- Email: [email protected]
X-ray diffraction studies of fission fragment damage in uranium carbide and nitride
Letters to the editors
51
TABLE2
Series Wt. of U,O,
(mg)
1
2
3
35.12
53.77
70.19
Wt. of main solution (g)
64029
58334
66678
Wt. of portion taken for further dilution (g)
1.1481
0.4394
0.8852
Wt. of dilute solution (g)
Wt. of dilute solution taken for source (column A) and alpha activity in counts/min (column B)
Specific count rate of UIOs (counts/min/mg)
51.279
25.399
28.114
A
B
A
B
o-3154 0.3099 O-2470
818.2 795.8 640.9
0.4382 0.3734 0.3204 0.08270 0.05306
1,461 1,251 1,075 277.3 177.6
21,125 20,911 21,130
20,908 21,010 21,040 21,027 20,990
20,976 21,023 23,026 20,927 20,940
Mean specific count rate of UIOt, = 21,ooO f 70counts/mm/ mg. Percentage *YJ by weight in UIOll = 70.48 rt 0.07. Percentage of total alpha-particles due to xs*U = 99.86 f 0.01. Geometry factor of counter = 708.7 f 3.8. Therefore, specific activity of rsrU = (2.109 f 0.012) X 10’ dis/min/mg. Half-life of rrsU = (I.615 f 0.009) x 106 years, (This value agrees within experimental error with those. of HYDE (1957) and of DOKUCHAEV and Os~pov (1960)) in the source. Sufficient counts were recorded so that the random error was always less than 0.3 per cent and in many cases less than 0.1 per cent. The errors involved in weighing out lJ,Oa and the various solutions were negligible compared with other operations. Consequently in determining the mean specific count rate of UsOa each of the 13 sources has been given equal weight, and the standard deviation of the mean calculated in the usual way. The uncertainty in the final value for YJ half-life is almost entirely due to that in the geometry factor of the low geometry counter. Acknowle~~e/,lenrs-The author’s thanks are due to MR. D. FARRINGTON for performing the mass-spectrometric analysis, to MR. M. MORGAN for counting the alpha-sources, and to DR. R. G. MONK for his interest in the work. D. S. POPPLEWELL
Atomic Weapons Research Establishment Aldermaston, Berks. REFERENCES DOKUCHAEV IA. P. and OSIPOV I. S. (1960) J. Nrrcl. Etteygy 11, 194. DUVAL C. (1953) Inoyganic Therttlo~ravitnetric Analysis p. 502 et seq., Elsevier, Amsterdam. HURST R.. HALL G. R. and GLOVER K. M. (1951) A.E.R.E. Report C/R 647. HYDE E. K. (1957) U.S.A.E.C. Report TID-5223. HYIX E. K., HA(;EMANN F. and KATZIN L. I. (1945) U.S.A.E.C. Report CC-2636.
X-ray diffraction studies of fission fragment damage in uranium carbide and nitride (Receioed 24 October 1960) A RELEASEof approximately 190 MeV of energy is associated with every fission event and about 85 per cent of this energy is carried by a pair of heavy, highly-ionized atoms which move through matter and lose their energy by collisions with electrons and atoms. When the fission fragments pass through a solid, a large number of atoms are knocked out from their positions in the crystal. The configuration produced by the passage of the fast particle will not be frozen in but some rearrangement will take place while the disturbed region of the crystal remains agitated, furthermore those defects which are mobile at the irradiation temperature will anneal out. The residual damage in the crystal will consist of interstitial atoms and vacancies either single or grouped and in materials containing more than one type of atom disordering or replacement of atoms is possible. TEWORDT (1958) showed theoretically that in copper, single interstitial atoms have a marked effect upon the unit cell size and it is probably reasonable to assume that in general ;Lnexpansion of the unit cell of an irradiated crystal will reflect the concentration of single and possibly small groups of interstitial atoms and the disordering of the structure if this is possible. If a given crystal can contain only a certain concentration of defects of a given type at least until it becomes heavily damaged. the number of atoms alTected by a single damage producing cvcnt can be deduced from the measurements of unit cell expansion.
52
Letters to the editors
FIG. I.-Fractional
incrkase in unit cell size for irradiated
uranium carbide as a function of dose. UN Q
I TmEamuz~tmEn(~crr) FIG. 2.-Fractional
I 4
1 4
increase in unit cell size for irradiated
I 4
4
mm+
uranium nitride as a function of dose.
