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Transmission characteristics of Cf252 neutrons passing through rare earth and boron loaded concrete slab shields
Nuclear Engineering and Design 117 (1989) 325-331 North-Holland, Amsterdam
325
TRANSMISSION CHARACTERISTICS O F C f 25z N E U T R O N S EARTH AND BORON LOADED CONCRETE SLAB SHIELDS B.S. S E S H A D R I
PASSING
THROUGH
RARE
a n d R. V E N K A T E S A N
Safety Research and Analysis Division, Indira Gandhi Center for Atomic Research, Kalpakkam-603 102, India Received 3 October 1988
Experiments that measure the transport of fission neutrons through gadolinium, samarium and boron loaded slab shields were carried out using a Cf 252 source. The effectiveness of rare earths in comparision to boron as suitable additives in concrete is investigated. A NE-213 liquid scintillation detector with pulse shape discrimination to simultaneously resolve neutron and gamma ray events was employed. The measurements were compared with the spectra calculated using the 'MORSE' Monte Carlo code. The calculated and measured spectra are in favourable agreement in the energy range 0.8 MeV up to 12 MeV depending on the concrete slab thickness. Gadolinium mixed concretes exhibit a much higher attenuation as compared to samarium as well as boron loaded types.
I. Introduction Although an estimation of transmitted neutron dose and flux is of primary interest from the shielding point of view, a knowledge of the spectral distribution of neutrons inside the shield would provide a valuable insight in determining the suitability of the shield material. It is c o m m o n practice to add boron in concretes in order to enhance neutron attenuation. Since the neutron removal is by (n, a) reactions, the problem of secondary gamma radiation does not exist. The possible disadvantages could be the radiation damage due to the intense local energy deposition by the (n, a) reaction, and helium production. Reported work on the transport of fission neutrons through rare-earth loaded concretes is very sparse. The effect of rare earth additions on concretes for the attenuation of ( D - T ) neutrons has already been reported elsewhere [1]. The objective of the work reported here is to study the attenuation properties and spectral degradation of Cf 252 neutrons in concretes of different densities and investigate the relative effects of rare earths and boron as additives in concrete. The absorption cross sections of some of the rareearths like gadolinium, samarium and europium as compared with boron is given in fig. 1. It is clear that the rare-earths in particular gadolinium have substantially high cross sections as compared to boron. As rare-earth oxides also show good ceramic properties [2] it was felt that they could be used as constituents of concrete to increase its shield effectiveness. 0029-5493/89/$03.50
Energy distributions of fast neutrons transmitted through slabs of normal and high density ilmenite concretes obtained by time of flight methods were reported by Adams et al. [3]. Schmidt has made a thorough
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
-
10 0
-
Samarium
........
Europium
...................
Gadolinium
.......
Boron
3 \
lo-1
"
\?',.
"'~'-~ - . ~ _ 1
........
I--
~o-2 ,.% "'...,...,.
""...... --...
o
10-4 0
1
I
I
2
4
G Energy
10
-....
-.....
I
I
12
14
(MeV)
Fig. 1. A b s o r p t i o n cross-section curves for different elements.
326
B.S. Seshadri, R. Venkatesan / Transmission characteristics of Cf 252 neutrons
review of fast neutron transmission through concretes [4]. Details of different types of concretes and their compositions used for the studies, transmitted spectra and dose rate evaluation of relaxation lengths (both experimental and computed) have been reported. Pictet et al. [5] have reported the use of rare-earths to increase the efficiency of a biological shield made of concrete. Rare-earth oxides have also been used in the concrete (density: 2.7 g / c m 3) of the biological shield for Rapsodie [6]. From the review of earlier work, it is observed that the use of rare earth oxides in concretes as additives, the available information is very limited with respect to their attenuation characteristics. N o work has been reported on the spectral degradation in concretes containing rare-earths as additives. Such a study is of particular interest to India because of the easy availability of rare-earth oxides as a byproduct in the Indian monozite industry.
