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v¯p Measurements for 239Pu below 2 MeV
Annals Df Nuclear Selene» and EngIneerIng. Vol. I. pp. 353 10355. Perl/amon Pres. 1974. Printed In Northern Ireland
Pp
MEASUREMENTS FOR
239Pu
BELOW 2 MeV
R. L. WALSH and J. W. BOLDEMAN AAECRE, Physics Division, Private Mailbag, Sutherland NSW 2232, Australia (Received 15 November 1973; in revised form 4 February 1974)
Abstract-The large liquid scintillator method has been used to measure the energy dependence of No fine structure in the low energy region was observed. An analysis of all eXisting 231lpU V" data below 5 MeV suggests a two-line energy dependence with a change of slope at about 800 keV.
v" for .3·PU below 2 MeV.
1. INTRODUCTION
3. RESULTS AND ANALYSIS
A number of studies of the energy dependence of v!Il' the average number of prompt neutrons emitted per fission, for 239pU have been made in recent years (see Manero and Konshin, 1972, pp. 695, 696). A lot of this interest has been generated by the controversy over the existence of fine structure in the v!Il energy dependence of 235U and to a lesser extent, 233U, as well as by the need for data of this type in fast reactor calculations. The present work examines the iip behaviour of 239pU below 2 MeV and follows up previous 235U and 233U work (Boldeman and Walsh,1970; Walsh and Holdeman, 1971). Because no ii" fine structure was evident in our 285U data and because the available 239pU ii" data also seemed to contain no significant structure in the low energy region, the search for fine structure in the present 239pU work was limited. Preliminary data were reported in Walsh and Holdeman (1971) and have been included in the recent iijJ evaluation of Manero and Konshin (1972).
The experimental results are listed in Table 1. ii p (262Cf) = 3'724 (JAEA, 1972). Figure 1 shows all 239pU data available to the end of 1972, between zero and 5 MeV. This data again is normalized to v!Il (252Cf) = 3·724. The published liquid scintillator results of Soleilhac et af. (1969, 1970) and Mather et al. (1965, 1970) have been reduced by 0'17 per cent to take account of the delayed gamma ray contribution (Walton and Sund, 1969; Ajitanand, 1971; Hanna et al., 1969, p. 19). In these experiments the neutron detection efficiency was higher than that used in this study. Also, the original data of Soleilhac et al. (1969) has been modified by the authors themselves (Manero and Konshin, 1972). A single line least squares fit to the data of Fig . I gives poor agreement with the low energy data. Therefore, in similar fashion to Mather and Bampton (1970), we have adopted a two line dependence as the 'best' description of the lip behaviour for 239PU below 5 MeV,
All values are given relative to
viz. 2. EXPERTIMENTAL METHOD
"p =
The large liquid scintillator method was used. Experimental details and corrections to data have been discussed previously in Boldeman and Dalton (1967) and Walsh and Bcldeman (1971). The 239pU target contained 1'4 per cent by weight 24DpU, which produced a spontaneous fission background of 2'7 fissions/min. The results were corrected for this contamination by regular monitoring of the MOpU rate throughout the experimental runs and by assuming a lip value for 240pU of 2'133 [Manero and Konshin, 1972-the value here is relative to ii:Jl (252Cf) = 3,724]. No correction for delayed gamma rays was made, as this was estimated to be negligible for the scintillator tank neutron detection efficiency used (74 per cent). 1
+ (0,112 ± O'013)E"
(1)
= 2'799 ± 0·007 + (0'170 ± 0'003)E" for 0·78 < E" s 5 MeV.