TABLE 1.---ESTIMATES OF THE NUMBER OF ATOMS AFFECTED BY A FLSStONEVENT
Number of atoms affected
Material
Reference
-
u
metal
U-MO alloy U-MO alloy (zrU)o, UaOs UC UN
Electrical resistance Electrical resistance Diffusion Phase transformation Stored energy Cell expansion Cell expansion
Measurements of changes in unit cell size were conducted on UC and UN powders made from natural uranium and irradiated for various periods of time in BEPO reactor in a flux of approximately IOr* neutrons cm-* se& at pile ambient temperature .(about 60°C). Specimens for X-ray examination were prepared before irradiation by pressing UC and UN powders into annealed high-purity aluminium disks. These disks mm then irradiated together with cobalt neutron dose monitors in evacuated and cold-welded aluminium cans. Sina X-ray detectors are sensitive to radiations emitted by radioactive materials standard X-ray diffraction techniques cannot be used for the examination of irradiated samples. A diffractometer used in this work constructed at A.E.R.E., Hanvell (ADAM, 1958) employs the principle of monochromatisation of the diffracted beam (CUMMINGS. KAULTiZ and SANDERSON, 1955). This experimental arrangement with a
QUERE and N~KACHE
(1959)
-3
x IO’
KONOBEEVSKtIf?f (II.
(1959)
-1
YI IO‘
KONOBEEVSKIIet al.
( 1959)
Wrrr~ts
(1959)
- 10’
N
IO‘
and SHERtLL
N 3.4 x IO’
CHILDS and MCGURN
N 2.8 x IO’ w 35 x IO’
Present work Present work
(1959)
proportional counter as a detector and a single channel analyset reduces the background due to the sample activity and Ruorescenoz to negligible proportions. Satisfactory reproducibility in unit all size determinations was obtained from the measurements of positions of diffraction lint peaks, using tungsten L-a radiation. The separation of L-a doublets is reasonably large and the weak ax-component can be easily eliminated by careful adjustment of slits placed in front of the counter. Figures I and 2 show fractional changes in unit all sixe in neutron irradiated UC and UN as functions of neutron dose. Within the accuracy of experiment the changes follow [I - exp (-m)] law where n is the neutron dose and y is constant for a given material. The damage in neutron irradiated materials containing tissik atoms is mainly caused by fission fragments. Assuming that the rate at which the fraction of affected atoms Y in the crystal
Letters to the editors increases with dose n is proportional to the fraction of atoms in the crystal which have not been affected, we have
g and therefore
= y(1 -
Y)
53
QUERE Y. and NAKACHEF. (1959) J. Nucl. Mat. 1,203. TEWORDTL. (1958) Phys. Rev. 109,61. WITTEL~M. C. and SHERRILLF. A. (1959) Phys. Rev. (Letters) 3, 176.
y = l - e-yn*
If the fractional change in the unit cell size is proportional to the fraction of atoms affected by the passage of fission fragments, we have AU - = A(1 - e-y”). a The number of fission events per atom of material irradiated to a neutron dose n is equal to u,n where a, is the fission crosssection. We can write thus yn = kqn where k = y/a, is the number of atoms affected by a single fission event. Experimental curves obtained for UC and UN show saturation effects at high neutron doses and values for k obtained from these curves are 2.8 x lo6 atoms per fission event for UC and 3.5 x lo6 atoms per fission event for UN. The fraction of uranium atoms which were destroyed by fission at the highest neutron doses in this investigation was approximately 4 x 1O-6, this corresponds to a burn-up of the order of 3.5 MWD/Te. Table 1 summarizes the available estimates of the number of atoms affected by a fission event. Although none of these estimates can be regarded as highly accurate the differences between certain materials are large and undoubtedly significant. Further studies are required to relate the numbers of atoms affected by a fission event to the properties of materials and their irradiation behaviour. J. ADAM M. D. ROGERS Atomic Energy Research Establishment Harwell. Didcot, Berks. REFERENCES ADAM J. (1958) A.E.R.E. M and C/R 2751. CHILDS B. G. and MCGURN J. (1959) Canadian Report, CRMet-868. CUMMINGSW. V. et al. (1955) Rev. Sci. Instrum. 26, 5. KONOBEEVSK~ S. T. et a/. (1959) J. Nucl. Energy 9,75.