2. Source and spectrometer The Cf 252 source is sealed in a cylindrical aluminium can about 5 cm in diameter and 8 cm in height. It emits 107 n / s with an average energy of 2 MeV. The neutron gamma ray detector consists of NE-213 liquid scintillator contained in a cylindrical plastic container (38 mm × 38 mm) coated on the inside with titanium dioxide paint to increase light reflection. It is mounted on a high gain photomultiplier tube R C A 8850. Neutron and gamma ray events in the detector were separated using pulse shape discrimination methods and stored in separate memory locations in PDP-11 computer. The pulse height data were permanently stored on magnetic disks. The neutron energy measurements range from 1.5 MeV up to 1l MeV. Although it was possible to make measurements down to 0.8 MeV, it was found that gamma ray discrimination of NE-213 spectrometer below 1.8 MeV was not satisfactory. G a m m a to neutron ratio of 1 : 1000 was chosen as the minimum ratio to be maintained in all the measurements. Description of experimental shields as well as the concentration of rare-earths and boron as additives is discussed in detail in ref. [1].
3. Experiment The general schematic of the experimental arrangement is shown in fig. 2. The Cf 2s2 source is mounted at
one end of a mild steel support 125 cm above the ground level in order to minimise background due to ground scattered neutrons. The NE-213 detector and photomultiplier tube assembly was kept at a distance of 125 cm in front of the source. The centres of the concrete blocks were aligned to be in line with the source and detector. The first slab of concrete was placed at a distance of 10 cm from the detector. The subsequent slabs were successively placed in front of the slabs already present. In this way, the shielding thickness could be increased while the detector remained in the same position. Consequently no inverse square law correction had to be made. The slab thickness was increased in steps of 10 cm up to a maximum of 40 cm and counting was done for 1000 seconds for each slab. The experiment was repeated for three types of concretes mentioned earlier. Energy calibration of the NE-213 spectrometor was carried out using the procedure of Verbinski et al. [7]. Since monoenergetic proton sources are not easily available, the spectrometer was calibrated using electrons of known energies. C o m p t o n scattered electrons from the specific energy gamma rays are the most convenient sources in this regard. The compton edge of the gamma rays from CS137, M n 54, N a 22 and TI 2°8 were used as reference points. During calibration and subsequent checks for reproducibility, the system was tested using a 5 Ci A m - B e neutron source kept at a distance of 40 cm from the detector. The recoil proton data was unscrambled using a derivative unfolding code ' D U S T ' based on cubic bell splines [8]. Here the smoothing of input data is obtained by least squares polynomial representation along with continuity conditions on the neutron and recoil proton spectra. The scheme employed leads to enhanced resolution as well as to an improved estimate of the peak positions.
4. Experimental errors The major errors in the measurements are due to counting statistics. Uncertainties in the recorded data are in the range of + 1.5 to + 6.0%, the maximum being the value for 40 cm of concrete and the lower end of measured energy 1.2 MeV. The error in the energy calibration is + 0.5%. Next, the error due to unfolding which includes error in the intrinsic efficiencies and detector resolution. Based on the large number of measurements with the known spectra, the maximum error in the peak values (due to unfolding) is estimated to be + 8%. Hence, the total errors are of the order of + 14%
B.S. Seshadri, R. Venkatesan / Transmission characteristics of C f 252 neutrons
327
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SPECTROMETER
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/
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7.2 _
10 c m
........
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.....
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_
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.......
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0
i 3
3
5
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Energy in MeV Fig. 3. Incident spectrum of Cf 252 neutrons.
11
5
7
9
Energy in MeV
Fig. 4. Transmitted spectra of Cf 252 neutrons through ord concrete (2.4 g / c m 3) of diff. thickness.
328
B.S. Seshadri, R. Venkatesan
around 1.2 MeV and about +10% around 10-12 MeV region.
