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Prompt neutron spectrum measurement of 238U fission induced by 10.17 and 12.12 MeV neutrons
Nuclear Instruments and Methods in Physics Research A 370 ( 1996) 477-483
NUCLEAR INSYRUMENTS & METIN PHYSICS
Li Anli*, Bai Xixiang, Wang Xiaozhong, Wang Yufeng, Meng Jiangchen, Yuan Yuan, Chih Tahai, Huang Shenghian China
Institute
of Atomic
Energy,
P.O.
Box 275-46
Beijing
102413.
Liu Weiping,
China
Received 24 January 1995; revised form received 2 May 1995 Abstract The prompt neutron spectra of *?J fission induced by 10.17 and 12. I2 MeV neutrons have been measured using a multi-segment fission chamber and two ST-451 liquid scintillator neutron detectors in conjunction with a double-TOF technique. Use of coincidence between the fission fragments and neutrons enabled the fission neutrons to be distinguished from other secondary ones. The detection efficiencies of the ST-451 liquid scintillators were determined by combining Monte Carlo calculation using a standard code NEFF4 [l] with experimental calibration using a low-mass “‘Cf ionization chamber. The measured spectra can be roughly described by Maxwellian distributions. The Maxwellian temperatures are obtained by fitting equal 1.40t0.04 MeV for 10.17 MeV incident and 1.47?0.04 MeV for 12.12 MeV, respectively. The experimental spectra are compared with theoretical calculated ones by code FINESSE [2] and the evaluation by ENDFIB-VI
w
1. Introduction Most investigations concerning the fission neutron spectrum have been concentrated in the fission by thermal neutrons and spontaneous fission. The prompt neutron spectrum of fission induced by fast neutrons has been insufficiently studied experimentally. This situation is mainly due to the following difficulties: (a) considerably small fission cross section for fast neutrons; (b) large number of the background neutrons produced in the same energy regions as the fission neutrons. So far a few measurements of the neutron spectra of ‘?_J tission induced by fast neutrons have been carried out by Baba et al. [4] at 2.0 MeV, Bernard et al. [5] at 2.1 and 4.9 MeV, Didiev et al. [6] a 13.9 MeV, Vasil’ev et al. [7] at 14.3 MeV, Almen et al. [8] at 1.35 and 2.02 MeV, Knitter et al. [9] at 1.90 and 2.30 MeV, Bertin et al. at 14.0 MeV [IO] and 7.0 MeV [ 1I]. In most of these experiments TOF technique was used and the measured spectra were described by Maxwellian distributions. The results are listed in Table 1. As can be seen, the data are not in good agreement with each other and some of them gave only l? or T (the Maxwellian temperature) without detailed fission neutron spectrum data. More important, in the incident energy region from 8 to 13 MeV there are no experimental
* Corresponding author. Elsevier Science B.V. SSDl 0168.9002(95)00796-2
data because of lack of monoenergetic neutron source. In the case of incident energy greater than 6 MeV, second chance fission (n, n’f) might occur which has perhaps an observable influence on fission neutron spectrum. In present work the prompt neutron spectra of ‘?I fission induced by 10.17 and 12.12 MeV neutrons have been measured at HI- 13 Tandem Van de Graaff Accelerator Laboratory of CIAE. One of the key points in this Table 1 Experimental measurements of prompt neutron spectrum from U-238 fission induced by fast neutrons Refs.
Incident energy [MeV]
Fission neutron E [MeV]
T [MeV]
WI
1.35
1.94%0.06
[91
1.90
2.02~0.10
I .29?0.04 I .35 20.06
[41
2.0
1.88
1.25
[81
2.02
[51
2.086
1.94-to.04 I .928-tO.O46
191
2.30
1.85~0.10
I .29f0.03 I.285~0.03 I I .23 20.07
[51
4.908
2.133+0.047
1.422~0.031
[Ill
7.0
2.07t0.07
I .38+0.046 I .40?0.04 I .47 +0.04 I .85+0.28
Present work [61
[lOI [71
10.17 12.12 13.9 14.0
2.10+0.06 2.21?0.06 2.7820.42 2.30*0.07
14.3
2.19kO.09
1.53 *o.os
I .46-1-0.06
Li Anli et al. I Nucl. Instr. and Meth. in Phys. Res. A 370 (1996) 477-483
478
experiment is how to distinguish the fission neutrons from serious background ones. So a multi-segment fission chamber was utilized and fission fragment signals were used in coincidence with fission neutron signals to reduce the background to 10m3 of one without the coincidence. The double TOF technique similar to Bertin [ 1 l] had to be used to separate the incident monoenergetic neutron peak and the continuous breakup neutrons. A compromise between the counting rate and the time resolution had to be made in a limited beam time. The amount of material seen by neutron detectors was much larger than the quantity of the uranium, so that the random coincidence background was still comparable to the effect and it was measured simultaneously. That made the experiment more complicated.
2. Experimental
arrangement
experimental arrangement shown in 1. The primary neutrons produced at direction via *H(d, n)3He The neutron-protarget was 2.5 cm cylindrical cell a window 0.25 mil Havar foil pressurized with atm absolute deuterium gas. energies of deuteron beam 7.44 MeV 10.17 MeV and 9.40 for 12.12 neutrons respectively. design principle the fission was to material around fissile sample little as The ionization chamber was steel cylinder. contained parallel plates 9 cm diameter in of 1.5 The natural was both-sides on the mm thick of stainless with deposilayer of 0.50 mg/cm* which was enough to most of fission fragments pass through. diameter of deposited uranium was 8 The total of the natural uranium about 5.1 As shown Fig. 1, fission chamber
0”
Fig. 1. The experimental
Momfor arrangement.