Measurements of some resonance activation integrals (Received 3 1 Januury 196 I ) THE resonance
integrals of sodium, manganese, cobalt, BsC~, ‘%u and of 08M~ have been determined from cadmium ratio measurements in a neutrqn beam. The method was the same as the one used earlier by JIRLOW and JOHAN~~~N(1960), so only a brief outline is given here. The cadmium ratio of the actual substance and of a I/v absorber (a thin BF, counter) were measured in a neutron beam from the reactor Rl. The neutron spectrum in the beam was accurately known from measurements with a fast chopper, as described by JOHAN~~~Net al. (1960). Corrections could thus be made for the deviation of the flux from l/E dependence. Also the effective cut-off energy to be used for the 1.1 mm thick cadmium could be measured directly. The results of the measurements are given in Table 1. The lower limit of the resonance integral has been fixed at 0.5 eV and the I/v part is not included. The factor k shows the corrections which have been made in the resonance integrals for scattering and self-absorption of neutrons in the foils. Multiple scattering as well as energy changes in the collisions have been taken into account. The errors given in k arise only from the calculation methods. The uncertainties caused by incomplete knowledge of resonance parameters and by effect of Doppler broadening have been included in the final values for the resonance integrals. The 00253 eV cross-sections given in Table I with the exception of that for OBMohave been taken from HUGHESand SCHWARTZ(1958) and from HUGHESet al. (1960). The thermal cross-section of BBM~given there appeared too high so it was determined in a separate measurement. Gold and molybdenum foils were irradiated in the thermal column of the reactor RI and the photo peaks at 0.411 and 0.740 MeV were compared using a scintillation counter. The cross-section obtained is only about one third of that given in the cited references. Probably
TABLE1.-RESULTS FROMTHE MEASUREMENTS u (0.0253) (barns)
Foil thickness (cm x lo-‘)
k
Na
0.520 f 0.010
30
0983 f 0.015
0.07 * 0.01
Mn
13.3 f 0.2
1.53
0.974 f 0.022
8.15 i 0.60
co
38.0 f 0.7
2.27
0.950 f 0.033
55.2 f 4.5
Yu
4.50 * 0.15
20
1.04 f- 0.01
3.09 f 0.15
*scu
2.0 f 0.3
100
1.015 f 0.010
1.38 f 0.23
“MO
O-18 f 0.02
100
1.13 & 0.01
10.7 f 2.5
Substance
Resonance integral (barns)
51
TABLE2
Series Wt. of U,O,
(mg)
1
2
3
35.12
53.77
70.19
Wt. of main solution (g)
64029
58334
66678
Wt. of portion taken for further dilution (g)
1.1481
0.4394
0.8852
Wt. of dilute solution (g)
Wt. of dilute solution taken for source (column A) and alpha activity in counts/min (column B)
Specific count rate of UIOs (counts/min/mg)
51.279
25.399
28.114
A
B
A
B
o-3154 0.3099 O-2470
818.2 795.8 640.9
0.4382 0.3734 0.3204 0.08270 0.05306
1,461 1,251 1,075 277.3 177.6
21,125 20,911 21,130
20,908 21,010 21,040 21,027 20,990
20,976 21,023 23,026 20,927 20,940
Mean specific count rate of UIOt, = 21,ooO f 70counts/mm/ mg. Percentage *YJ by weight in UIOll = 70.48 rt 0.07. Percentage of total alpha-particles due to xs*U = 99.86 f 0.01. Geometry factor of counter = 708.7 f 3.8. Therefore, specific activity of rsrU = (2.109 f 0.012) X 10’ dis/min/mg. Half-life of rrsU = (I.615 f 0.009) x 106 years, (This value agrees within experimental error with those. of HYDE (1957) and of DOKUCHAEV and Os~pov (1960)) in the source. Sufficient counts were recorded so that the random error was always less than 0.3 per cent and in many cases less than 0.1 per cent. The errors involved in weighing out lJ,Oa and the various solutions were negligible compared with other operations. Consequently in determining the mean specific count rate of UsOa each of the 13 sources has been given equal weight, and the standard deviation of the mean calculated in the usual way. The uncertainty in the final value for YJ half-life is almost entirely due to that in the geometry factor of the low geometry counter. Acknowle~~e/,lenrs-The author’s thanks are due to MR. D. FARRINGTON for performing the mass-spectrometric analysis, to MR. M. MORGAN for counting the alpha-sources, and to DR. R. G. MONK for his interest in the work. D. S. POPPLEWELL
Atomic Weapons Research Establishment Aldermaston, Berks. REFERENCES DOKUCHAEV IA. P. and OSIPOV I. S. (1960) J. Nrrcl. Etteygy 11, 194. DUVAL C. (1953) Inoyganic Therttlo~ravitnetric Analysis p. 502 et seq., Elsevier, Amsterdam. HURST R.. HALL G. R. and GLOVER K. M. (1951) A.E.R.E. Report C/R 647. HYDE E. K. (1957) U.S.A.E.C. Report TID-5223. HYIX E. K., HA(;EMANN F. and KATZIN L. I. (1945) U.S.A.E.C. Report CC-2636.