The incident Cf 252 neutron spectrum as measured at the distance of 40 cm from the source is shown in fig. 3. A dominant peak at 2.4 MeV and two humps at 6.5 MeV and 8 MeV are present. Fig. 4 shows the transmitted spectra for concrete without any additives (density 2.4 g / c m 3) for thickness varying from 10 to 40 cm. similar spectra for concretes of this density with the addition of 1% by weight of gadolinium concentrate, samarium concentrate and boron are shown in figs. 5, 6 and 7 respectively. In addition to overall fall in intensity with increasing thickness, considerable spectral changes take place in the measured energy range. While in the energy range from 1.5 MeV to 4 MeV the peaks become broader, above 4 MeV peaks begin to appear. For 10 cm thick ordinary concrete (2.4 g / c m 3) a peak is observed at 2.3 MeV. The addition of gadolinium, samarium or boron does not change the position or shape of this peak. However boron (20%) causes maximum fall in intensity followed by gadolinium (15%) and then samarium (10%) in that order. Although significant change in spectral behaviour is observed
neutrons
3.6 lOom
3.0
' /
.........
2.4
t,,,,l
30cm .................. 40cm
5. Results and discussion
20cm
.......
g
~1.2 Z
,
o.o
.
1
"","~.~r,~~
,
3
7
5 Energy
in
9
,
11
MeV
Fig. 6. Transmitted spectra of Cf 252 neutrons through concrete (2.4 g / c m 3 + 1% Sm).
addition of gadolinium and samarium reduces the flux marginally up to 6 MeV. For 20 cm thick concrete, the peak position shifts to 2.5 MeV. Here again the addition of boron lowers the fluxes most in the peak region (by 15%) although to a lesser extent than in the case of 10 cm concrete.
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3.6
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.........
~ ~: 2.4
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.
.
.
.
.
.
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.
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"'
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V
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Z
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~-~ 1.2 P
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/ Transmission characteristics of
..... ~,','.;:~7,. 5 7
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9
11
Energy in iMeV
Fig. 5. Transmitted spectra of Cf 252 neutrons through concrete (2.4 g/cm 3 + 1% Gd).
0.0
F
','\'~,
i
I
3
5
7
9
11
Energy in MeV
Fig. 7. Transmitted spectra of Cf 252 neutrons through concrete (2.4 g/cm 3 + 1% B).
329
B.S. Seshadri, R. genkatesan / Transmission characterisncs of Cf" " neutrons
When the thickness is increased to 30 cm, the peak shifts to 2.6 MeV and is considerably broadened. Boron is marginally more effective than gadolinium or samarium in reducing the flux. For still larger thickness of 40 cm, it is found that gadolinium is superior compared to boron in reducing the neutron flux. In figs. 4 and 5, even at 1.8 MeV a peak is observed, this is due to the interference of gamma rays. The C f 252 source has a large gamma ray background and hence it became necessary to increase the discrimination level. The strobe window was further reduced in the pulse shape discriminator of the NE-213 spectrometer in order to reduce the effect of gamma ray interference. A small shift in the position of 2.3 MeV peak is also attributed to this interference. It may be noted that the appearance of distinct peaks arround 2 MeV, 4.5 MeV, 6.5 MeV and 7.5 MeV is understandable in the light of the energy dependance of the absorption cross section for concrete. The absorption cross section data from DLC-2 [9] for ordinary concrete is shown in fig. 8. The typical transmitted spectrum (for 40 cm thick) with the same
a
o
•- . . . . . . .
io I
group structure is also shown. It may be observed that the peaks in the transmitted spectrum are nearly coincident with the valleys in the cross section. Because of the inherent uncertainties of the relative intensities of the unfolded spectra, only a qualitative comparison is possible. Even in the case of concretes with additives, correlation between the spectral behaviour and the absorption cross section of the corresponding additives is present. In particular, the observed higher effectiveness of gadolinium versus boron in the lower energy peak region matches well with the higher absorption cross section of gadolinium as compared to boron.
6. Comparison with the calculated spectra Transmitted spectra of C f 252 neutrons through the different types of concretes were calculated using the code MORSE-CG'. This is a monte carlo programme with combinatorial geometry option which enables faithful description of the experimental configuration. A point isotropic source was assumed for the calculations. The cross-section data for the constituent ele-
CONC. 2 "
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Fig. 8. Comparison of spectra with cross section (sigma absorption).
