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A source of error in the determination of uranium-235 by gamma-spectrometry
Notes
373
Dimethylchlorophosphine (IV) was prepared for the first time recently by cleavage of dimethylaminodimethylphosphine with hydrogen chloride. ~4~ Tetramethyldiphosphine disulphide (I/I), obtainable in 85 per cent yield from methylmagnesium bromide and thiophosphoryl chloride, ~6~ has now been converted to dimethylchlorophosphine in 46 per cent yield by simultaneous reduction and cleavage with phenyldichlorophosphine. (CHa)2P--P(CHs)2 + ~PC12 --* 2(CHa)2PC1 + (~PS) fl N Iv S S ]II
Experimental Methyldichlorophosphine. The complex (I) prepared by shaking 12 g of methyl chloride, 31 g of phosphorus trichloride and 29 g of aluminium chloride in a sealed glass tube ta~ was placed in a 200 ml flask connected to a 6 in. Vigreaux column. Addition of 21 ml of phosphorus oxychloride produced an exothermic reaction and most of the complex dissolved. Phenyldichlorophosphine (31 mi) was added and the mixture was heated to 130 °. At that temperature the mixture became homogeneous and 23 ml of a clear, colorless liquid distilled at 80-98 °. Redistillation in a nitrogen atmosphere gave 21 g (82 per cent) of methyldichiorophosphine, b.p. 80-84 °. The infra-red spectrum was identical to that of an authentic sample supplied by the U.S. Army Chemical Cows and contained a strong band for the methyl group at 7-75 p, as reported by CASON and BAXTER.tS~ The 31p magnetic resonance was a sha W peak at -- 191 p.p.m., relative to 85 per cent phosphoric acid (literature value tTj - 191-2 p.p.m.). Substitution of tributylphosphine for the phenyldichlorophosphine used in this experiment gave methyldichlorophosphine in only 61 per cent yield. Dimethylchlorophosphine. A mixture of 9.3 g of tetramethyldiphosphine disulphide and 20"3 ml of phenyldichlorophosphine was heated in a nitrogen atmosphere until the mixture became homogeneous at about 200 °. The clear, yellow solution was distilled to give 6.8 ml of a clear, colorless liquid b.p. 62-70 °. Redistillation gave 4"2 g (46 per cent) of clear, colorless dimethylchlorophosphine, b.p. 77 °, m.p. --4 to 0 ° (literature values: c~> b.p. 73 °, m.p. --1.4 to --1'0°). The infra-red spectrum contained bands assignable to saturated CH bonds at 3.35/~ and to a methyl group at 7.75/~. The proton magnetic resonance contained a symmetrical doublet in the methyl region. The 31p magnetic resonance was a single, sha W peak with a chemical shift of --93.0 p.p.m., relative to 85 per cent phosphoric acid. The mass spectrum contained major peaks at 81, 83, 96 and 98 units in the proper ratio for the presence of one chlorine atom in the molecule. Central Research Department Experimental Station E. I. du Pont de Nemours and Company Wilmington, Delaware
G . W . PARSHALL
~4~A. B. BURGand P. J. SLOTA,J. Amer. Chem. Soc. 80, 1107 (1958). tsJ H. REINHARDT,D. BIANCHIand D. MOLLE,Ber. 90, 1657 (1957). ¢6) j. CASONand W. N. BAXTER,J. Org. Chem. 23, 1303 (1958). iv} N. MULLER,P. C. LAUTERBURand I. GOLDENSON,J. Amer. Chem. Soc. 78, 3557 (1956).
A source of error in the determination of uranium-235 by gamma-spectrometry (Received 14 September, 1959; in revisedform 15 October 1959) URANIUM-235 emits y-rays at 184 keV which can easily be resolved from other y-rays emitted by uranium by using a scintillation spectrometer; data on radiation emitted by uranium and its daughters are given in the Table 1. A number of workers have measured the intensity of y-rays in an energy band at 184 keV to determine ~ssU concentrations in uranic samples.