was placed at 0” direction with respect to the beam, and the distance between the centers of the gas cell and the samples was 64 cm. This was the minimum distance necessary to obtain adequate time separation between the arrival of the monoenergetic neutrons and the continuum neutrons from the breakup processes. The 103 plates were divided into eight separate segments for the following reasons: 1) To reduce the uncertainty of the flight path, because the distance from the first plate to the last plate was as large as 17.1 cm. 2) To reduce the interplate capacitances and to make the rise time of fission fragment pulses faster. Each segment was connected to individual preamplifier, fast filter amplifier and constant fraction discrimination as shown in Fig. 2. The eight timing outputs were mixed together and then fed into a TAC (time to amplitude converter). Therefore eight primary neutron TOF spectra could be measured with a single ADC (analog-to-digital converter), each of them had their own flight path, while these timing outputs were also fed into connectors 2 to 9 of an IR (input register) in the computer interface to distinguish which segment the fission event belongs to. The fission neutrons were recorded by two heavily shielded detectors, consisting of ST-45 1
Data acquisition
and treatment
Two thresholds at E, = 0.65 and 1.65 MeV were set for each neutron detector by electronics and software, respectively. The higher threshold was aimed at upgrading the effect-to-background ratio in higher energy region of the fission neutron TOF spectrum. Moreover, pulse shape discrimination was used only for the higher threshold to eliminate gamma ray background. In measuring fission neutron TOF spectrum the pickoff signals were employed instead of the fission fragment signals because the timing characteristic of the latter was not well enough. In this case the timing outputs of the pickoff signals had to pass through a gate set by fission fragment timing signals before being fed into TACs. As an example, a measured primary neutron TOF spectrum for the sixth segment of the chamber is shown in Fig. 3(a). The monoenergetic neutron peak with an obtained FWHM of 8.6 ns could not be clearly separated from the continuum neutrons coming from the three-body breakup process ‘H(d, n)pd for the flight path of 64 cm. If only monoenergetic neutrons were selected out by setting a software gate A indicated by the arrows in Fig. 3(a), then a lot of monoenergetic neutrons would have been lost. Alternatively a new method was adopted by us to improve
Li Anli et al. I Nucl. Instr. and Meth. in Phy. Res. A 370 (1996) 477-483
Pick off Y Left
479
0’ moniter
1
Right detector
detector Fission chamber
s
FD
T P
, ti AC
I
AC’
c DC I
I
Fig. 2. The block diagram of electronics.
it. For constant fraction discriminators connected with the fission chamber the zero-crossing time was a function of the amplitude of the input signals. The linear spectrum of fission fragments was divided into eight groups according to their amplitudes by setting gates in offline analyses, the corresponding primary neutron TOF spectra are shown in Fig. 3(b). As can be seen from the figure, the positions of the TOF spectra were shifted along the X axis with the variation of the amplitude. In such case, the primary neutron peaks with typical FWHM of 5.7 ns could clearly be separated from the continuum neutrons. The backgrounds were treated as follows: At first, in the monoenergetic peak of incident neutron beam the random background must be subtracted. Then, the accidental random coincidence events between signals from fission chambers and neutron detectors should also be subtracted. As shown in Fig. 3(b), gate A and B were set on each of these partial primary neutron TOF spectra. Gate A only covered the monoenergetic peak and rejected the continuum breakup neutrons. Gate B, with the same width as gate A, was set at the right of the monoenergetic peak for fissions induced by scattered neutrons randomly in time (background). Corresponding to the software gate A and B on the primary neutron TOF spectra, an effect plus background spectrum A and a background spectrum B
were obtained respectively for each segment of the fission chamber and for each neutron detector. A spectrum called (A - B), was obtained by subtracting B from A. The spectra A, B, and (A - B) for the left neutron detector and sixth segment are shown in Fig. 4. Use of fast-fast coincidence between the signals from fission fragments and those from neutron detectors enabled the fission neutrons to be distinguished from other secondary ones. But a small amount of the background leaked in via random accidental coincidence with fission events, which was still comparable to the effect. Such a random coincidence background TOF spectrum was measured simultaneously by the following means. The timing output of fission chamber was delayed by several integral cycles of pulsed beam and then fed into a CFD (constant fraction discriminator) as a gate input, through which the pickoff signals passed and led into the same TACs used to measure fission neutron TOF spectra. The TOF spectra obtained in this operation reproduced fairly the random coincidence in the same cycle. The random spectra were for all of the segments together, because no corresponding grouping signals were fed into the input register for the delayed fission events. The pickoff signals gated by delayed fission timing outputs were gated again by TAC output signals of either of the neutron detectors and then
480
c/t
Li Anli et al. I Nucl. Instr. and Meth. in Phys. Rex A 370 (19%) 477-483
a
IAT”
A
A__ (b)
group 8
400
200
200
r*TBJ
100
Wl group 5 II
A-B 400 -
Fig. 3. The primary neutron TOF spectrum at 10 MeV incident for sixth segment of the fission chamber: (a) total spectrum; (b) partial spectra corresponding to different amplitude ranges on the fragment linear spectrum. Gate A only covered monoenergetic peak and gate B for background measurement; (c) sum spectrum of eight partial spectra after they were moved along the X axis for compensating the shift.