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I 15- O0
330
B.S. Seshadri, R. Venkatesan / Transmission characteristics of Cf 252 neutrons
lo4
10
9-
.......
8-
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........
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>~6-
I
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5
I
¢_ uJ z z
3
4 E N ERGY
5
6
7
8
2
9
IN MeV
Fig. 9. Measured and calculated spectra for ord. concrete (2.4 g/cm 3).
ments of the concrete and additives was taken from the 100 group DLC-2 library. The first 30 groups covering the range between 0.273 MeV to 12 MeV was used, as the measured spectra were from 1.5 MeV upwards. Fig. 9 shows the comparison between measured and calculated spectra for concrete of density 2.4 g / c m 3. Figs. 10, 11 and 12 represent comparisons between measured and calculated spectra for concrete (2.4 g / c m 3) containing 1% by weight of B, Gd and Sm respectively. A typical concrete thickness of 40 cm was chosen to represent the comparison between measured and calculated spectra. Compared to the corresponding measured spectra, this also reveals the changing nature with the different types of concrete. The main peak at 2.35 MeV is very well resolved. The peaks and valleys as observed in experimental measurements are also seen. But in view of the slight difference in group structure in computations, the positions of peaks and valleys are not exactly matched between computations and measurements. The agreement is found to be quite significant in the main peak region of 2.35 MeV. At higher energies (5-10 MeV), the calculated values are higher as compared with measurements by about 20%. This could be due to
3 ENERGY
IN
MeV
Fig. 10. Measured and calculated spectra for concrete (2.4 g/cm 3 + 1% B).
9
8i
...........
NORSE
__MEASURED 7
~6 ~E
LL
s
Ld Z
ENERGY IN NleV
Fig. l l. Measured and calculated spectra for concrete (2.4 g/cm 3 + 1% Gd).
B.S. Seshadri, R. Venkatesan / Transmission characteristics of Cf 252 neutrons
331
the fact t h a t anisotropic b e h a v i o u r of the source has not b e e n included in the calculation. Fx
9] 8.
Acknowledgement .
.
.
.
.
.
.
.
.
_
MORSE
_
MEASURED
~6
T h e a u t h o r s are t h a n k f u l to D.V. G o p i n a t h , Head, Safety Research P r o g r a m m e for guidance a n d encouragement.
References
~3 f3~
5
ENERGY
IN
MeV
Fig. 12. Measured and calculated spectra for concrete (2.4 g/cm 3 + 1% Sm).
[1] B.S. Seshadri, V. Venkatesam and V. Sundararaman, Transmitted spectra of (D-T) neutrons through rare-earth mixed concretes, Nucl. Engrg. Des. 97, No. 2 (1986) 223-232. [2] V. Lach, Engg. compendium on Rad. Shielding, Vol. II (Jaeger-Verlas, 1975). [3] Adams et al., Health Physics 36 (1979) 671. [4] Fritz A.R. Schmidt, Report ORNL-RSIC 26 (1970). [5] Pictet et al., Reactor Shielding, I.A.E.A. Technical report series 34 (1964) 152. [6] Safety report for Rapsodie, Vol. I, chap. 1, C.E.A. France (in French). [7] Verbinski et al., Calibration of an organic scintillator for neutron spectrometry, Nucl. Inst. and Meth. 65 (1968) 8-25. [8] K.V. Subbiah and A. Natarajan, Derivative unfolding by spline technique, Nucl. Inst. and Meth. 228 (1985) 377-386. [9] DLC-2, 100 group cross-section data based on ENDF/B, RSIC library, Oak Ridge (1974).