(2)
2·844 ± 0'007
for E" :s:; 0'78 MeV, and ii"
For the particular pair of lines (1) and (2), X2 (123 degrees of freedom) has a slight minimum. Lines (1) and (2) are reproduced in Fig. 1. This description is comparable with our previous work on 235U and 233U (Walsh and Boldeman, 1971) in that the li!ll behaviour for those two nuclides was also found to be best fitted by two straight lines below 5 MeV. The two line fit given by equations (1) and (2) is in good agreement with the fifth order polynomial
353
R. L. WALSH: and J. W.
354
BOLDEMAN
JIOO r----.,---~-__,r_-___,r_-___,--__;_r---r-----r-----r----,-----,-----r--I
30100
• u........ ,t II a !'WIN.. II ,I
Vp"2'199"O-170
en
Ili7lI) lit,,)
, P.""" ""'" ,
.
o
tfJPt
A 1ol'11;[R .t II [IU5)
MAHU .1 at 111101
,
,00
J. 50l!lUiAC II ,I ,,,,,.
+
o •
e
••
5QLEkJ-lAC .tll Ilt70 I DOMY.R£.NI
•
• iMIfI:E.N\IJtI It l{ ~ ti!UIl
'\. 4~.~:
J-ooo
4~
....
o
• •
I·
",,;V
II 'V" ........ t+
...t.
.~.
...
~~'H"'O'"Hn .'"
2-700
05
0'0
H
2-5
a
I-I
NE\I1RCIN EI£RGY Cw. Vl
Fig. 1.
v" vs incident neutron energy,
Table 1. v" results for 2SlPU between zero and 2 MeV. Values are relative to v,,(UOCf) = 3'724.
Neutron Energy (keV) 200 350 550 700 900 1300 1600 1900
± 55 ± 52 ± 36 ± 36 ± 48 ± 50 ± 50
± 50
2-849 2-869 2-893 2-915 2-938 2-976 3'029 3·102
± ±
±
±
± ±
± ±
0·013 0'017 0·017 0·017 0'014 0·020 0,021 0·019
fit of Manero and Konshin (1972), viz.
v:p
= 2·83916 + 0'121669E + 0·0200483E2 - 0·00387316E3 + 0·295825 X 10-3 E4 - 0·801081 X 10-6 E5.
(3)
The difference between the two is nowhere greater than 0·5 per cent over the range 0-4'5 MeV. This agreement is to be expected, of course, since both fits are based on very nearly the same data [we have excluded the Mather et al. (1970), 50 keV energy band values. Also Manero and Konshin (1972), includes no delayed gamma ray corrections.] However, equation (3) does not fit the 240pU spontaneous fission v" value [v,,(240PU) = 2·133 at En = -6·3 MeV], whereas equation (1) does. 4. DISCUSSION
The data of the present experiment contain no obvious fine structure in the region 0-2 MeV. This
2S0PU.
All measurements.
result agrees with the conclusion of no fine structure for 239pU below 2 MeV contained in the most recent IAEA review of v values (Manero and Konshin, 1972). The absence of fine structure for 289PU is consistent with our earlier suggestion (Boldeman and Walsh, 1970; Walsh and Boldeman, 1971) that for compound nuclei of mass> 234 the second hump of the fission potential barrier is lower than the first. Accordingly, any weak coupling effects that may be present at the saddle point will be smeared out in the passage to scission. Such a change in relative barrier heights at mass ,...." 234 has been recently shown by (d,pf) and (t,pf) measurements (Back et al., 1973). The energy difference between the 289PU fission threshold (at ,....,,-1·5 MeV, Back et al., 1969) and the position where the slope change suggested in Section 3 occurs, represents a compound nucleus excitation E* '" 2·3 MeV. This value corresponds to the excitation energy at which a change in the angular distribution of 239pU fission fragments has been observed (Griffin, 1963; Britt et al., 1963; Huizenga et al., 1968). These authors interpreted this change as due to the onset of two-quasiparticle excitations, which implied a transition state nucleonnucleon pairing energy gap 2t1f ,...." 2·3 MeV. However, other workers (Ignatyuk and Smirenkin, 1969; Androsenko et al., 1970) have pointed out that the above analyses failed to take into account the doublehumped nature of the fission potential barrier (Strutinsky, 1967), concluding that 26.f ,...." 1'5 MeV, which is close to the equilibrium value 2t1o ,...." 1·3