Fig. 4. The fission neutron TOF spectra for the sixth segment of the fission chamber and left neutron detector with low threshold at 10 MeV The effect plus background spectrum A corresponding to the gate A on the primary neutron TOF spectra and background spectrum B corresponding to the gate B shown in Fig. 3.
the “real plus random” specalone (C - D). As can be seen from the figure, elastic scattering neutrons leaked in by random coincidence and formed an elastic scattering peak on the spectrum. The peak disappeared after subtracting the random coincidence as shown in Fig. detector trum
fed into connector 12 of the IR to identify the random coincidence events. The primary neutron TOF spectra were obtained for the random coincidence measurements and two gates, C and D, were set on it, which covered monoenergetic neutron peaks and background respectively and were the same as gate A and B in effect measurements. Corresponding to the gate C and D, two random coincidence spectra, called spectra C and D, were obtained respectively. The random spectrum (C - D) were obtained by subtracting spectrum D from spectrum C for each of the neutron detectors. The spectra C, D for the left detector are shown in Fig. 5. The random spectra (C - D) were for eight segments together and (A - B) spectra were for each of the segments so that eight (A-B) spectra should be summed and formed a “real plus random” spectrum. Fig. 6(a) shows the fission neutron TOF spectra for the left neutron
at low
threshold:
(A - B) and the random spectrum
6(b). Logic output signals of TACs (TAC-2 and TAC-3, see Fig. 2) used in measurements of fission neutron TOF spectra for either left or right neutron detector, were fed into MPP (multi-parameter plate) as general triggers. It implied that coincidence between neutron signals and fission signals was employed to trigger data acquisition. One advantage of using outputs of TACs as triggers was to reduce the counting rate at the computer interface, and consequently decrease the computer dead time and save recording tapes. A bit output on the real-panel of the MPP was connected with connector 11 of the IR to indicate which neutron detector recorded the fission neutron.
Li Anli et al. I Nucl. Instr. and Meth. in Phys. Rer. A 370 (1996) 477-483
01 *
481
The data accumulation time was about 120 h for each incident neutron energy. The multi-parameter events were stored event by event into buffer tapes in online data acquisition and the events recorded in tapes would be sorted in offline analyses. The efficiency curves of two neutron detectors were determined by combining Monte Carlo calculations using a standard code NEFF4 with experimental calibration using a low mass “‘Cf ionization chamber [ 121. The experimental calibration of neutron detection efficiency will be described in detail elsewhere 1121. The energy range of the measured fission neutron spectra were 0.7 to 11.5 MeV at 10.17 MeV incident and 0.9 to 9.5 MeV at 12.12 MeV incident respectively.
*
4. Error estimations
01
-100
200
.
300
I
-.-*A--..
400
500
The diameter of the deposited natural uranium layers was 8 cm and the thickness of the neutron detectors was 5 cm, which caused an energy uncertainty in the fission neutron TOF spectrum calculated by AE 2AT 2AL _~=7+~=2.6%,
Fig. 5. The random coincidence TOF spectra for left neutron detector with low threshold at 10 MeV Spectra C, D correspond to gates C, D on the primary neutron TOF spectrum respectively.
(4
4
Red
Random
plus random
alone
“0 5
-
“4/ 32. 1
100
200 300 Channels
400
500
Fig. 6. The fission neutron TOF spectra for left neutron detector at low threshold with 10 MeV. (a) The “real plus random” (A - B) and random alone (C - D); (b) real spectrum (A - B) (C - D).
(1)
here flight path L is 2.5 m. The effect-to-background ratio was about 2 in major part of the measured fission neutron TOF spectra. But this ratio got smaller in the high energy regions, especially under the elastic scattering peaks, where it was less than 1. That is the reason why the statistical errors of the data in high energy regions were rather large. The directions of the detected fragments affect significantly the shape of fission neutron spectrum, so in present work the fission fragments were detected at almost 217 solid angles except a small solid angle parallel with the backing sheets. The inhomogeneity of the deposited natural uranium layers was within 25% with average density so that self-absorption of the fission of 0.50 mglcm’, fragments in the layers was negligible. About 96% of the fission events were recorded. Estimates indicate that the average neutron energy will be changed by less than 1% by these effects. In the primary neutron TOF spectrum measured by 0” monitor detector there exists a tail at the low energy side of the monoenergetic peak, which results mainly from monoenergetic neutrons elastically scattered from surrounding material such as the shielding. The portion of these scattered neutrons is 1 or 2% of the area under the monoenergetic neutron peak. The fission events induced by these scattered neutrons were recorded by the fission chamber, if they sat in the monoenergetic peak. This effect made the fission neutron TOF spectra shift to the low energy and resulted in a little softening of the fission neutron spectra. Fission neutrons scattered from shielding
Li Anli et al. I Nucl. Instr. and Meth. in Phys. Res. A 370 (1996) 477-483
482
material of the neutron detectors had the same influence on the fission neutron spectra and they changed the average energy of the fission neutron spectra by less than 1%.
5. Results and discussion For most of the existing data the experimental energy distributions of the prompt fission neutrons can be fitted approximately with a Maxwellian distribution
\ N(E) = A&
exp(-E/T)
,
(2)
here A is a constant for normalization, and 7’ is an energy parameter analogous to nuclear temperature of the fission fragments. These experimental observations are consistent with a theoretical concept of the statistical model [ 131, that is, neutrons are emitted from highly excited and fully accelerated fragments. When incident neutron energy exceeds 6.5 MeV, experimentally measured fission neutrons are coming from both the reactions (n, f) and (n, n’f). The neutrons emitted from compound nuclei prior to fission (pre-fission neutrons) are included, which could not be separated experimentally from those emitted from fission fragments (post-fission neutrons). These pre-fission neutrons should be subtracted from the measured fission neutron spectra before they are fitted with Maxwellian distributions. The pre-fission neutron spectrum is a sum of evaporated (i.e. equilibrium emitted) and pre-equilibrium emitted neutrons. Using pre-equilibrium theory based on exciton model developed by Griffin [14] and the statistical theory, the pre-fission neutron spectra were calculated by Zhang Jingshang [15] and normalized to the measured ones. After subtracting the pre-fission neutrons, the prompt neutron spectra emitted from fission fragments were obtained and can be described by Maxwellian distribution. The optimum fitting yields T=
1.40~0.04MeVorE=2.10+0.06MeV for Ei = 10.17 MeV
T = 1.4720.04
MeV or E = 2.2lkO.06
MeV
forE, = 12.12MeV, here I? is the average energy of the prompt neutrons, E, is the energy of incident neutrons respectively. The ratios of measured spectra to Maxwellian distributions are shown in Fig. 7. The error includes the statistical uncertainty and that from determination of detection efficiency. Some deviation of the experimental data from the Maxwellian distributions appears in low energy range. As mentioned above, the Maxwellian spectrum is a rough approximation. It neglects three important physical effects: (1) the distribution f residual nuclear temperature T of fission fragments, that results from the initial distribution of fission fragment excitation energy and the
\
0.6-
I I 0. 8
I :I, 1.5 2.0
3.0
\
I
I
I
II_
.I
tj
8
lo
E./M&
Fig. 7. Comparisons of measured post-fission neutron spectra with FINESSE calculations and ENDF/B-VI evaluations. All spectra are shown in the form of ratios to Maxwellian distribution with a nuclear temperature T = 1.40 MeV for 10.17 MeV and T = 1.47 MeV for 12.12 MeV respectively.