325
TRANSMISSION CHARACTERISTICS O F C f 25z N E U T R O N S EARTH AND BORON LOADED CONCRETE SLAB SHIELDS B.S. S E S H A D R I
PASSING
THROUGH
RARE
a n d R. V E N K A T E S A N
Safety Research and Analysis Division, Indira Gandhi Center for Atomic Research, Kalpakkam-603 102, India Received 3 October 1988
Experiments that measure the transport of fission neutrons through gadolinium, samarium and boron loaded slab shields were carried out using a Cf 252 source. The effectiveness of rare earths in comparision to boron as suitable additives in concrete is investigated. A NE-213 liquid scintillation detector with pulse shape discrimination to simultaneously resolve neutron and gamma ray events was employed. The measurements were compared with the spectra calculated using the 'MORSE' Monte Carlo code. The calculated and measured spectra are in favourable agreement in the energy range 0.8 MeV up to 12 MeV depending on the concrete slab thickness. Gadolinium mixed concretes exhibit a much higher attenuation as compared to samarium as well as boron loaded types.
I. Introduction Although an estimation of transmitted neutron dose and flux is of primary interest from the shielding point of view, a knowledge of the spectral distribution of neutrons inside the shield would provide a valuable insight in determining the suitability of the shield material. It is c o m m o n practice to add boron in concretes in order to enhance neutron attenuation. Since the neutron removal is by (n, a) reactions, the problem of secondary gamma radiation does not exist. The possible disadvantages could be the radiation damage due to the intense local energy deposition by the (n, a) reaction, and helium production. Reported work on the transport of fission neutrons through rare-earth loaded concretes is very sparse. The effect of rare earth additions on concretes for the attenuation of ( D - T ) neutrons has already been reported elsewhere [1]. The objective of the work reported here is to study the attenuation properties and spectral degradation of Cf 252 neutrons in concretes of different densities and investigate the relative effects of rare earths and boron as additives in concrete. The absorption cross sections of some of the rareearths like gadolinium, samarium and europium as compared with boron is given in fig. 1. It is clear that the rare-earths in particular gadolinium have substantially high cross sections as compared to boron. As rare-earth oxides also show good ceramic properties [2] it was felt that they could be used as constituents of concrete to increase its shield effectiveness. 0029-5493/89/$03.50
Energy distributions of fast neutrons transmitted through slabs of normal and high density ilmenite concretes obtained by time of flight methods were reported by Adams et al. [3]. Schmidt has made a thorough
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
-
10 0
-
Samarium
........
Europium
...................
Gadolinium
.......
Boron
3 \
lo-1
"
\?',.
"'~'-~ - . ~ _ 1
........
I--
~o-2 ,.% "'...,...,.
""...... --...
o
10-4 0
1
I
I
2
4
G Energy
10
-....
-.....
I
I
12
14
(MeV)
Fig. 1. A b s o r p t i o n cross-section curves for different elements.
326
B.S. Seshadri, R. Venkatesan / Transmission characteristics of Cf 252 neutrons
review of fast neutron transmission through concretes [4]. Details of different types of concretes and their compositions used for the studies, transmitted spectra and dose rate evaluation of relaxation lengths (both experimental and computed) have been reported. Pictet et al. [5] have reported the use of rare-earths to increase the efficiency of a biological shield made of concrete. Rare-earth oxides have also been used in the concrete (density: 2.7 g / c m 3) of the biological shield for Rapsodie [6]. From the review of earlier work, it is observed that the use of rare earth oxides in concretes as additives, the available information is very limited with respect to their attenuation characteristics. N o work has been reported on the spectral degradation in concretes containing rare-earths as additives. Such a study is of particular interest to India because of the easy availability of rare-earth oxides as a byproduct in the Indian monozite industry.
2. Source and spectrometer The Cf 252 source is sealed in a cylindrical aluminium can about 5 cm in diameter and 8 cm in height. It emits 107 n / s with an average energy of 2 MeV. The neutron gamma ray detector consists of NE-213 liquid scintillator contained in a cylindrical plastic container (38 mm × 38 mm) coated on the inside with titanium dioxide paint to increase light reflection. It is mounted on a high gain photomultiplier tube R C A 8850. Neutron and gamma ray events in the detector were separated using pulse shape discrimination methods and stored in separate memory locations in PDP-11 computer. The pulse height data were permanently stored on magnetic disks. The neutron energy measurements range from 1.5 MeV up to 1l MeV. Although it was possible to make measurements down to 0.8 MeV, it was found that gamma ray discrimination of NE-213 spectrometer below 1.8 MeV was not satisfactory. G a m m a to neutron ratio of 1 : 1000 was chosen as the minimum ratio to be maintained in all the measurements. Description of experimental shields as well as the concentration of rare-earths and boron as additives is discussed in detail in ref. [1].
