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A strong dynamic neutron source based on the 209Bi(γ, n) reaction
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
Nuclear Instruments and Methods in Physics Research A309 (1991) 503-507 North-Holland
Section A
A strong dynamic neutron source based on the R. Moreh
a,
R. Fedorowicz
b,
W.V. Prestwich
b,
T.J. Kennett
b
209Bi(y,
n) reaction
and M.A. Islam
b
Physics Department, Ben-Gurion University, and Nuclear Research Center-Neger, Beer-Shera, Israel n Department of Physics, McMaster University, Hamilton, Ontario, Canada
Received 6 May 1991 and in revised form 8 July 1991
An intense dynamic n-source, I = 10 5 n/s having two n-groups En = 177 keV and 114 keV is reported . It is based on the Bi(y, n) reaction where a chance overlap occurs between a -y-line of the Cu(n, y) reaction and a nuclear level at 7637 keV in 209 13i. The thermal neutrons which induces the Cu(n, y) reaction are obtained from a reactor. The spin, parity and total and radiative widths of the 7637 keV level in Bi were determined . The resonance cross sections for n-production are: o'yn=275±40 nib and 93±14 nib for the 177 keV and 114 keV groups respectively . The resulting dynamic n-source is the most intense which can be produced by this method . 1. Introduction A fast n-source based on the Bi(y, n) reaction is obtained using a photon source generated by the Cu(n, y) reaction using thermal neutrons . It has been known [1] that one of the y-lines at 7637 keV emitted by the Cu(n, y) reaction photoexcites by chance an unbound resonance level in 2°9Bi . This resonance process was first reported by Ben-David et al . [1] who also measured the absolute scattering cross section of the elastically scattered photons, o-yy = 4 nib. The same resonance event was later studied using a Ge(Li) detector by Cesareo et al . [2] and an attempt to determine the spin of this level by measuring the angular distribution of the elastically scattered radiation was made . A more careful study of the spectroscopic properties of this level in 2o9Bi was carried out by Wolf et al . [3] who made an attempt to determine F0 , Fy, and F (being the ground state radiative width, the total radiative width and the neutron width of the level) . This was carried out by measuring: (1) The absolute photon elastic resonance scattering cross section yielding ayy = 4.0 ± 0.7 nib with F,/F, = 1.0 . (2) The angular distribution of the 7637 keV elastically scattered photons which yielded an uncertain determination, J= z, of the spin of the 7637 keV resonance level in 2°9Bi; (3) The ratio of the scattering cross section at T = 297 K and T= 78 K (liquid nitrogen temperature) . (4) The nuclear self-absorption ratio, and (5) the neutrons emitted by the 2°9 Bi(y, n) reaction were also detected, but the energy resolution of the 3He detector used did not permit a good energy separation between the thermal peak and the neutron groups emitted by the effect
of the 7637 keV -y-line on the Bi target . From the above measurements only lower limits on the values of the ground state radiative width Ft and the total radiative width F have been deduced. In the present work, a commercial high resolution 3He neutron detector produced by Seforad was used and the (y, n) cross section was measured using a transmission method and also by comparing the neutron yield emitted in the Bi(y, n) reaction with that of another resonance reaction, 2° 'Pb(y, n) of known cross section [4]. In this latter reaction, the incident y-line, at 7632 keV is generated by a photon source based on the Fe(n, ^l) reaction ; the cross section was reported to be o-y = 0.37 b, with a neutron energy of E = 86 keV. It should be remarked that the (y, n) reaction is a well-known method for generating fast neutron sources. In particular the 'Be(-y, n) reaction was used [5,6] for producing neutrons in the keV range where the photons inducing the reaction were obtained from the 124Sb radioactive source . 2. Experimental method Fig. 1 shows a schematic diagram of the experimental system . The y-source was generated by the (n, y) reaction on 3 kg of metallic Cu filings contained in an aluminum box 5.4 cm square by 18 .4 cm high placed within the core of the McMaster Nuclear Reactor. The -y-beam was neutron filtered by passing it through 56 cm of borated wax, and collimated to a 2 .5 cm diameter at the target position, where the intensity of the 7637 keV -y-line was about 10 5 photons/s. A 2.5 cm
0168-9002/91/$03.50 © 1991 - Elsevier Science Publishers B .V . All rights reserved
504
R Moreh et al . / A strong dynamic neutron source
RNS
wnM1 .G
;ZVI=
0
Reactor Wall Reactor Pool
Cast Iron
Concrete
He-3
Counter
VZOO lead
Paraffin
Plywood
Fig. 1. Schematic diagram (not to scale) of the experimental system showing the photon source, the n-detector, the reactor core, and the shielding. The target which lies below the n-detector is not shown. diameter, 4 cm long cylindrical target of metallic 2°9Bi was used . It was positioned with its axis perpendicular to that of the -y-beam. The angular distribution of the neutrons was measured at three laboratory angles of s
4"lO i
3
CHANNEL NUMBER Fig. 2. Photoneutron spectrum of the 2'Bi(y, n) reaction . The photon source is obtained from the Cu(n, y) reaction . The n-groups at 177 keV and 114 keV are produced by the 7637 keV y-line and correspond to transitions to the ground state and the 63 keV level in 2°8Bi . The n-group at 455 keV is produced by the effect of the 7915 keV -y-line of the Cu -y-source. The channel number of the thermal peak is 270.