subsequent cooling of the fragments as neutrons are emitted; (2) the energy dependence of the cross section gC for the inverse process of compound nucleus formation; and (3) center-of-mass motion of the fission fragments from which the neutrons are emitted. Due to the complexity of fission neutron emission, calculation of the fission neutron spectrum should be based on an adequate statistical model approach in conjunction with fission theory to deduce the intricate fragment distribution. There exists a widely used Madland-Nix-Model and its computer code MNM [ 131 on the basis of standard nuclear evaporation theory, with which prompt fission neutron spectrum can be calculated as a function of both the fission nuclei and its excitation energy. The distribution of residual nuclear temperature of fission fragments is taken to be triangular in shape, extending linearly from zero to a maximum value T,,,. Tm is determined from the average energy release, the separation energy and kinetic energy of the neutron inducing fission, the total average fission fragment kinetic energy, and the level density parameter of the Fermi-gas model. The neutron energy spectrum for fixed residual nuclear temperature is integrated over this triangular distribution to obtain the neutron energy spectrum in the center-of-mass system of a given fission fragment, which is then transformed to the laboratory system. The energy dependence of a, is simulated with an approximate way, in which’ a slightly readjusted value of the level-density parameter was adopted. The MNM was applied to systematic fission neutron data calculation for fissile nuclei in ENDF/B-VI. The corresponding post-fission neutron spectra evaluated by ENDFIB-VI are also shown in Fig. 7 as dashed lines. The MNM has been modified by Marten [2] considering the dependence on fragment mass number A. This modified
Li Anli et al. I Nucl. Instr. and Meth. in Phys. Res. A 370 (1996) 477-483
code is called FINESSE. In contrast to the original MNM, the different center of mass system spectra for light and heavy fragment groups are taken into account in the FINESSE code and the level density parameter is described by realistic effective data obtained within a more complex statistical model approach. The post-fission neutron spectra of “U fission induced by 10 and 12 MeV neutrons were calculated respectively by Marten [ 171 using FINESSE code without any parameter fit. These neutron spectra from all fission chances did not include the prefission neutrons due to the (n, n’f) reaction. The calculated results are shown in Fig. 7 as solid lines. As seen in the figure, FINESSE calculated results are in better agreement with our experimental points than the evaluations of ENDFIB-VI, especially in the high energy range. We have collected the results of experimental measurements of prompt neutron spectrum of ?J fission induced by fast neutrons. The fitting average energies ,J? of the fission neutron spectra are shown in Fig. 8 as a function of incident neutron energy E,. The solid line is calculated .?? with FINESSE. In Fig. 8 a dashed line close to FINESSE calculation results represents E calculated with FUPl by CAi Chonghai et al. [16], FUPI is a unified program for calculating various fast neutron data of fissile nuclei, which can also
I
2
:
8
I
I
I
I
6
8
IO
12
c
%/MeV Fig. 8. Dependence neutrons on incident
of average energy neutron energy.
,?? of ‘?J
post-fission
483
be used to calculate fission neutron spectrum and partial fission cross sections by using pre-equilibrium and statistical theories.
Acknowledgement This work was supported partly by International Atomic Energy Agency under contract F4. CPR. 5 188 and the Third World Academy of Sciences under Research Grant No. 86-74. We would like to thank them. We wish to thank Prof. H.Klein for his help in supplying the Monte Carlo code NEFF4. We also wish to acknowledge the FINESSE calculation by Dr. H. Marten.
References [I] G. Dietze and H. Klein, PTB-Report
ND-22 (1982). [2] H. Marten, A. Ruben and D. Seeliger, Proc. Int. Conf. on 50 Years with Nuclear Fission, 25-28 April, 1989, Gaithersburg, vol. 2, p. 743. [3] Cited from ENDFIB-VI. [4] M. Baba et al., Fission spectrum measurement of Th-232 and U-238 for 2 MeV Neutrons, INDC(NDS)-220. Physics of Neutron Emission in Fission, MIT0 (1988) p. 149. [S] E. Barnard et al., Nucl. Phys. 71 (1965) 228. [6] D. didiev et al., Antwerp Int. Conf. on the Study of Nuclear Structure with Neutrons, Antwerp (1965). [7] Vasil’ev et al., JETP 38 (1960) 671. [8] E. Almen et al.. AE-429 (1971). [9] H.H. Knitter et al., IAEA Consultants Meeting on Prompt Fission Neutron Spectra, Vienna ( 1971) p. 41, [lo] A. Bertin and G. Cleyeux, Proc. 3rd Conf. on Neutron Cross-section and Tech., Knoxville ( 1971) p. 286. [I I] A. Bertin, R. Bois and J. Ftehaut, NEANDC-INDC Report CEA-R-4913 (1978). [12] L. Anli et al., At. Energy Sci. and Technol. 28 (1994) 38. [13] D.G. Madland and J.R. Nix, Nucl. Sci. Eng. 81 (1982) 213. [14] J.J. Griffin, Phys. Rev. Lett. 17 (1966). [ 151 Zhang Jingshang, to be published. [16] Cai Chonghai and Shen Qingbiao, Commun. Nucl. Data Prog., CNIC-00412, No. 3 (1990) 29. [17] H. Marten, private communication.