374
Notes TABLE 1.--PRINCIPAL RADIATIONSFROMURANIUMAND ITS SHORT-LIVEDDAUGHTERS(1,2)
Half-life
Principal particle energies (MeV) and occurrence percentages
Principal y-energies (MeV) and 70 ~/disintegration
238U
4.5 × 102 years
4'182 (77~), 4.134 (2370)
ga4Th
24'1 days
0.100 (35~), 0"191 (65%)
2u"Pa (UX~)
1.175 months
0"58 (1%), 1"50 (9%) 2.31 (9070)
22sU
7.13 × 10 2 years 4.58 (10%), 4.40 (83%)
231Th
26.5 hr
0.134 (20%), 0.218 (33%)
aa4U
2.47 × 105 years
4"768 (72%), 4"71 (28%)
0"045 mostly internally converted 0.029, 0.0628 (eL/y = 0.45), 0.0914 (ez/7 = 2.0) 0"043, 0"230, 0"255, 0'770, 0"803, 1"01, 1"24, 1"44, 1"69, 1"83 ( < 1%) 0"110 (5%), 0"146 (12%), 0"184 (55 %), 0"209 ( < 4 % ) 0"022 (13%), 0"085 (770), 0"122 (0"1 70), 0"167 (0"1 70), 0.208 (0-02 %) 0"118 (<0"3%)
Isotope
~
Note: ely = ratio of conversion electrons to 7 quanta emitted. Such measurements at low 2a~U abundances have been made by MOR~SON and COSGROVE(a), who estimated the intensity of the 184 keV line from 285U from a spectrum, and by MILLER,'4~ M~YFR,~5~ REYNOLDSand ELDRrDGEte~ and JACOE,tT) who measured the total radiation at 184 keV with a singlechannel analyser. While some of these workers concluded that part of the radiation at 184 keV was attributable to 284Pa (a daughter of 28sU) and used techniques to overcome errors caused by it, it does not appear to have been appreciated that the background radiation is the result of bremsstrahlung emitted by ~-rays from ~84Pa. It has been calculated, for the case of a thick natural uranium metal source in which the daughters are in equilibrium, that the intensity of bremsstrahlung at 184 keV is of the same order as the intensity from the 7-rays produced by 235U. The work described here confirms this prediction and examines some of the factors which influence this background.
Experimental Determination of the Origin of the Background Gamma Radiation at 184 keV Apparatus A conventional scintillation gamma spectrometer was used for the measurements. This consisted of a NaI (T1) crystal, 1 in. thick and 1½ in. dia., mounted directly on an E.M.L type 6097F photomultiplier. Pulses from the multiplier were amplified by a standard linear amplifier and fed to a single-channel pulse analyser. Two scaling units coupled in series were used to count pulses from the analyser. Samples were rigidly mounted on top of the crystal. To remove the effect of the 2-3 MeV ~/-ray from ~s4Pa, a/~-filter of Perspex was inserted between the sample and the crystal. tx) D. STROMINGER,J. M. HOLLANDERand G. T. SEABORG,Rev. Mod. Phys. 30, 585 (1958). (2) V. S. DZELEPOVand L. K. PEKER,Atomic Energy of Canada Ltd., AECL-457 (1957). (8) G. H. MORmSONand J. F. COS6ROVE,Analyt. Chem. 29, 1770 (1957). (') D. G. MILLER,General Electric Co., HW-39969 (1955). ¢~J R. C. MEYER,NBL 127. U.S.A.E.C. Research and Development Progress Report for the Period July 1955 to December 1955 (1956). (6) S. A. R~OLDS and J. S. ELDmDGE, Oak Ridge National Laboratory, CF-57-1-3 (1957). IT) M. E. JAcons, Goodyear Atomic Corporation, GAT-254 (1958).
Notes
375
Radiation from uranium rods--variation with concentration A typical spectrum of the low-energy gamma radiation from a 1 in. thick uranium metal rod, in which the daughter activity is in equilibrium, is shown in Fig. 1. There is a complex peak at 90-100 keV, caused mainly by "4Th and uranium X-radiation. There is apparently a continuous background under the 235U gamma line at 184 keV. (The counter background has been subtracted in all curves
7000 6000 ~n c
5000 4000 5000 2000
/
I000
0
i
50
i
I
I00 150 Pulse height,
I
200 keY
i
250
i
300
FIG. 1.--Gamma spectrum of natural uranium metal ¼ in. thick. shown.) It was shown that the magnitude of this background was independent of the " s U concentration over the range 7.2 × 10-3 to 9.2 × 10-3 and amounts to 78 per cent of the radiation from natural uranium metal in the energy band 164-196 keV. Fig. 2 shows the spectra from uranyl nitrate crystals with a ~35U concentration of 4 × 10-~, and from solutions containing different amounts of this salt. The background increases with the atomic stopping power of the source. Fig. 3 shows the spectra from sources of different thickness of natural uranium metal; when the thickness of metal is less than the range of 2"3 MeV fl-radiation caused by the daughter of "sU, the background decreases. These experiments have shown that this background is the result of bremsstrahlung caused by E-radiation stopped in the source, and not to Compton background from high-energy y-rays from "4Pa nor to a high-energy "tail" caused by poor resolution of the complex line at 100 keV. The uranium salts and metal used in the experiments described were in equilibrium with the E-active daughter products 234Th and "4Pa. Spectra were also recorded from a freshly purified sample of U3Os, and it was found that the bremsstrahlung background increased as expected with the 24 day half-life of ~84Th, as shown in Fig. 4. Conclusions The radiation at 184 keV from a sample of uranium is a mixture of radiation from " s U and bremsstrahiung. This bremsstrahlung results mainly from E-rays from "4Pa (a daughter of "*U) being stopped in the sample. The amount of bremsstrahlung, relative to the " s U line, depends on the 2ssU concentration and the size, nature and chemical history of the sample. Thus, at near natural level, measurement of the intensity of y-radiation at 184 keV can only provide a good measure of 23~U content when the daughter activity is removed (e.g. MEYER (5)) o r when a correction for the background is applied (e.g. MOm~SON and CoscRow(8)). This correction is constant for samples of similar size and composition only when the daughters are in equilibrium; it is small for dilute solutions and for samples which are thin compared with the range of the E-rays.