57 ° , 90 ° , and 123 ° , by using a cylindrical 3He neutron detector placed with its axis in a horizontal direction and parallel to that of the 2°9Bi target ; it could be rotated around the target at a constant distance of 13 cm . The detector was shielded by a sheet of Cd and 5 mm Pb against the effect of thermal neutrons and soft -y-rays. The energy resolution (FWHM) of the n-detector (manufactured by Seforad Applied Radiation Ltd., of Emek Hayarden, Israel) was around 15 keV for thermal neutrons increasing to around 17 keV at a neutron energy of 177 keV. Fig. 2 shows the energy spectrum of neutrons emitted by the 2°9Bi target where the incident photons are produced by the Cu(n, y) reaction . It shows two strong intensity n-groups at E = 177 keV and 114 keV, identified as being produced by the 7637 keV y-line and leading to the ground and first excited states of 2°sBi . Another n-group at 455 keV was observed which is produced by the intense -y-line at 7915 keV of the Cu(n, y) reaction . 3. Spin and parity of 7637 keV level in 209 Bi Since the 2°9Bi ground state has JO' = 9 -, then the spin of the 7637 keV resonance level, being very likely
R. Moreh et al. / A strong dynamic neutron source
50 5
4. The (y, n) cross section
U') 1 .1 z
w 1-
1 .0
w Q LLJ
9/2~~7637
Po
1 .2 1 .1 - 9/2+
0.9
50
209 Bi
70
En=177
keV
5+" -0 0 208Bi
90
110
130
150
ANGLE[DEG) Fig. 3. Angular distribution of the 177 keV and the 114 keV n-groups as measured using the ; He n-spectrometer . The generation process of the two n-groups is indicated. The solid line passing through the experimental points corresponds to 1 =0.
photoexcited by dipole transition, is expected to have one of the spin and parity values : J~ = z t, z }, or t. The results of the angular distribution of the elastically scattered photons of ref. [3] made an uncertain assignment of J = 9 to this level. This assignment seems to be correct because the resonance level was found to proceed by an isotropic angular distribution, I n = 0, to both the ground, 5 +, and the 63 keV, 4 +, excited level in 2°9 Bi as shown in fig. 3. The fact that I n = 0 also implies that the parity of the resonance level is even and hence J' = z +. It should be emphasized that any other spin assignment, say J = z or ~z would require the neutrons to proceed by 1n > 0, leading to a drastic reduction in the intensity of one of the emitted neutron groups and to a possible non-isotropic angular distribution . The generation process of the two neutron groups is shown in fig. 3.
The (y, n) cross section was measured by a transmission experiment through a Bi absorber and composed to that of a Pb absorber . The result was: o, (y, n,,) = 275 f 40 mb and o-(y, n,) = 93 ± 14 mb . This neutron production cross section is high and leads to the most intense source produced hitherto using neutron capture -y-rays. The high neutron yield is caused by the high intensity of the 7637 keV -y-line producing the n-groups and also by the high natural abundance of the 2°9 Bi isotope emitting the neutrons, being 100% . For comparison, table 1 lists the parameters of another resonance (y, n) reaction which produces another intense source of neutrons. The table shows that the present source is more intense by a factor of 3.4 relative to that of the 207Pb(y, n) reaction which provides a nearly monoenergetic neutron source with an energy of 86 keV . The present source is distinct in the sense that it has a higher energy, higher intensity and has a much lower background of high energy photons relative to that of the 2°7 Pb(1', n) source. This point is discussed in more detail below. It is of interest to note that the ratio of experimental cross sections o-yn,, and o,,n of the two n-groups leading to the ground and first excited state in 2°s Bi may be understood by considering the n-energies and the spins of the final states . This is because the nuclear structure of the ground and the first excited states in 2°sBi are essentially identical, hence the neutron decay intensity of the 7637 keV resonance level in 209 13i is governed by the volume in phase space and by the statistical factor . Thus, the cross-section ratio may be written as : o-Yno
R =-= 0',n
(2Jo±1)El.5 (2Jt ± 1)Elt ~
=2 .3 6
where the subscripts 0 and 1 refer to the ground and first excited states in 2°s Bi and E, refer to the corresponding n-energies . This value of R overlaps, within one standard deviation, the experimental ratio 2.96 ±
Table 1 Comparison between the values of the thermal n-cross section Q, Y , the yield Y of the -y-line (expressed in units of number of photons per 100 neutron captures [9]), the natural abundance a of the emitting isotope in the (y, n) reaction cross section and the neutron yield In . The n-yield was normalized in such a way that In was taken as 100 for the yield from the 2°9 Bi(y, n) reaction . anY
Y [no ./100]
3.8
14 .5
a
2.6
27 .2
a
[b]
2°9 Bi(y,
n)
207pb(y n) Ref. [9]. n Ref. [4]. a
a [%]
100 22.6
QYn,,
QYnI
In
275±40
93±16
100
[mb] 370±50 b
[mb]
29
506
R. Moreh et al. / A strong dynamic neutron source
0.67 obtained from table 1 . This is regarded as a nice agreement in view of all uncertainties involved in the above comparison . 5. Width of the 7637 keV level in
2°9Bî