3. Experiment The general schematic of the experimental arrangement is shown in fig. 2. The Cf 2s2 source is mounted at
one end of a mild steel support 125 cm above the ground level in order to minimise background due to ground scattered neutrons. The NE-213 detector and photomultiplier tube assembly was kept at a distance of 125 cm in front of the source. The centres of the concrete blocks were aligned to be in line with the source and detector. The first slab of concrete was placed at a distance of 10 cm from the detector. The subsequent slabs were successively placed in front of the slabs already present. In this way, the shielding thickness could be increased while the detector remained in the same position. Consequently no inverse square law correction had to be made. The slab thickness was increased in steps of 10 cm up to a maximum of 40 cm and counting was done for 1000 seconds for each slab. The experiment was repeated for three types of concretes mentioned earlier. Energy calibration of the NE-213 spectrometor was carried out using the procedure of Verbinski et al. [7]. Since monoenergetic proton sources are not easily available, the spectrometer was calibrated using electrons of known energies. C o m p t o n scattered electrons from the specific energy gamma rays are the most convenient sources in this regard. The compton edge of the gamma rays from CS137, M n 54, N a 22 and TI 2°8 were used as reference points. During calibration and subsequent checks for reproducibility, the system was tested using a 5 Ci A m - B e neutron source kept at a distance of 40 cm from the detector. The recoil proton data was unscrambled using a derivative unfolding code ' D U S T ' based on cubic bell splines [8]. Here the smoothing of input data is obtained by least squares polynomial representation along with continuity conditions on the neutron and recoil proton spectra. The scheme employed leads to enhanced resolution as well as to an improved estimate of the peak positions.
4. Experimental errors The major errors in the measurements are due to counting statistics. Uncertainties in the recorded data are in the range of + 1.5 to + 6.0%, the maximum being the value for 40 cm of concrete and the lower end of measured energy 1.2 MeV. The error in the energy calibration is + 0.5%. Next, the error due to unfolding which includes error in the intrinsic efficiencies and detector resolution. Based on the large number of measurements with the known spectra, the maximum error in the peak values (due to unfolding) is estimated to be + 8%. Hence, the total errors are of the order of + 14%
B.S. Seshadri, R. Venkatesan / Transmission characteristics of C f 252 neutrons
327
7000
"/////////////////////,4
o
/
//
//
// //
/
// // // //
I00 ,400 , 750
2550
§ / /
/
/ /
// // // // // // //
/
// •
/
N E - 213 ~
/
SPECTROMETER
Cf-252
SOURCE
BLOCKS
/
/--SH,ELO WA~L
/
// //, 0.5 mR/ht
3mR/h~
Fig. 2. Experimental arrangement.
3.6
7.2 _
10 c m
........
20 cm
.....
30 cm
_
3.0
E ~. 5.4
.......
~E
L0 cm
2.4
~.
1.8
z
1.2
3.6 Z
\
r
4.8 0.6
0
i 3
3
5
7
g
Energy in MeV Fig. 3. Incident spectrum of Cf 252 neutrons.
11
5
7
9
Energy in MeV
Fig. 4. Transmitted spectra of Cf 252 neutrons through ord concrete (2.4 g / c m 3) of diff. thickness.
328
B.S. Seshadri, R. Venkatesan
around 1.2 MeV and about +10% around 10-12 MeV region.