X-ray diffraction studies of fission fragment damage in uranium carbide and nitride (Receioed 24 October 1960) A RELEASEof approximately 190 MeV of energy is associated with every fission event and about 85 per cent of this energy is carried by a pair of heavy, highly-ionized atoms which move through matter and lose their energy by collisions with electrons and atoms. When the fission fragments pass through a solid, a large number of atoms are knocked out from their positions in the crystal. The configuration produced by the passage of the fast particle will not be frozen in but some rearrangement will take place while the disturbed region of the crystal remains agitated, furthermore those defects which are mobile at the irradiation temperature will anneal out. The residual damage in the crystal will consist of interstitial atoms and vacancies either single or grouped and in materials containing more than one type of atom disordering or replacement of atoms is possible. TEWORDT (1958) showed theoretically that in copper, single interstitial atoms have a marked effect upon the unit cell size and it is probably reasonable to assume that in general ;Lnexpansion of the unit cell of an irradiated crystal will reflect the concentration of single and possibly small groups of interstitial atoms and the disordering of the structure if this is possible. If a given crystal can contain only a certain concentration of defects of a given type at least until it becomes heavily damaged. the number of atoms alTected by a single damage producing cvcnt can be deduced from the measurements of unit cell expansion.
52
Letters to the editors
FIG. I.-Fractional
incrkase in unit cell size for irradiated
uranium carbide as a function of dose. UN Q
I TmEamuz~tmEn(~crr) FIG. 2.-Fractional
I 4
1 4
increase in unit cell size for irradiated
I 4
4
mm+
uranium nitride as a function of dose.
TABLE 1.---ESTIMATES OF THE NUMBER OF ATOMS AFFECTED BY A FLSStONEVENT
Number of atoms affected
Material
Reference
-
u
metal
U-MO alloy U-MO alloy (zrU)o, UaOs UC UN
Electrical resistance Electrical resistance Diffusion Phase transformation Stored energy Cell expansion Cell expansion
Measurements of changes in unit cell size were conducted on UC and UN powders made from natural uranium and irradiated for various periods of time in BEPO reactor in a flux of approximately IOr* neutrons cm-* se& at pile ambient temperature .(about 60°C). Specimens for X-ray examination were prepared before irradiation by pressing UC and UN powders into annealed high-purity aluminium disks. These disks mm then irradiated together with cobalt neutron dose monitors in evacuated and cold-welded aluminium cans. Sina X-ray detectors are sensitive to radiations emitted by radioactive materials standard X-ray diffraction techniques cannot be used for the examination of irradiated samples. A diffractometer used in this work constructed at A.E.R.E., Hanvell (ADAM, 1958) employs the principle of monochromatisation of the diffracted beam (CUMMINGS. KAULTiZ and SANDERSON, 1955). This experimental arrangement with a
QUERE and N~KACHE
(1959)
-3
x IO’
KONOBEEVSKtIf?f (II.
(1959)
-1
YI IO‘
KONOBEEVSKIIet al.
( 1959)
Wrrr~ts
(1959)
- 10’
N
IO‘
and SHERtLL
N 3.4 x IO’
CHILDS and MCGURN
N 2.8 x IO’ w 35 x IO’
Present work Present work
(1959)
proportional counter as a detector and a single channel analyset reduces the background due to the sample activity and Ruorescenoz to negligible proportions. Satisfactory reproducibility in unit all size determinations was obtained from the measurements of positions of diffraction lint peaks, using tungsten L-a radiation. The separation of L-a doublets is reasonably large and the weak ax-component can be easily eliminated by careful adjustment of slits placed in front of the counter. Figures I and 2 show fractional changes in unit all sixe in neutron irradiated UC and UN as functions of neutron dose. Within the accuracy of experiment the changes follow [I - exp (-m)] law where n is the neutron dose and y is constant for a given material. The damage in neutron irradiated materials containing tissik atoms is mainly caused by fission fragments. Assuming that the rate at which the fraction of affected atoms Y in the crystal
Letters to the editors increases with dose n is proportional to the fraction of atoms in the crystal which have not been affected, we have
g and therefore
= y(1 -
Y)
53
QUERE Y. and NAKACHEF. (1959) J. Nucl. Mat. 1,203. TEWORDTL. (1958) Phys. Rev. 109,61. WITTEL~M. C. and SHERRILLF. A. (1959) Phys. Rev. (Letters) 3, 176.