ii. Measurements for mpu below 2 MeV
MeV. More recently, data on 239PU fission isomers (Limkilde and Sletten, 1973) and on fission potential barrier heights (Back et al., 1973) also suggest 21:1,""" 1·5 MeV. 5. REFERENCES
Ajitanand N. N. (1971) Nucl. Phys. AI64, 300. AndrosenkoK. D., ErmagambetovS. B., IgnatyukA. V., Rabotnov N. S., Smirenkin G. N., Soldatov A. S., Usachev L. N., Shpak D. L. , Kapitsa S. P., Tsipenuik Iu. M. and Kovach 1. (1970) LASL Report No. LA· 4369-TR (unpublished). Back B. B., Bondorf J. P., Otroshenko G. A., Pedersen J . and Rasmussen B. (1969) 2nd /AEA Symp, on Physics and Chemistry ofFission, Vienna, July 1969, p. 351. Back B. B., Britt H. C., Garrett J. D., Hansen O. and Leroux B. (1973) Proc. 3rd [AEA Symp. Physics and Chemistry of Fission, Rochester, paper SM-174/201. Boldeman J. W. and Dalton A. W. (1967) AAEC/E172. Boldeman J. W. and Walsh R. L. (1970) J. Nucl . En. 24, 191. Bondarenko r. 1., Kuzminov B. D ., Kutsaeva L. S., Prokhorova L. 1. and Smirenkin G . N. (1958) Proc. 2nd U.N. Int. Conf on Peaceful Uses ofAtomic Energy, New York, 15,353. Britt H. C., Stokes R. H. , Gibbs W. R. and Griffin J. J. (1963) Phys. Rev. Lett. 11. 343. Conde H ., Hansen J. and Holmberg N. (1968) J. Nuc/. En . 22, 53. Graves E. R. (1963) ANL-S800, 2nd Edit. Griffin J. J. (1963) Phys. Rev. 132(5), 2204.
355
Hanna G. C., Westcott C. H., Lemmel H. D., Leonard B. R., Story J. S. and Attree P. M. (1969) Atom. Energy Rev. 7(4), 3. Hopkins J. C. and Diven B. C. (1963) Nucl. Phys. 48, 433. Huizenga J. R., Behkaroi A. N., Meadows J. W., Jr. and Klema E. D. (1968) Phys. Rev. 174, 1539. IAEA (1972) 2nd IAEA Panel Meeting on Neutron Standard Reference Data, Vienna, Nov. 1972. Ignatyuk A. V. and Smirenkin G. N. (1969) Phys. Lett. 29B, 3, 159. Limkilde P. and Sletten G. (1973) Nucl. Phys. A199, 504. Manero F. and Konshin V. A. (1972) Atom. Energy Rev. 10(4). Mather D . S., Fieldhouse P. and Moat A. (1965) Nuel. Phys. 66, 149. Mather D. S. , Bampton P. F., James G. and Nind P. J. (1970) AWRE 0 42/70. Mather D. S. and Bampton P. F. (1970) AWRE 0 86/70. Savin M. V., Khokhlov Yu. A., Zamyatnin Yu. S. and Paramonova I. N. (1970) Znd [AEA Conf on Nuclear Data for Reactors, Helsinki, June 1970, Paper CN-
26/40. Smirenkin G. N., Boudarenko I. 1., Kutsaeva L. S., Mishenko Kh. D., Prokhorova 1. I. and Shemetenko B. P. (1959) J. nucl. Energy 9, 155. Soleilhac M., Frehait J. and Gauriau J. (1969) J. nucl. EI/ergy 23, 257. Soleilhac M., Decarsin J. P., Frehaut J., Gauriau I., Mosinski G., Beau M., Bertrand R., Chardine Mme. and Ottoz A. (1970) Report 69-10 D.O. 0025 U.P. Strutinsky V. M. (1967) Nllc/. Phys. A95,420. Walsh R. L. and Boldeman J. W. (1971) J. nucl. Energy 25, 321. Walton R. B. and Sund R. E. (1969) Nucl . Instrum. Meth. 68, 163.
