- Email: [email protected]
Development of a new scintillator containing uranium
NUCLEAR
INSTRUMENTS
AND
METHODS
143 ( 1 9 7 7 )
61-62;
©
NORTH-HOLLAND
PUBLISHING
CO.
DEVELOPMENT OF A NEW SCINTILLATOR CONTAINING URANIUM*
E D W A R D C A T A L A N O and JOHN B. CZIRR
Lawrence Livermore Laboratory, University ol Cal~lornia, Livermore, Cal(/brnia 94550, U.S.A. Received 17 January 1977 We built and measured the performance of a prototype fission detector. This detector, based on an inorganic solid-solution single-crystal scintillator containing uranium, provides several advantages over parallel-plate fission chambers: an orderof-magnitude improvement in uranium-to-substrate ratio, no fission-fragment losses, and ease of construction.
Fission-chamber efficiency is severely limited by the short range of fission fragments in the thin metallic or oxide films commonly employed. Also, fragments emitted near the foil plane can cause energy-dependent losses of several percent. The prototype detector we built overcomes the above problems by incorporating uranium in a homogeneous inorganic scintillator matrix. Our goals were to develop a system that a) would provide a substitute for common (parallel-plate) fission chambers, b) have an effective uranium fraction superior to typical fission-fragment detectors, c) have nanosecond timing capabilities, and d) be relatively inexpensive. To achieve these goals, we settled on solid-solution single crystals that use Ce(III) as the fluorescing species. Ce(III) was chosen to satisfy the nanosecond optical emission lifetime criterion. [Oxide crystals and glasses were not considered because of the necessity to elimintate Ce(IV), whose intense optical absorption overlaps Ce(III) emission.] Other criteria were that the uranium species, the Ce(III) species, and all other components have a) mutual solid solubilities, b) non-interfering absorption and emission spectra, c) simple neutron reactions and absorption cross sections, d) stability in ambient room condition (nonhygroscopic), and e) the capability of being easily grown as single crystals. These criteria led to a study of the crystal growth and phase stability of MF2:UF4:CeF3
(where M = Ca, Sr, and Ba) solid solutions in the UF 4- rich, CeF3-poor portion of the ternary phase diagrams. Samples taken from the latter regions of the phase field were unsuccessful as neutron detectors, mainly because the U(IV) quenched energy transfer between the MF2 (major component) and the CeF 3 (very low concentrations). (CaF2:CeF3 with 0.01<[CeF3]_
L 300
I0
20
1
ALPHAS
30
40
50
FISSION FRAGMENTS ° o
d
200 == o
o
o
I00
0
0
1O0
200
300
400
500
CHANNEL NUMBER * Work performed under the auspices of the U.S. Energy Research and Development Administration under contract no. W-7405-Eng-48.
Fig. 1. Measured pulse-height distribution for 0.6 m m thick scintillator exposed to 0.25-1 MeV neutrons. The ordinate of the left-hand (alpha) curve should be multiplied by 500.
62
E
CATALANO
beam from the Livermore 100 MeV Linac. Fig. 1 shows the scintillator response to neutrons in the 0.25-1.0 MeV range. The observed characteristics of this prototype are a) scintillator decay time about 20 ns, b) pulse-height ratio of fission fragments-to4.75-MeV alpha particles equal to 4.0, c) alpha-particle pulse height well resolved from phototube noise, d) fission-fragment pulse-height resolution equal to 40% (full width at half maximum), e) fission-fragment pulse height moderately well resolved from alpha particles, t) a nonhygroscopic, rugged, inorganic crystal, and g) an observed transmission for Ce(III) optical emission greater than 60% per mm of crystal thickness. The 1% 234U contamination resulted in a 30-fold increase in alpha decay rate compared to pure 235U.
AND J. B. C Z I R R
Detectors that use a l o w - 2 3 4 U content will reduce the alpha pileup problem and improve the fissionfragment-to-alpha-particle pulse-height separation. We believe that a reasonable upper limit on the U F 4 content of future detectors is 6% (tool percent), a three-fold improvement over the present prototype. Increasing the UFa content will probably require a reduction in CeF 3 content by about the same amount. These changes will probably result in a modest pulse-height reduction. The present prototype with 1.8 mol% U F 4 is, in terms of uranium-to-substrate ratio, an order-ofmagnitude improvement over typical modern fission chambers.
Reference 1) E. Catalano and E. W. Wrenn, J. Crystal Growth 30 (1975) 54.
