cadmium of each disc had a 4 ° sector removed which acted as a window for slow neutrons. The two discs were mounted, 50 cm apart, on a central axle connected to a d.c. motor capable ofrotatlng the two discs at up to 60 cycles/see. The disc nearest to the source led the other disc in phase by 1°. The calculated distributions of neutron velocities allowed through the selector at various speeds of revolution are shown in Fig. I for 10 and 20 cycles/see. At each speed the selector allows all the neutrons with a velocity above a well defined minimum value to reach the detector. The number of neutrons having velocities in the region between two of these minimum velocities is obtained by subtracting the responses of the detector for the two corresponding rotor speeds, the velocity resolution of the selector being determined by the rotor speed difference
considered. In the present case differences of 10 cycles/see between the respective rotor speeds were used in order to keep the differences in detector response to a statistically meaningful level.
Results and discussion The measured neutron velocity spectra are shown in Fig. 2a for neutrons which were moderated respectively by 30 cm of paraffin wax and 30 cm of water. Here it is seen that the spectrum of water moderated neutrons has the expected thermal neutron peak at 2200 m/see whereas the paraffin wax moderated neutron spectrum has a maximum at a neutron velocity of only 1050 m/see. We have taken a transmission Laue X-ray photo-
graph shown in Fig. 2b which indicates that the paraffin wax has an unexpectedly well ordered structure, the largest atomic spacing of the carbon atoms being 3.98 A. However, because neutrons, unlike X-rays arc coherently scattered by both the hydrogen and the carbon atoms in the paraffin wax, the relevant atomic spacing for neutrons is 1.99 A. Our paraffin results may be explained, in the same way as the phenomenon of cold neutron production (4), by considering the wave properties of neutrons,
150
l '1OO' NEUTRON No. DIFFERENCEISEC.
A ANDB NORMALISED so, .ERE I
o
16oo
s~oo
2000
NEUTRON VELOCITY
M/S
•
Fio. 2a. Spectra of moderated neutrons. Curve (A) shows the spectrum of neutrons moderated by paraffin wax, Curve (B) shows the spectrum of neutrons moderated by water.
FIO. 2b. Transmission X-ray Laue photograph of paraffin wax.
200
Technical notes which when incident on a structure of atomic spacing d will be diffracted from their initial direction and the detector providing that n 2 = Y d s i n O . Hence for first order diffraction only wavelengths longer than 2d will reach the detector. Using the de Broglie relation we find that this corresponds to a neutron velocity of 1050 m/sec. Figure 2a shows that the numbers of neutrons reaching the detector with velocities greater than 1050 m/sec are greatly reduced in the case of paratFm which agrees with the diffraction theory.
t~onclusion The results of this work shows that paraffin wax cannot be considered to be without structure and that when used to moderate neutrons from an Am/Be source the spectrum of these neutrons is considerably modified from the expected thermal distribution. The neutrons moderated in water have as expected a near thermal spectrum.
Physics Department University of Aston in Birmingham
A . J . Cox S . A . NEw
References l. COX A. J., FRANCOIS P. E. and GATRELL R. P. Int. J. appl. Radiat. Isotopes 19, 541 (1968). 2. Du~INO J. R., PEOm~t G. B., Free G. A., MrrCH~LL D. P. and SE~R~ E. Phys. Rev. 48, 704 (1935). 3. THOMPSON M. N. and TAYLOR J. M. Nucl. Inst. Meth. 37, 306 (1965). 4. ANDERSON H. L., FEm~I E. and MARSHALL L. Phys. Rev. 70, 815 (1946).
InternationalJournalof AppliedRadiationand Isotopes,1969,VoL20, pp. 201-203. PergamonPress. Printedin NorthernIreland
Emission Spectra of Thermolnminescent Lithium Fluoride and Lithium Borate: Manganese IN THE study of thermoluminescent phosphors valuable information may be obtained by determining the emission spectrum of the light evolved on heating. A recent example is the work on the thermoluminescence process in rare-earth activated CaFy{1), at temperatures up to 300°K, where the emission spectra for different glow peaks were found to be line spectra characteristic of the rare-earth dopant. Some thermoluminescent phosphors have properties which render them particularly suitable as radiation dosemeters{ 2) and in these cases a knowledge of their emission spectra is of value not only as an aid
2 01
to the basic understanding of the thermoluminescence process, but also to enable the phosphor to be matched to the most suitable photomultiplier for greatest accuracy in the read-out of absorbed dose. This note reports the use of a system for the determination of the emission spectra of thermoluminescent phosphors above ambient temperature, based on a grating monochromator with photomultiplier detector and calibrated to correct for wavelength dependence of sensitivity. The system has been used in the first instance for the common radlothermoluminescent phosphors lithium fluoride (TLD-100)* and lithium borate: Mn. Similar work has recently been reported by GORBmS(a) who used a quartz-prism spectrograph with photographic detection and who determined the variation of response with wavelength by means of a standardised tungsten filament quartz-iodlne lamp. The same type of lamp has been used in this work in order to determine the wavelength response of our detecting assembly.
Equipment and response calibration procedure The apparatus consisted of a small furnace and monochromator (Bausch and Lomb, model 33-8645-01) with photomultiplier housing and shutter at the exit slit. These items were contained in a light-tight box and leads passed from the photomultiplier to an E H T supply and d.c. amplifier (A.E.R.E., Type 1388A). The output of the d.c. amplifier was fed on to the Y-axis of an X/Y,T plotter (Bryans, model 22020). The furnace consisted of an aluminium rod on which the heater coils were wound directly. The flat end of the rod had a groove which held closely a small silica sample tube 0-4 cm in dia., but left a large portion of its surface exposed so that light from the heated phosphor could emerge. The tube, into which a fine thermocouple could be inserted, was positioned close to the inlet slit of the monochromator. The wavelength drum was driven by a small synchronous motor. Calibration of the variation in response with wavelength was achieved by the use of a quartziodine tungsten filament lamp operated at a known colour temperature. The entrance slit of the monochromator was uniformly illuminated by reflection from an opal screen. The current necessary to operate the lamp at a colour temperature of 3000°K and the variation of the reflection coefficient with wavelength of the opal screenft were determined by NPL Metrology Centre. Also supplied was a wavelength dependent factor f2 to correct for the deviation of the lamp from a true black-body radiator. The spectral distribution of radiant energy * Harshaw Chemical Co.