Pp
MEASUREMENTS FOR
239Pu
BELOW 2 MeV
R. L. WALSH and J. W. BOLDEMAN AAECRE, Physics Division, Private Mailbag, Sutherland NSW 2232, Australia (Received 15 November 1973; in revised form 4 February 1974)
Abstract-The large liquid scintillator method has been used to measure the energy dependence of No fine structure in the low energy region was observed. An analysis of all eXisting 231lpU V" data below 5 MeV suggests a two-line energy dependence with a change of slope at about 800 keV.
v" for .3·PU below 2 MeV.
1. INTRODUCTION
3. RESULTS AND ANALYSIS
A number of studies of the energy dependence of v!Il' the average number of prompt neutrons emitted per fission, for 239pU have been made in recent years (see Manero and Konshin, 1972, pp. 695, 696). A lot of this interest has been generated by the controversy over the existence of fine structure in the v!Il energy dependence of 235U and to a lesser extent, 233U, as well as by the need for data of this type in fast reactor calculations. The present work examines the iip behaviour of 239pU below 2 MeV and follows up previous 235U and 233U work (Boldeman and Walsh,1970; Walsh and Holdeman, 1971). Because no ii" fine structure was evident in our 285U data and because the available 239pU ii" data also seemed to contain no significant structure in the low energy region, the search for fine structure in the present 239pU work was limited. Preliminary data were reported in Walsh and Holdeman (1971) and have been included in the recent iijJ evaluation of Manero and Konshin (1972).
The experimental results are listed in Table 1. ii p (262Cf) = 3'724 (JAEA, 1972). Figure 1 shows all 239pU data available to the end of 1972, between zero and 5 MeV. This data again is normalized to v!Il (252Cf) = 3·724. The published liquid scintillator results of Soleilhac et af. (1969, 1970) and Mather et al. (1965, 1970) have been reduced by 0'17 per cent to take account of the delayed gamma ray contribution (Walton and Sund, 1969; Ajitanand, 1971; Hanna et al., 1969, p. 19). In these experiments the neutron detection efficiency was higher than that used in this study. Also, the original data of Soleilhac et al. (1969) has been modified by the authors themselves (Manero and Konshin, 1972). A single line least squares fit to the data of Fig . I gives poor agreement with the low energy data. Therefore, in similar fashion to Mather and Bampton (1970), we have adopted a two line dependence as the 'best' description of the lip behaviour for 239PU below 5 MeV,
All values are given relative to
viz. 2. EXPERTIMENTAL METHOD
"p =
The large liquid scintillator method was used. Experimental details and corrections to data have been discussed previously in Boldeman and Dalton (1967) and Walsh and Bcldeman (1971). The 239pU target contained 1'4 per cent by weight 24DpU, which produced a spontaneous fission background of 2'7 fissions/min. The results were corrected for this contamination by regular monitoring of the MOpU rate throughout the experimental runs and by assuming a lip value for 240pU of 2'133 [Manero and Konshin, 1972-the value here is relative to ii:Jl (252Cf) = 3,724]. No correction for delayed gamma rays was made, as this was estimated to be negligible for the scintillator tank neutron detection efficiency used (74 per cent). 1
+ (0,112 ± O'013)E"
(1)
= 2'799 ± 0·007 + (0'170 ± 0'003)E" for 0·78 < E" s 5 MeV.