376
Notes
7[
Intensities are normalized at
]
>, o
c
.30
35 40 45 Pulse height, orbitrory units
50
30
FIG. 2.--Gamma spectra of uranium compounds and solutions (0"43 ~).
35 40 45 Pulse height, arbitrary units
50
FIG. 3.--Variation of gamma spectrum of natural uranium metal with thickness of sample.
5.0 c
E 2.5 ~
2.O
~
t.5
1.0
t
][
J
I
I0
~
~
I
eur~
I
20 50 Time after I~eparation, days
40
FIG. 4.--Growth of 184 keV line in freshly prepared UaOa (channel width 164-196 keV).
Acknowledgements--This note is published by permission of Sir WILLIAMCOOK, Managing Director, and Dr. H. KRO~,mERGER,Director of Research and Development of the United Kingdom Atomic Energy Authority (D. & E. Group). Messrs. J. SI-mPn~RD, M. J. TODD and B. G. FERGOSONassisted with the experiments. Dr. A. N. HAMER prepared the sample of U3Os. Two pieces of uranium metal were prepared by Operations Branch, Springtields, of the U.K.A.E.A. D. K. CARTWlU~n'r U.K.A.E.A. (Development & Engineering Group) E . J . ROBmNS Research and Development Branch, Capenhurst, Chester
373
Dimethylchlorophosphine (IV) was prepared for the first time recently by cleavage of dimethylaminodimethylphosphine with hydrogen chloride. ~4~ Tetramethyldiphosphine disulphide (I/I), obtainable in 85 per cent yield from methylmagnesium bromide and thiophosphoryl chloride, ~6~ has now been converted to dimethylchlorophosphine in 46 per cent yield by simultaneous reduction and cleavage with phenyldichlorophosphine. (CHa)2P--P(CHs)2 + ~PC12 --* 2(CHa)2PC1 + (~PS) fl N Iv S S ]II
Experimental Methyldichlorophosphine. The complex (I) prepared by shaking 12 g of methyl chloride, 31 g of phosphorus trichloride and 29 g of aluminium chloride in a sealed glass tube ta~ was placed in a 200 ml flask connected to a 6 in. Vigreaux column. Addition of 21 ml of phosphorus oxychloride produced an exothermic reaction and most of the complex dissolved. Phenyldichlorophosphine (31 mi) was added and the mixture was heated to 130 °. At that temperature the mixture became homogeneous and 23 ml of a clear, colorless liquid distilled at 80-98 °. Redistillation in a nitrogen atmosphere gave 21 g (82 per cent) of methyldichiorophosphine, b.p. 80-84 °. The infra-red spectrum was identical to that of an authentic sample supplied by the U.S. Army Chemical Cows and contained a strong band for the methyl group at 7-75 p, as reported by CASON and BAXTER.tS~ The 31p magnetic resonance was a sha W peak at -- 191 p.p.m., relative to 85 per cent phosphoric acid (literature value tTj - 191-2 p.p.m.). Substitution of tributylphosphine for the phenyldichlorophosphine used in this experiment gave methyldichlorophosphine in only 61 per cent yield. Dimethylchlorophosphine. A mixture of 9.3 g of tetramethyldiphosphine disulphide and 20"3 ml of phenyldichlorophosphine was heated in a nitrogen atmosphere until the mixture became homogeneous at about 200 °. The clear, yellow solution was distilled to give 6.8 ml of a clear, colorless liquid b.p. 62-70 °. Redistillation gave 4"2 g (46 per cent) of clear, colorless dimethylchlorophosphine, b.p. 77 °, m.p. --4 to 0 ° (literature values: c~> b.p. 73 °, m.p. --1.4 to --1'0°). The infra-red spectrum contained bands assignable to saturated CH bonds at 3.35/~ and to a methyl group at 7.75/~. The proton magnetic resonance contained a symmetrical doublet in the methyl region. The 31p magnetic resonance was a single, sha W peak with a chemical shift of --93.0 p.p.m., relative to 85 per cent phosphoric acid. The mass spectrum contained major peaks at 81, 83, 96 and 98 units in the proper ratio for the presence of one chlorine atom in the molecule. Central Research Department Experimental Station E. I. du Pont de Nemours and Company Wilmington, Delaware
G . W . PARSHALL
~4~A. B. BURGand P. J. SLOTA,J. Amer. Chem. Soc. 80, 1107 (1958). tsJ H. REINHARDT,D. BIANCHIand D. MOLLE,Ber. 90, 1657 (1957). ¢6) j. CASONand W. N. BAXTER,J. Org. Chem. 23, 1303 (1958). iv} N. MULLER,P. C. LAUTERBURand I. GOLDENSON,J. Amer. Chem. Soc. 78, 3557 (1956).