NUCLEAR INSYRUMENTS & METIN PHYSICS
Li Anli*, Bai Xixiang, Wang Xiaozhong, Wang Yufeng, Meng Jiangchen, Yuan Yuan, Chih Tahai, Huang Shenghian China
Institute
of Atomic
Energy,
P.O.
Box 275-46
Beijing
102413.
Liu Weiping,
China
Received 24 January 1995; revised form received 2 May 1995 Abstract The prompt neutron spectra of *?J fission induced by 10.17 and 12. I2 MeV neutrons have been measured using a multi-segment fission chamber and two ST-451 liquid scintillator neutron detectors in conjunction with a double-TOF technique. Use of coincidence between the fission fragments and neutrons enabled the fission neutrons to be distinguished from other secondary ones. The detection efficiencies of the ST-451 liquid scintillators were determined by combining Monte Carlo calculation using a standard code NEFF4 [l] with experimental calibration using a low-mass “‘Cf ionization chamber. The measured spectra can be roughly described by Maxwellian distributions. The Maxwellian temperatures are obtained by fitting equal 1.40t0.04 MeV for 10.17 MeV incident and 1.47?0.04 MeV for 12.12 MeV, respectively. The experimental spectra are compared with theoretical calculated ones by code FINESSE [2] and the evaluation by ENDFIB-VI
w
1. Introduction Most investigations concerning the fission neutron spectrum have been concentrated in the fission by thermal neutrons and spontaneous fission. The prompt neutron spectrum of fission induced by fast neutrons has been insufficiently studied experimentally. This situation is mainly due to the following difficulties: (a) considerably small fission cross section for fast neutrons; (b) large number of the background neutrons produced in the same energy regions as the fission neutrons. So far a few measurements of the neutron spectra of ‘?_J tission induced by fast neutrons have been carried out by Baba et al. [4] at 2.0 MeV, Bernard et al. [5] at 2.1 and 4.9 MeV, Didiev et al. [6] a 13.9 MeV, Vasil’ev et al. [7] at 14.3 MeV, Almen et al. [8] at 1.35 and 2.02 MeV, Knitter et al. [9] at 1.90 and 2.30 MeV, Bertin et al. at 14.0 MeV [IO] and 7.0 MeV [ 1I]. In most of these experiments TOF technique was used and the measured spectra were described by Maxwellian distributions. The results are listed in Table 1. As can be seen, the data are not in good agreement with each other and some of them gave only l? or T (the Maxwellian temperature) without detailed fission neutron spectrum data. More important, in the incident energy region from 8 to 13 MeV there are no experimental
* Corresponding author. Elsevier Science B.V. SSDl 0168.9002(95)00796-2
data because of lack of monoenergetic neutron source. In the case of incident energy greater than 6 MeV, second chance fission (n, n’f) might occur which has perhaps an observable influence on fission neutron spectrum. In present work the prompt neutron spectra of ‘?I fission induced by 10.17 and 12.12 MeV neutrons have been measured at HI- 13 Tandem Van de Graaff Accelerator Laboratory of CIAE. One of the key points in this Table 1 Experimental measurements of prompt neutron spectrum from U-238 fission induced by fast neutrons Refs.
Incident energy [MeV]
Fission neutron E [MeV]
T [MeV]
WI
1.35
1.94%0.06
[91
1.90
2.02~0.10
I .29?0.04 I .35 20.06
[41
2.0
1.88
1.25
[81
2.02
[51
2.086
1.94-to.04 I .928-tO.O46
191
2.30
1.85~0.10
I .29f0.03 I.285~0.03 I I .23 20.07
[51
4.908
2.133+0.047
1.422~0.031
[Ill
7.0
2.07t0.07
I .38+0.046 I .40?0.04 I .47 +0.04 I .85+0.28
Present work [61
[lOI [71
10.17 12.12 13.9 14.0
2.10+0.06 2.21?0.06 2.7820.42 2.30*0.07
14.3
2.19kO.09
1.53 *o.os
I .46-1-0.06
Li Anli et al. I Nucl. Instr. and Meth. in Phys. Res. A 370 (1996) 477-483
478
experiment is how to distinguish the fission neutrons from serious background ones. So a multi-segment fission chamber was utilized and fission fragment signals were used in coincidence with fission neutron signals to reduce the background to 10m3 of one without the coincidence. The double TOF technique similar to Bertin [ 1 l] had to be used to separate the incident monoenergetic neutron peak and the continuous breakup neutrons. A compromise between the counting rate and the time resolution had to be made in a limited beam time. The amount of material seen by neutron detectors was much larger than the quantity of the uranium, so that the random coincidence background was still comparable to the effect and it was measured simultaneously. That made the experiment more complicated.
2. Experimental
arrangement
experimental arrangement shown in 1. The primary neutrons produced at direction via *H(d, n)3He The neutron-protarget was 2.5 cm cylindrical cell a window 0.25 mil Havar foil pressurized with atm absolute deuterium gas. energies of deuteron beam 7.44 MeV 10.17 MeV and 9.40 for 12.12 neutrons respectively. design principle the fission was to material around fissile sample little as The ionization chamber was steel cylinder. contained parallel plates 9 cm diameter in of 1.5 The natural was both-sides on the mm thick of stainless with deposilayer of 0.50 mg/cm* which was enough to most of fission fragments pass through. diameter of deposited uranium was 8 The total of the natural uranium about 5.1 As shown Fig. 1, fission chamber
0”
Fig. 1. The experimental
Momfor arrangement.