(2)
2·844 ± 0'007
for E" :s:; 0'78 MeV, and ii"
For the particular pair of lines (1) and (2), X2 (123 degrees of freedom) has a slight minimum. Lines (1) and (2) are reproduced in Fig. 1. This description is comparable with our previous work on 235U and 233U (Walsh and Boldeman, 1971) in that the li!ll behaviour for those two nuclides was also found to be best fitted by two straight lines below 5 MeV. The two line fit given by equations (1) and (2) is in good agreement with the fifth order polynomial
353
R. L. WALSH: and J. W.
354
BOLDEMAN
JIOO r----.,---~-__,r_-___,r_-___,--__;_r---r-----r-----r----,-----,-----r--I
30100
• u........ ,t II a !'WIN.. II ,I
Vp"2'199"O-170
en
Ili7lI) lit,,)
, P.""" ""'" ,
.
o
tfJPt
A 1ol'11;[R .t II [IU5)
MAHU .1 at 111101
,
,00
J. 50l!lUiAC II ,I ,,,,,.
+
o •
e
••
5QLEkJ-lAC .tll Ilt70 I DOMY.R£.NI
•
• iMIfI:E.N\IJtI It l{ ~ ti!UIl
'\. 4~.~:
J-ooo
4~
....
o
• •
I·
",,;V
II 'V" ........ t+
...t.
.~.
...
~~'H"'O'"Hn .'"
2-700
05
0'0
H
2-5
a
I-I
NE\I1RCIN EI£RGY Cw. Vl
Fig. 1.
v" vs incident neutron energy,
Table 1. v" results for 2SlPU between zero and 2 MeV. Values are relative to v,,(UOCf) = 3'724.
Neutron Energy (keV) 200 350 550 700 900 1300 1600 1900
± 55 ± 52 ± 36 ± 36 ± 48 ± 50 ± 50
± 50
2-849 2-869 2-893 2-915 2-938 2-976 3'029 3·102
± ±
±
±
± ±
± ±
0·013 0'017 0·017 0·017 0'014 0·020 0,021 0·019
fit of Manero and Konshin (1972), viz.
v:p
= 2·83916 + 0'121669E + 0·0200483E2 - 0·00387316E3 + 0·295825 X 10-3 E4 - 0·801081 X 10-6 E5.
(3)
The difference between the two is nowhere greater than 0·5 per cent over the range 0-4'5 MeV. This agreement is to be expected, of course, since both fits are based on very nearly the same data [we have excluded the Mather et al. (1970), 50 keV energy band values. Also Manero and Konshin (1972), includes no delayed gamma ray corrections.] However, equation (3) does not fit the 240pU spontaneous fission v" value [v,,(240PU) = 2·133 at En = -6·3 MeV], whereas equation (1) does. 4. DISCUSSION
The data of the present experiment contain no obvious fine structure in the region 0-2 MeV. This
2S0PU.
All measurements.
result agrees with the conclusion of no fine structure for 239pU below 2 MeV contained in the most recent IAEA review of v values (Manero and Konshin, 1972). The absence of fine structure for 289PU is consistent with our earlier suggestion (Boldeman and Walsh, 1970; Walsh and Boldeman, 1971) that for compound nuclei of mass> 234 the second hump of the fission potential barrier is lower than the first. Accordingly, any weak coupling effects that may be present at the saddle point will be smeared out in the passage to scission. Such a change in relative barrier heights at mass ,...." 234 has been recently shown by (d,pf) and (t,pf) measurements (Back et al., 1973). The energy difference between the 289PU fission threshold (at ,....,,-1·5 MeV, Back et al., 1969) and the position where the slope change suggested in Section 3 occurs, represents a compound nucleus excitation E* '" 2·3 MeV. This value corresponds to the excitation energy at which a change in the angular distribution of 239pU fission fragments has been observed (Griffin, 1963; Britt et al., 1963; Huizenga et al., 1968). These authors interpreted this change as due to the onset of two-quasiparticle excitations, which implied a transition state nucleonnucleon pairing energy gap 2t1f ,...." 2·3 MeV. However, other workers (Ignatyuk and Smirenkin, 1969; Androsenko et al., 1970) have pointed out that the above analyses failed to take into account the doublehumped nature of the fission potential barrier (Strutinsky, 1967), concluding that 26.f ,...." 1'5 MeV, which is close to the equilibrium value 2t1o ,...." 1·3