The incident Cf 252 neutron spectrum as measured at the distance of 40 cm from the source is shown in fig. 3. A dominant peak at 2.4 MeV and two humps at 6.5 MeV and 8 MeV are present. Fig. 4 shows the transmitted spectra for concrete without any additives (density 2.4 g / c m 3) for thickness varying from 10 to 40 cm. similar spectra for concretes of this density with the addition of 1% by weight of gadolinium concentrate, samarium concentrate and boron are shown in figs. 5, 6 and 7 respectively. In addition to overall fall in intensity with increasing thickness, considerable spectral changes take place in the measured energy range. While in the energy range from 1.5 MeV to 4 MeV the peaks become broader, above 4 MeV peaks begin to appear. For 10 cm thick ordinary concrete (2.4 g / c m 3) a peak is observed at 2.3 MeV. The addition of gadolinium, samarium or boron does not change the position or shape of this peak. However boron (20%) causes maximum fall in intensity followed by gadolinium (15%) and then samarium (10%) in that order. Although significant change in spectral behaviour is observed
neutrons
3.6 lOom
3.0
' /
.........
2.4
t,,,,l
30cm .................. 40cm
5. Results and discussion
20cm
.......
g
~1.2 Z
,
o.o
.
1
"","~.~r,~~
,
3
7
5 Energy
in
9
,
11
MeV
Fig. 6. Transmitted spectra of Cf 252 neutrons through concrete (2.4 g / c m 3 + 1% Sm).
addition of gadolinium and samarium reduces the flux marginally up to 6 MeV. For 20 cm thick concrete, the peak position shifts to 2.5 MeV. Here again the addition of boron lowers the fluxes most in the peak region (by 15%) although to a lesser extent than in the case of 10 cm concrete.
3.6
3.6
/~ ~3.0
[ /
.........
~ ~: 2.4
.
~
.
.
.
.
.
.
.
lOom
~ 1 0 c m
20cm
........
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30crn ................ 40cm
. . . . . . . . . . . . . &Ocm
.
rl
u ilJ
1.8
20cm
3.0
3ocm
~ 2.4
/ ",,,"~
i ' =1.2
'
"'
;\, \ "..' ~,
V
3
Z
]
~-~ 1.2 P
~ ....\~\
,i.............
1" 0.0
Cf252
/ Transmission characteristics of
..... ~,','.;:~7,. 5 7
0.6 -- .-
9
11
Energy in iMeV
Fig. 5. Transmitted spectra of Cf 252 neutrons through concrete (2.4 g/cm 3 + 1% Gd).
0.0
F
','\'~,
i
I
3
5
7
9
11
Energy in MeV
Fig. 7. Transmitted spectra of Cf 252 neutrons through concrete (2.4 g/cm 3 + 1% B).
329
B.S. Seshadri, R. genkatesan / Transmission characterisncs of Cf" " neutrons
When the thickness is increased to 30 cm, the peak shifts to 2.6 MeV and is considerably broadened. Boron is marginally more effective than gadolinium or samarium in reducing the flux. For still larger thickness of 40 cm, it is found that gadolinium is superior compared to boron in reducing the neutron flux. In figs. 4 and 5, even at 1.8 MeV a peak is observed, this is due to the interference of gamma rays. The C f 252 source has a large gamma ray background and hence it became necessary to increase the discrimination level. The strobe window was further reduced in the pulse shape discriminator of the NE-213 spectrometer in order to reduce the effect of gamma ray interference. A small shift in the position of 2.3 MeV peak is also attributed to this interference. It may be noted that the appearance of distinct peaks arround 2 MeV, 4.5 MeV, 6.5 MeV and 7.5 MeV is understandable in the light of the energy dependance of the absorption cross section for concrete. The absorption cross section data from DLC-2 [9] for ordinary concrete is shown in fig. 8. The typical transmitted spectrum (for 40 cm thick) with the same
a
o
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group structure is also shown. It may be observed that the peaks in the transmitted spectrum are nearly coincident with the valleys in the cross section. Because of the inherent uncertainties of the relative intensities of the unfolded spectra, only a qualitative comparison is possible. Even in the case of concretes with additives, correlation between the spectral behaviour and the absorption cross section of the corresponding additives is present. In particular, the observed higher effectiveness of gadolinium versus boron in the lower energy peak region matches well with the higher absorption cross section of gadolinium as compared to boron.