was placed at 0” direction with respect to the beam, and the distance between the centers of the gas cell and the samples was 64 cm. This was the minimum distance necessary to obtain adequate time separation between the arrival of the monoenergetic neutrons and the continuum neutrons from the breakup processes. The 103 plates were divided into eight separate segments for the following reasons: 1) To reduce the uncertainty of the flight path, because the distance from the first plate to the last plate was as large as 17.1 cm. 2) To reduce the interplate capacitances and to make the rise time of fission fragment pulses faster. Each segment was connected to individual preamplifier, fast filter amplifier and constant fraction discrimination as shown in Fig. 2. The eight timing outputs were mixed together and then fed into a TAC (time to amplitude converter). Therefore eight primary neutron TOF spectra could be measured with a single ADC (analog-to-digital converter), each of them had their own flight path, while these timing outputs were also fed into connectors 2 to 9 of an IR (input register) in the computer interface to distinguish which segment the fission event belongs to. The fission neutrons were recorded by two heavily shielded detectors, consisting of ST-45 1
Data acquisition
and treatment
Two thresholds at E, = 0.65 and 1.65 MeV were set for each neutron detector by electronics and software, respectively. The higher threshold was aimed at upgrading the effect-to-background ratio in higher energy region of the fission neutron TOF spectrum. Moreover, pulse shape discrimination was used only for the higher threshold to eliminate gamma ray background. In measuring fission neutron TOF spectrum the pickoff signals were employed instead of the fission fragment signals because the timing characteristic of the latter was not well enough. In this case the timing outputs of the pickoff signals had to pass through a gate set by fission fragment timing signals before being fed into TACs. As an example, a measured primary neutron TOF spectrum for the sixth segment of the chamber is shown in Fig. 3(a). The monoenergetic neutron peak with an obtained FWHM of 8.6 ns could not be clearly separated from the continuum neutrons coming from the three-body breakup process ‘H(d, n)pd for the flight path of 64 cm. If only monoenergetic neutrons were selected out by setting a software gate A indicated by the arrows in Fig. 3(a), then a lot of monoenergetic neutrons would have been lost. Alternatively a new method was adopted by us to improve
Li Anli et al. I Nucl. Instr. and Meth. in Phy. Res. A 370 (1996) 477-483
Pick off Y Left
479
0’ moniter
1
Right detector
detector Fission chamber
s
FD
T P
, ti AC
I
AC’
c DC I
I
Fig. 2. The block diagram of electronics.
it. For constant fraction discriminators connected with the fission chamber the zero-crossing time was a function of the amplitude of the input signals. The linear spectrum of fission fragments was divided into eight groups according to their amplitudes by setting gates in offline analyses, the corresponding primary neutron TOF spectra are shown in Fig. 3(b). As can be seen from the figure, the positions of the TOF spectra were shifted along the X axis with the variation of the amplitude. In such case, the primary neutron peaks with typical FWHM of 5.7 ns could clearly be separated from the continuum neutrons. The backgrounds were treated as follows: At first, in the monoenergetic peak of incident neutron beam the random background must be subtracted. Then, the accidental random coincidence events between signals from fission chambers and neutron detectors should also be subtracted. As shown in Fig. 3(b), gate A and B were set on each of these partial primary neutron TOF spectra. Gate A only covered the monoenergetic peak and rejected the continuum breakup neutrons. Gate B, with the same width as gate A, was set at the right of the monoenergetic peak for fissions induced by scattered neutrons randomly in time (background). Corresponding to the software gate A and B on the primary neutron TOF spectra, an effect plus background spectrum A and a background spectrum B
were obtained respectively for each segment of the fission chamber and for each neutron detector. A spectrum called (A - B), was obtained by subtracting B from A. The spectra A, B, and (A - B) for the left neutron detector and sixth segment are shown in Fig. 4. Use of fast-fast coincidence between the signals from fission fragments and those from neutron detectors enabled the fission neutrons to be distinguished from other secondary ones. But a small amount of the background leaked in via random accidental coincidence with fission events, which was still comparable to the effect. Such a random coincidence background TOF spectrum was measured simultaneously by the following means. The timing output of fission chamber was delayed by several integral cycles of pulsed beam and then fed into a CFD (constant fraction discriminator) as a gate input, through which the pickoff signals passed and led into the same TACs used to measure fission neutron TOF spectra. The TOF spectra obtained in this operation reproduced fairly the random coincidence in the same cycle. The random spectra were for all of the segments together, because no corresponding grouping signals were fed into the input register for the delayed fission events. The pickoff signals gated by delayed fission timing outputs were gated again by TAC output signals of either of the neutron detectors and then
480
c/t
Li Anli et al. I Nucl. Instr. and Meth. in Phys. Rex A 370 (19%) 477-483
a
IAT”
A
A__ (b)
group 8
400
200
200
r*TBJ
100
Wl group 5 II
A-B 400 -
Fig. 3. The primary neutron TOF spectrum at 10 MeV incident for sixth segment of the fission chamber: (a) total spectrum; (b) partial spectra corresponding to different amplitude ranges on the fragment linear spectrum. Gate A only covered monoenergetic peak and gate B for background measurement; (c) sum spectrum of eight partial spectra after they were moved along the X axis for compensating the shift.
Fig. 4. The fission neutron TOF spectra for the sixth segment of the fission chamber and left neutron detector with low threshold at 10 MeV The effect plus background spectrum A corresponding to the gate A on the primary neutron TOF spectra and background spectrum B corresponding to the gate B shown in Fig. 3.