A source of error in the determination of uranium-235 by gamma-spectrometry (Received 14 September, 1959; in revisedform 15 October 1959) URANIUM-235 emits y-rays at 184 keV which can easily be resolved from other y-rays emitted by uranium by using a scintillation spectrometer; data on radiation emitted by uranium and its daughters are given in the Table 1. A number of workers have measured the intensity of y-rays in an energy band at 184 keV to determine ~ssU concentrations in uranic samples.
374
Notes TABLE 1.--PRINCIPAL RADIATIONSFROMURANIUMAND ITS SHORT-LIVEDDAUGHTERS(1,2)
Half-life
Principal particle energies (MeV) and occurrence percentages
Principal y-energies (MeV) and 70 ~/disintegration
238U
4.5 × 102 years
4'182 (77~), 4.134 (2370)
ga4Th
24'1 days
0.100 (35~), 0"191 (65%)
2u"Pa (UX~)
1.175 months
0"58 (1%), 1"50 (9%) 2.31 (9070)
22sU
7.13 × 10 2 years 4.58 (10%), 4.40 (83%)
231Th
26.5 hr
0.134 (20%), 0.218 (33%)
aa4U
2.47 × 105 years
4"768 (72%), 4"71 (28%)
0"045 mostly internally converted 0.029, 0.0628 (eL/y = 0.45), 0.0914 (ez/7 = 2.0) 0"043, 0"230, 0"255, 0'770, 0"803, 1"01, 1"24, 1"44, 1"69, 1"83 ( < 1%) 0"110 (5%), 0"146 (12%), 0"184 (55 %), 0"209 ( < 4 % ) 0"022 (13%), 0"085 (770), 0"122 (0"1 70), 0"167 (0"1 70), 0.208 (0-02 %) 0"118 (<0"3%)
Isotope
~
Note: ely = ratio of conversion electrons to 7 quanta emitted. Such measurements at low 2a~U abundances have been made by MOR~SON and COSGROVE(a), who estimated the intensity of the 184 keV line from 285U from a spectrum, and by MILLER,'4~ M~YFR,~5~ REYNOLDSand ELDRrDGEte~ and JACOE,tT) who measured the total radiation at 184 keV with a singlechannel analyser. While some of these workers concluded that part of the radiation at 184 keV was attributable to 284Pa (a daughter of 28sU) and used techniques to overcome errors caused by it, it does not appear to have been appreciated that the background radiation is the result of bremsstrahlung emitted by ~-rays from ~84Pa. It has been calculated, for the case of a thick natural uranium metal source in which the daughters are in equilibrium, that the intensity of bremsstrahlung at 184 keV is of the same order as the intensity from the 7-rays produced by 235U. The work described here confirms this prediction and examines some of the factors which influence this background.