y = l - e-yn*
If the fractional change in the unit cell size is proportional to the fraction of atoms affected by the passage of fission fragments, we have AU - = A(1 - e-y”). a The number of fission events per atom of material irradiated to a neutron dose n is equal to u,n where a, is the fission crosssection. We can write thus yn = kqn where k = y/a, is the number of atoms affected by a single fission event. Experimental curves obtained for UC and UN show saturation effects at high neutron doses and values for k obtained from these curves are 2.8 x lo6 atoms per fission event for UC and 3.5 x lo6 atoms per fission event for UN. The fraction of uranium atoms which were destroyed by fission at the highest neutron doses in this investigation was approximately 4 x 1O-6, this corresponds to a burn-up of the order of 3.5 MWD/Te. Table 1 summarizes the available estimates of the number of atoms affected by a fission event. Although none of these estimates can be regarded as highly accurate the differences between certain materials are large and undoubtedly significant. Further studies are required to relate the numbers of atoms affected by a fission event to the properties of materials and their irradiation behaviour. J. ADAM M. D. ROGERS Atomic Energy Research Establishment Harwell. Didcot, Berks. REFERENCES ADAM J. (1958) A.E.R.E. M and C/R 2751. CHILDS B. G. and MCGURN J. (1959) Canadian Report, CRMet-868. CUMMINGSW. V. et al. (1955) Rev. Sci. Instrum. 26, 5. KONOBEEVSK~ S. T. et a/. (1959) J. Nucl. Energy 9,75.
Measurements of some resonance activation integrals (Received 3 1 Januury 196 I ) THE resonance
integrals of sodium, manganese, cobalt, BsC~, ‘%u and of 08M~ have been determined from cadmium ratio measurements in a neutrqn beam. The method was the same as the one used earlier by JIRLOW and JOHAN~~~N(1960), so only a brief outline is given here. The cadmium ratio of the actual substance and of a I/v absorber (a thin BF, counter) were measured in a neutron beam from the reactor Rl. The neutron spectrum in the beam was accurately known from measurements with a fast chopper, as described by JOHAN~~~Net al. (1960). Corrections could thus be made for the deviation of the flux from l/E dependence. Also the effective cut-off energy to be used for the 1.1 mm thick cadmium could be measured directly. The results of the measurements are given in Table 1. The lower limit of the resonance integral has been fixed at 0.5 eV and the I/v part is not included. The factor k shows the corrections which have been made in the resonance integrals for scattering and self-absorption of neutrons in the foils. Multiple scattering as well as energy changes in the collisions have been taken into account. The errors given in k arise only from the calculation methods. The uncertainties caused by incomplete knowledge of resonance parameters and by effect of Doppler broadening have been included in the final values for the resonance integrals. The 00253 eV cross-sections given in Table I with the exception of that for OBMohave been taken from HUGHESand SCHWARTZ(1958) and from HUGHESet al. (1960). The thermal cross-section of BBM~given there appeared too high so it was determined in a separate measurement. Gold and molybdenum foils were irradiated in the thermal column of the reactor RI and the photo peaks at 0.411 and 0.740 MeV were compared using a scintillation counter. The cross-section obtained is only about one third of that given in the cited references. Probably
TABLE1.-RESULTS FROMTHE MEASUREMENTS u (0.0253) (barns)
Foil thickness (cm x lo-‘)
k
Na
0.520 f 0.010
30
0983 f 0.015
0.07 * 0.01
Mn
13.3 f 0.2
1.53
0.974 f 0.022
8.15 i 0.60
co
38.0 f 0.7
2.27
0.950 f 0.033
55.2 f 4.5
Yu
4.50 * 0.15
20
1.04 f- 0.01
3.09 f 0.15
*scu
2.0 f 0.3
100
1.015 f 0.010
1.38 f 0.23
“MO
O-18 f 0.02
100
1.13 & 0.01
10.7 f 2.5
Substance
Resonance integral (barns)