the “real plus random” specalone (C - D). As can be seen from the figure, elastic scattering neutrons leaked in by random coincidence and formed an elastic scattering peak on the spectrum. The peak disappeared after subtracting the random coincidence as shown in Fig. detector trum
fed into connector 12 of the IR to identify the random coincidence events. The primary neutron TOF spectra were obtained for the random coincidence measurements and two gates, C and D, were set on it, which covered monoenergetic neutron peaks and background respectively and were the same as gate A and B in effect measurements. Corresponding to the gate C and D, two random coincidence spectra, called spectra C and D, were obtained respectively. The random spectrum (C - D) were obtained by subtracting spectrum D from spectrum C for each of the neutron detectors. The spectra C, D for the left detector are shown in Fig. 5. The random spectra (C - D) were for eight segments together and (A - B) spectra were for each of the segments so that eight (A-B) spectra should be summed and formed a “real plus random” spectrum. Fig. 6(a) shows the fission neutron TOF spectra for the left neutron
at low
threshold:
(A - B) and the random spectrum
6(b). Logic output signals of TACs (TAC-2 and TAC-3, see Fig. 2) used in measurements of fission neutron TOF spectra for either left or right neutron detector, were fed into MPP (multi-parameter plate) as general triggers. It implied that coincidence between neutron signals and fission signals was employed to trigger data acquisition. One advantage of using outputs of TACs as triggers was to reduce the counting rate at the computer interface, and consequently decrease the computer dead time and save recording tapes. A bit output on the real-panel of the MPP was connected with connector 11 of the IR to indicate which neutron detector recorded the fission neutron.
Li Anli et al. I Nucl. Instr. and Meth. in Phys. Rer. A 370 (1996) 477-483
01 *
481
The data accumulation time was about 120 h for each incident neutron energy. The multi-parameter events were stored event by event into buffer tapes in online data acquisition and the events recorded in tapes would be sorted in offline analyses. The efficiency curves of two neutron detectors were determined by combining Monte Carlo calculations using a standard code NEFF4 with experimental calibration using a low mass “‘Cf ionization chamber [ 121. The experimental calibration of neutron detection efficiency will be described in detail elsewhere 1121. The energy range of the measured fission neutron spectra were 0.7 to 11.5 MeV at 10.17 MeV incident and 0.9 to 9.5 MeV at 12.12 MeV incident respectively.
*
4. Error estimations
01
-100
200
.
300
I
-.-*A--..
400
500
The diameter of the deposited natural uranium layers was 8 cm and the thickness of the neutron detectors was 5 cm, which caused an energy uncertainty in the fission neutron TOF spectrum calculated by AE 2AT 2AL _~=7+~=2.6%,
Fig. 5. The random coincidence TOF spectra for left neutron detector with low threshold at 10 MeV Spectra C, D correspond to gates C, D on the primary neutron TOF spectrum respectively.
(4
4
Red
Random
plus random
alone
“0 5
-
“4/ 32. 1
100
200 300 Channels
400
500
Fig. 6. The fission neutron TOF spectra for left neutron detector at low threshold with 10 MeV. (a) The “real plus random” (A - B) and random alone (C - D); (b) real spectrum (A - B) (C - D).
(1)
here flight path L is 2.5 m. The effect-to-background ratio was about 2 in major part of the measured fission neutron TOF spectra. But this ratio got smaller in the high energy regions, especially under the elastic scattering peaks, where it was less than 1. That is the reason why the statistical errors of the data in high energy regions were rather large. The directions of the detected fragments affect significantly the shape of fission neutron spectrum, so in present work the fission fragments were detected at almost 217 solid angles except a small solid angle parallel with the backing sheets. The inhomogeneity of the deposited natural uranium layers was within 25% with average density so that self-absorption of the fission of 0.50 mglcm’, fragments in the layers was negligible. About 96% of the fission events were recorded. Estimates indicate that the average neutron energy will be changed by less than 1% by these effects. In the primary neutron TOF spectrum measured by 0” monitor detector there exists a tail at the low energy side of the monoenergetic peak, which results mainly from monoenergetic neutrons elastically scattered from surrounding material such as the shielding. The portion of these scattered neutrons is 1 or 2% of the area under the monoenergetic neutron peak. The fission events induced by these scattered neutrons were recorded by the fission chamber, if they sat in the monoenergetic peak. This effect made the fission neutron TOF spectra shift to the low energy and resulted in a little softening of the fission neutron spectra. Fission neutrons scattered from shielding
Li Anli et al. I Nucl. Instr. and Meth. in Phys. Res. A 370 (1996) 477-483
482
material of the neutron detectors had the same influence on the fission neutron spectra and they changed the average energy of the fission neutron spectra by less than 1%.
5. Results and discussion For most of the existing data the experimental energy distributions of the prompt fission neutrons can be fitted approximately with a Maxwellian distribution
\ N(E) = A&
exp(-E/T)
,
(2)
here A is a constant for normalization, and 7’ is an energy parameter analogous to nuclear temperature of the fission fragments. These experimental observations are consistent with a theoretical concept of the statistical model [ 131, that is, neutrons are emitted from highly excited and fully accelerated fragments. When incident neutron energy exceeds 6.5 MeV, experimentally measured fission neutrons are coming from both the reactions (n, f) and (n, n’f). The neutrons emitted from compound nuclei prior to fission (pre-fission neutrons) are included, which could not be separated experimentally from those emitted from fission fragments (post-fission neutrons). These pre-fission neutrons should be subtracted from the measured fission neutron spectra before they are fitted with Maxwellian distributions. The pre-fission neutron spectrum is a sum of evaporated (i.e. equilibrium emitted) and pre-equilibrium emitted neutrons. Using pre-equilibrium theory based on exciton model developed by Griffin [14] and the statistical theory, the pre-fission neutron spectra were calculated by Zhang Jingshang [15] and normalized to the measured ones. After subtracting the pre-fission neutrons, the prompt neutron spectra emitted from fission fragments were obtained and can be described by Maxwellian distribution. The optimum fitting yields T=
1.40~0.04MeVorE=2.10+0.06MeV for Ei = 10.17 MeV
T = 1.4720.04
MeV or E = 2.2lkO.06
MeV
forE, = 12.12MeV, here I? is the average energy of the prompt neutrons, E, is the energy of incident neutrons respectively. The ratios of measured spectra to Maxwellian distributions are shown in Fig. 7. The error includes the statistical uncertainty and that from determination of detection efficiency. Some deviation of the experimental data from the Maxwellian distributions appears in low energy range. As mentioned above, the Maxwellian spectrum is a rough approximation. It neglects three important physical effects: (1) the distribution f residual nuclear temperature T of fission fragments, that results from the initial distribution of fission fragment excitation energy and the
\
0.6-
I I 0. 8
I :I, 1.5 2.0
3.0
\
I
I
I
II_
.I
tj
8
lo
E./M&
Fig. 7. Comparisons of measured post-fission neutron spectra with FINESSE calculations and ENDF/B-VI evaluations. All spectra are shown in the form of ratios to Maxwellian distribution with a nuclear temperature T = 1.40 MeV for 10.17 MeV and T = 1.47 MeV for 12.12 MeV respectively.