ii. Measurements for mpu below 2 MeV
MeV. More recently, data on 239PU fission isomers (Limkilde and Sletten, 1973) and on fission potential barrier heights (Back et al., 1973) also suggest 21:1,""" 1·5 MeV. 5. REFERENCES
Ajitanand N. N. (1971) Nucl. Phys. AI64, 300. AndrosenkoK. D., ErmagambetovS. B., IgnatyukA. V., Rabotnov N. S., Smirenkin G. N., Soldatov A. S., Usachev L. N., Shpak D. L. , Kapitsa S. P., Tsipenuik Iu. M. and Kovach 1. (1970) LASL Report No. LA· 4369-TR (unpublished). Back B. B., Bondorf J. P., Otroshenko G. A., Pedersen J . and Rasmussen B. (1969) 2nd /AEA Symp, on Physics and Chemistry ofFission, Vienna, July 1969, p. 351. Back B. B., Britt H. C., Garrett J. D., Hansen O. and Leroux B. (1973) Proc. 3rd [AEA Symp. Physics and Chemistry of Fission, Rochester, paper SM-174/201. Boldeman J. W. and Dalton A. W. (1967) AAEC/E172. Boldeman J. W. and Walsh R. L. (1970) J. Nucl . En. 24, 191. Bondarenko r. 1., Kuzminov B. D ., Kutsaeva L. S., Prokhorova L. 1. and Smirenkin G . N. (1958) Proc. 2nd U.N. Int. Conf on Peaceful Uses ofAtomic Energy, New York, 15,353. Britt H. C., Stokes R. H. , Gibbs W. R. and Griffin J. J. (1963) Phys. Rev. Lett. 11. 343. Conde H ., Hansen J. and Holmberg N. (1968) J. Nuc/. En . 22, 53. Graves E. R. (1963) ANL-S800, 2nd Edit. Griffin J. J. (1963) Phys. Rev. 132(5), 2204.
355
Hanna G. C., Westcott C. H., Lemmel H. D., Leonard B. R., Story J. S. and Attree P. M. (1969) Atom. Energy Rev. 7(4), 3. Hopkins J. C. and Diven B. C. (1963) Nucl. Phys. 48, 433. Huizenga J. R., Behkaroi A. N., Meadows J. W., Jr. and Klema E. D. (1968) Phys. Rev. 174, 1539. IAEA (1972) 2nd IAEA Panel Meeting on Neutron Standard Reference Data, Vienna, Nov. 1972. Ignatyuk A. V. and Smirenkin G. N. (1969) Phys. Lett. 29B, 3, 159. Limkilde P. and Sletten G. (1973) Nucl. Phys. A199, 504. Manero F. and Konshin V. A. (1972) Atom. Energy Rev. 10(4). Mather D . S., Fieldhouse P. and Moat A. (1965) Nuel. Phys. 66, 149. Mather D. S. , Bampton P. F., James G. and Nind P. J. (1970) AWRE 0 42/70. Mather D. S. and Bampton P. F. (1970) AWRE 0 86/70. Savin M. V., Khokhlov Yu. A., Zamyatnin Yu. S. and Paramonova I. N. (1970) Znd [AEA Conf on Nuclear Data for Reactors, Helsinki, June 1970, Paper CN-
26/40. Smirenkin G. N., Boudarenko I. 1., Kutsaeva L. S., Mishenko Kh. D., Prokhorova 1. I. and Shemetenko B. P. (1959) J. nucl. Energy 9, 155. Soleilhac M., Frehait J. and Gauriau J. (1969) J. nucl. EI/ergy 23, 257. Soleilhac M., Decarsin J. P., Frehaut J., Gauriau I., Mosinski G., Beau M., Bertrand R., Chardine Mme. and Ottoz A. (1970) Report 69-10 D.O. 0025 U.P. Strutinsky V. M. (1967) Nllc/. Phys. A95,420. Walsh R. L. and Boldeman J. W. (1971) J. nucl. Energy 25, 321. Walton R. B. and Sund R. E. (1969) Nucl . Instrum. Meth. 68, 163.