6. Comparison with the calculated spectra Transmitted spectra of C f 252 neutrons through the different types of concretes were calculated using the code MORSE-CG'. This is a monte carlo programme with combinatorial geometry option which enables faithful description of the experimental configuration. A point isotropic source was assumed for the calculations. The cross-section data for the constituent ele-
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4 EXPT. S P E C T R U M
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SIGMA
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ABSORPTION
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I 3-00
I
I 5-00
I
I
I
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I
I I1-00
I
I 13-00
ENERGY (MEV)
Fig. 8. Comparison of spectra with cross section (sigma absorption).
I
I 15- O0
330
B.S. Seshadri, R. Venkatesan / Transmission characteristics of Cf 252 neutrons
lo4
10
9-
.......
8-
MORSE
........
MORSE
- - M E A S U R E D
--MEASURED
'1
>~6-
I
! m
5
I
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4 E N ERGY
5
6
7
8
2
9
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Fig. 9. Measured and calculated spectra for ord. concrete (2.4 g/cm 3).
ments of the concrete and additives was taken from the 100 group DLC-2 library. The first 30 groups covering the range between 0.273 MeV to 12 MeV was used, as the measured spectra were from 1.5 MeV upwards. Fig. 9 shows the comparison between measured and calculated spectra for concrete of density 2.4 g / c m 3. Figs. 10, 11 and 12 represent comparisons between measured and calculated spectra for concrete (2.4 g / c m 3) containing 1% by weight of B, Gd and Sm respectively. A typical concrete thickness of 40 cm was chosen to represent the comparison between measured and calculated spectra. Compared to the corresponding measured spectra, this also reveals the changing nature with the different types of concrete. The main peak at 2.35 MeV is very well resolved. The peaks and valleys as observed in experimental measurements are also seen. But in view of the slight difference in group structure in computations, the positions of peaks and valleys are not exactly matched between computations and measurements. The agreement is found to be quite significant in the main peak region of 2.35 MeV. At higher energies (5-10 MeV), the calculated values are higher as compared with measurements by about 20%. This could be due to
3 ENERGY
IN
MeV
Fig. 10. Measured and calculated spectra for concrete (2.4 g/cm 3 + 1% B).
9
8i
...........
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__MEASURED 7
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LL
s
Ld Z
ENERGY IN NleV
Fig. l l. Measured and calculated spectra for concrete (2.4 g/cm 3 + 1% Gd).
B.S. Seshadri, R. Venkatesan / Transmission characteristics of Cf 252 neutrons
331
the fact t h a t anisotropic b e h a v i o u r of the source has not b e e n included in the calculation. Fx
9] 8.
Acknowledgement .
.
.
.
.
.
.
.
.
_
MORSE
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T h e a u t h o r s are t h a n k f u l to D.V. G o p i n a t h , Head, Safety Research P r o g r a m m e for guidance a n d encouragement.
References
~3 f3~
5
ENERGY
IN
MeV
Fig. 12. Measured and calculated spectra for concrete (2.4 g/cm 3 + 1% Sm).
[1] B.S. Seshadri, V. Venkatesam and V. Sundararaman, Transmitted spectra of (D-T) neutrons through rare-earth mixed concretes, Nucl. Engrg. Des. 97, No. 2 (1986) 223-232. [2] V. Lach, Engg. compendium on Rad. Shielding, Vol. II (Jaeger-Verlas, 1975). [3] Adams et al., Health Physics 36 (1979) 671. [4] Fritz A.R. Schmidt, Report ORNL-RSIC 26 (1970). [5] Pictet et al., Reactor Shielding, I.A.E.A. Technical report series 34 (1964) 152. [6] Safety report for Rapsodie, Vol. I, chap. 1, C.E.A. France (in French). [7] Verbinski et al., Calibration of an organic scintillator for neutron spectrometry, Nucl. Inst. and Meth. 65 (1968) 8-25. [8] K.V. Subbiah and A. Natarajan, Derivative unfolding by spline technique, Nucl. Inst. and Meth. 228 (1985) 377-386. [9] DLC-2, 100 group cross-section data based on ENDF/B, RSIC library, Oak Ridge (1974).