Experimental Determination of the Origin of the Background Gamma Radiation at 184 keV Apparatus A conventional scintillation gamma spectrometer was used for the measurements. This consisted of a NaI (T1) crystal, 1 in. thick and 1½ in. dia., mounted directly on an E.M.L type 6097F photomultiplier. Pulses from the multiplier were amplified by a standard linear amplifier and fed to a single-channel pulse analyser. Two scaling units coupled in series were used to count pulses from the analyser. Samples were rigidly mounted on top of the crystal. To remove the effect of the 2-3 MeV ~/-ray from ~s4Pa, a/~-filter of Perspex was inserted between the sample and the crystal. tx) D. STROMINGER,J. M. HOLLANDERand G. T. SEABORG,Rev. Mod. Phys. 30, 585 (1958). (2) V. S. DZELEPOVand L. K. PEKER,Atomic Energy of Canada Ltd., AECL-457 (1957). (8) G. H. MORmSONand J. F. COS6ROVE,Analyt. Chem. 29, 1770 (1957). (') D. G. MILLER,General Electric Co., HW-39969 (1955). ¢~J R. C. MEYER,NBL 127. U.S.A.E.C. Research and Development Progress Report for the Period July 1955 to December 1955 (1956). (6) S. A. R~OLDS and J. S. ELDmDGE, Oak Ridge National Laboratory, CF-57-1-3 (1957). IT) M. E. JAcons, Goodyear Atomic Corporation, GAT-254 (1958).
Notes
375
Radiation from uranium rods--variation with concentration A typical spectrum of the low-energy gamma radiation from a 1 in. thick uranium metal rod, in which the daughter activity is in equilibrium, is shown in Fig. 1. There is a complex peak at 90-100 keV, caused mainly by "4Th and uranium X-radiation. There is apparently a continuous background under the 235U gamma line at 184 keV. (The counter background has been subtracted in all curves
7000 6000 ~n c
5000 4000 5000 2000
/
I000
0
i
50
i
I
I00 150 Pulse height,
I
200 keY
i
250
i
300
FIG. 1.--Gamma spectrum of natural uranium metal ¼ in. thick. shown.) It was shown that the magnitude of this background was independent of the " s U concentration over the range 7.2 × 10-3 to 9.2 × 10-3 and amounts to 78 per cent of the radiation from natural uranium metal in the energy band 164-196 keV. Fig. 2 shows the spectra from uranyl nitrate crystals with a ~35U concentration of 4 × 10-~, and from solutions containing different amounts of this salt. The background increases with the atomic stopping power of the source. Fig. 3 shows the spectra from sources of different thickness of natural uranium metal; when the thickness of metal is less than the range of 2"3 MeV fl-radiation caused by the daughter of "sU, the background decreases. These experiments have shown that this background is the result of bremsstrahlung caused by E-radiation stopped in the source, and not to Compton background from high-energy y-rays from "4Pa nor to a high-energy "tail" caused by poor resolution of the complex line at 100 keV. The uranium salts and metal used in the experiments described were in equilibrium with the E-active daughter products 234Th and "4Pa. Spectra were also recorded from a freshly purified sample of U3Os, and it was found that the bremsstrahlung background increased as expected with the 24 day half-life of ~84Th, as shown in Fig. 4. Conclusions The radiation at 184 keV from a sample of uranium is a mixture of radiation from " s U and bremsstrahiung. This bremsstrahlung results mainly from E-rays from "4Pa (a daughter of "*U) being stopped in the sample. The amount of bremsstrahlung, relative to the " s U line, depends on the 2ssU concentration and the size, nature and chemical history of the sample. Thus, at near natural level, measurement of the intensity of y-radiation at 184 keV can only provide a good measure of 23~U content when the daughter activity is removed (e.g. MEYER (5)) o r when a correction for the background is applied (e.g. MOm~SON and CoscRow(8)). This correction is constant for samples of similar size and composition only when the daughters are in equilibrium; it is small for dilute solutions and for samples which are thin compared with the range of the E-rays.
376
Notes
7[
Intensities are normalized at
]
>, o
c
.30
35 40 45 Pulse height, orbitrory units
50
30
FIG. 2.--Gamma spectra of uranium compounds and solutions (0"43 ~).
35 40 45 Pulse height, arbitrary units
50
FIG. 3.--Variation of gamma spectrum of natural uranium metal with thickness of sample.
5.0 c
E 2.5 ~
2.O
~
t.5
1.0
t
][
J
I
I0
~
~
I
eur~
I
20 50 Time after I~eparation, days
40
FIG. 4.--Growth of 184 keV line in freshly prepared UaOa (channel width 164-196 keV).
Acknowledgements--This note is published by permission of Sir WILLIAMCOOK, Managing Director, and Dr. H. KRO~,mERGER,Director of Research and Development of the United Kingdom Atomic Energy Authority (D. & E. Group). Messrs. J. SI-mPn~RD, M. J. TODD and B. G. FERGOSONassisted with the experiments. Dr. A. N. HAMER prepared the sample of U3Os. Two pieces of uranium metal were prepared by Operations Branch, Springtields, of the U.K.A.E.A. D. K. CARTWlU~n'r U.K.A.E.A. (Development & Engineering Group) E . J . ROBmNS Research and Development Branch, Capenhurst, Chester