INSTRUMENTS
AND
METHODS
143 ( 1 9 7 7 )
61-62;
©
NORTH-HOLLAND
PUBLISHING
CO.
DEVELOPMENT OF A NEW SCINTILLATOR CONTAINING URANIUM*
E D W A R D C A T A L A N O and JOHN B. CZIRR
Lawrence Livermore Laboratory, University ol Cal~lornia, Livermore, Cal(/brnia 94550, U.S.A. Received 17 January 1977 We built and measured the performance of a prototype fission detector. This detector, based on an inorganic solid-solution single-crystal scintillator containing uranium, provides several advantages over parallel-plate fission chambers: an orderof-magnitude improvement in uranium-to-substrate ratio, no fission-fragment losses, and ease of construction.
Fission-chamber efficiency is severely limited by the short range of fission fragments in the thin metallic or oxide films commonly employed. Also, fragments emitted near the foil plane can cause energy-dependent losses of several percent. The prototype detector we built overcomes the above problems by incorporating uranium in a homogeneous inorganic scintillator matrix. Our goals were to develop a system that a) would provide a substitute for common (parallel-plate) fission chambers, b) have an effective uranium fraction superior to typical fission-fragment detectors, c) have nanosecond timing capabilities, and d) be relatively inexpensive. To achieve these goals, we settled on solid-solution single crystals that use Ce(III) as the fluorescing species. Ce(III) was chosen to satisfy the nanosecond optical emission lifetime criterion. [Oxide crystals and glasses were not considered because of the necessity to elimintate Ce(IV), whose intense optical absorption overlaps Ce(III) emission.] Other criteria were that the uranium species, the Ce(III) species, and all other components have a) mutual solid solubilities, b) non-interfering absorption and emission spectra, c) simple neutron reactions and absorption cross sections, d) stability in ambient room condition (nonhygroscopic), and e) the capability of being easily grown as single crystals. These criteria led to a study of the crystal growth and phase stability of MF2:UF4:CeF3
(where M = Ca, Sr, and Ba) solid solutions in the UF 4- rich, CeF3-poor portion of the ternary phase diagrams. Samples taken from the latter regions of the phase field were unsuccessful as neutron detectors, mainly because the U(IV) quenched energy transfer between the MF2 (major component) and the CeF 3 (very low concentrations). (CaF2:CeF3 with 0.01<[CeF3]_
L 300
I0
20
1
ALPHAS
30
40
50
FISSION FRAGMENTS ° o
d
200 == o
o
o
I00
0
0
1O0
200
300
400
500
CHANNEL NUMBER * Work performed under the auspices of the U.S. Energy Research and Development Administration under contract no. W-7405-Eng-48.
Fig. 1. Measured pulse-height distribution for 0.6 m m thick scintillator exposed to 0.25-1 MeV neutrons. The ordinate of the left-hand (alpha) curve should be multiplied by 500.
62
E
CATALANO
beam from the Livermore 100 MeV Linac. Fig. 1 shows the scintillator response to neutrons in the 0.25-1.0 MeV range. The observed characteristics of this prototype are a) scintillator decay time about 20 ns, b) pulse-height ratio of fission fragments-to4.75-MeV alpha particles equal to 4.0, c) alpha-particle pulse height well resolved from phototube noise, d) fission-fragment pulse-height resolution equal to 40% (full width at half maximum), e) fission-fragment pulse height moderately well resolved from alpha particles, t) a nonhygroscopic, rugged, inorganic crystal, and g) an observed transmission for Ce(III) optical emission greater than 60% per mm of crystal thickness. The 1% 234U contamination resulted in a 30-fold increase in alpha decay rate compared to pure 235U.
AND J. B. C Z I R R
Detectors that use a l o w - 2 3 4 U content will reduce the alpha pileup problem and improve the fissionfragment-to-alpha-particle pulse-height separation. We believe that a reasonable upper limit on the U F 4 content of future detectors is 6% (tool percent), a three-fold improvement over the present prototype. Increasing the UFa content will probably require a reduction in CeF 3 content by about the same amount. These changes will probably result in a modest pulse-height reduction. The present prototype with 1.8 mol% U F 4 is, in terms of uranium-to-substrate ratio, an order-ofmagnitude improvement over typical modern fission chambers.
Reference 1) E. Catalano and E. W. Wrenn, J. Crystal Growth 30 (1975) 54.