subsequent cooling of the fragments as neutrons are emitted; (2) the energy dependence of the cross section gC for the inverse process of compound nucleus formation; and (3) center-of-mass motion of the fission fragments from which the neutrons are emitted. Due to the complexity of fission neutron emission, calculation of the fission neutron spectrum should be based on an adequate statistical model approach in conjunction with fission theory to deduce the intricate fragment distribution. There exists a widely used Madland-Nix-Model and its computer code MNM [ 131 on the basis of standard nuclear evaporation theory, with which prompt fission neutron spectrum can be calculated as a function of both the fission nuclei and its excitation energy. The distribution of residual nuclear temperature of fission fragments is taken to be triangular in shape, extending linearly from zero to a maximum value T,,,. Tm is determined from the average energy release, the separation energy and kinetic energy of the neutron inducing fission, the total average fission fragment kinetic energy, and the level density parameter of the Fermi-gas model. The neutron energy spectrum for fixed residual nuclear temperature is integrated over this triangular distribution to obtain the neutron energy spectrum in the center-of-mass system of a given fission fragment, which is then transformed to the laboratory system. The energy dependence of a, is simulated with an approximate way, in which’ a slightly readjusted value of the level-density parameter was adopted. The MNM was applied to systematic fission neutron data calculation for fissile nuclei in ENDF/B-VI. The corresponding post-fission neutron spectra evaluated by ENDFIB-VI are also shown in Fig. 7 as dashed lines. The MNM has been modified by Marten [2] considering the dependence on fragment mass number A. This modified
Li Anli et al. I Nucl. Instr. and Meth. in Phys. Res. A 370 (1996) 477-483
code is called FINESSE. In contrast to the original MNM, the different center of mass system spectra for light and heavy fragment groups are taken into account in the FINESSE code and the level density parameter is described by realistic effective data obtained within a more complex statistical model approach. The post-fission neutron spectra of “U fission induced by 10 and 12 MeV neutrons were calculated respectively by Marten [ 171 using FINESSE code without any parameter fit. These neutron spectra from all fission chances did not include the prefission neutrons due to the (n, n’f) reaction. The calculated results are shown in Fig. 7 as solid lines. As seen in the figure, FINESSE calculated results are in better agreement with our experimental points than the evaluations of ENDFIB-VI, especially in the high energy range. We have collected the results of experimental measurements of prompt neutron spectrum of ?J fission induced by fast neutrons. The fitting average energies ,J? of the fission neutron spectra are shown in Fig. 8 as a function of incident neutron energy E,. The solid line is calculated .?? with FINESSE. In Fig. 8 a dashed line close to FINESSE calculation results represents E calculated with FUPl by CAi Chonghai et al. [16], FUPI is a unified program for calculating various fast neutron data of fissile nuclei, which can also
I
2
:
8
I
I
I
I
6
8
IO
12
c
%/MeV Fig. 8. Dependence neutrons on incident
of average energy neutron energy.
,?? of ‘?J
post-fission
483
be used to calculate fission neutron spectrum and partial fission cross sections by using pre-equilibrium and statistical theories.
Acknowledgement This work was supported partly by International Atomic Energy Agency under contract F4. CPR. 5 188 and the Third World Academy of Sciences under Research Grant No. 86-74. We would like to thank them. We wish to thank Prof. H.Klein for his help in supplying the Monte Carlo code NEFF4. We also wish to acknowledge the FINESSE calculation by Dr. H. Marten.
References [I] G. Dietze and H. Klein, PTB-Report
ND-22 (1982). [2] H. Marten, A. Ruben and D. Seeliger, Proc. Int. Conf. on 50 Years with Nuclear Fission, 25-28 April, 1989, Gaithersburg, vol. 2, p. 743. [3] Cited from ENDFIB-VI. [4] M. Baba et al., Fission spectrum measurement of Th-232 and U-238 for 2 MeV Neutrons, INDC(NDS)-220. Physics of Neutron Emission in Fission, MIT0 (1988) p. 149. [S] E. Barnard et al., Nucl. Phys. 71 (1965) 228. [6] D. didiev et al., Antwerp Int. Conf. on the Study of Nuclear Structure with Neutrons, Antwerp (1965). [7] Vasil’ev et al., JETP 38 (1960) 671. [8] E. Almen et al.. AE-429 (1971). [9] H.H. Knitter et al., IAEA Consultants Meeting on Prompt Fission Neutron Spectra, Vienna ( 1971) p. 41, [lo] A. Bertin and G. Cleyeux, Proc. 3rd Conf. on Neutron Cross-section and Tech., Knoxville ( 1971) p. 286. [I I] A. Bertin, R. Bois and J. Ftehaut, NEANDC-INDC Report CEA-R-4913 (1978). [12] L. Anli et al., At. Energy Sci. and Technol. 28 (1994) 38. [13] D.G. Madland and J.R. Nix, Nucl. Sci. Eng. 81 (1982) 213. [14] J.J. Griffin, Phys. Rev. Lett. 17 (1966). [ 151 Zhang Jingshang, to be published. [16] Cai Chonghai and Shen Qingbiao, Commun. Nucl. Data Prog., CNIC-00412, No. 3 (1990) 29. [17] H. Marten, private communication.