From the knowledge of the total (y, n) cross section, being 368 mb, and that of o,,,, = 4 mb, the branching ratio TOIT for the decay of the 7637 keV level was deduced using the relation : 0-,n/o-, =T /T = 0.0109, (2) where o-yy and o-, are the elastic photon scattering cross section and the total cross section for the emission of both photons and neutrons . In the present case o-, = Q because cr, = o, + o-yy and o-yy « o", When this value of To/h = 0.0109 is combined with the results of the measurements of the elastic scattering cross section o-yy, and the self absorption ration RT (reported in ref. [3]) it was possible to determine the level width. The details of this method of determination of the level width were described in detail elsewhere [3,5]; here we shall only give a short outline of the method . To do so we first note that in cases where the natural width of the level is larger than its Doppler width A,, namely I' >> D y, the variation of the temperature of the scatterer has no influence on the scattering cross section. Indeed this was found to be true for the present case because the measured ratio RT of the scattered intensities I(78 K)/I(279 K) at 78 K and 297 K was found [3] to be RT = 1 .00 ± 0.02. For such a case the value of S (being the energy distance between the peaks of the 7637 keV line shapes of the Cu(n, y) reaction and the nuclear level in 2°9Bî) can not be determined from a measurement of the temperature effect [3]. We may thus simplify matters by assuming S = 0. It can be shown that the choice of any other value below 3 = 20 eV does not change significantly the present conclusions . To determine the level width we draw a line in the (T , I') plane which contains all points for which the
computed value of the nuclear self-absorption ratio R is equal to the experimental value (reported in ref. [3] to be R = 0.02 + 0.01 obtained for a 10 g/cm 2 thick absorber placed at an angle of 60° and a detector angle of 135 ° with respect to the incident photon beam). Another line in the (To , I') plane is drawn which contains all points for which the computed value of o-yy is equal to the experimental value. It turns out that these two lines approach each other at large values of T and T but do not have a clear intersection region and thus do not yield a unique solution for the values of ro and T. It is only by including a third line in the (To, T) plane corresponding to the value of T,/I' measured in the present work that it was possible to obtain an intersection region representing a unique solution to T and I' with rather large errors : r=40+400
eV, T =0 .44+0
. .20 eV
Details of the calculations necessary for drawing the lines in the (I' , I') plane are described in detail in ref. In this connection it should be remarked that for bound levels, it is usually sufficient to carry out the experiments mentioned in section 1 for an unambiguous determination of To and I' . For unbound levels it is usually necessary to carry out another measurement namely of the absolute (y, n) cross section for the determination of the I', and T. 6. Photon background This dynamic n-source is not free of photon background . Low and high energy photons contribute to the background . The low energy photons arise from the Compton effect, pair production and the other interaction processes of the photon beam on the Bi target . At 90 ° to the photon beam direction, the energy of the scattered photons is in the 500 keV range and their intensity can be drastically reduced using a 5 mm thick Pb absorber, while keeping the n-yield relatively unaffected . The high energy photons have an energy of
Table 2 Factors governing the intensity of the "background" photons emitted by the effect of n-capture gamma rays on Bi and Pb where cr., denotes the resonance photon scattering cross section and L, the resulting intensity of the resonantly scattered 'Y-lines. The other listed parameters were defined in the caption to table 1 . y-Source 209 Bi(y, y) 208 Pb(y, y) a Ref. [9]. b Ref. [3]. ` Ref. [8].
Cu(n, y) Fe(n, y)
EY [keV] 7637 7279
~ny [b]
3.8 2.6
Y [no./100] 14 .5 4 .6
a
a
a [%]
100 52 .4
ayy [mb]
4.0 b 5560 `
I 0.64 100
R. Moreh et al. / A strong dynamic neutron source
7637 keV and are contributed by elastic resonance scattering on z° . Bi . The corresponding photon intensity is much lower than that of the 86 keV n-source based on the Pb(y, n) reaction [4]. In this latter source, the high energy background photons occur at 7279 keV and are generated by a chance overlap of the 7279 keV line of the Fe(n, y) source with a nuclear level in 208 Pb, which just happens to have the highest known resonance scattering intensity [6]. Table 2 lists the important parameters governing the intensity of the high energy photon background emitted by the above two neutron sources; the figures show that the intensity of the 7637 keV photons emitted by the Bi source is a factor 160 lower than that of the 7279 keV photons. In this respect the present n-source has a much better quality than that of the 86 keV source.
50 7
References [1] B. Ben-David, B. Arad, J. Balderman and Y. Schlesinger, Phys . Rev. 146 (1966) 852. [2] R. Cesareo, M. Giannini, P. Oliva, D. Prosperi and M.C . Ramorino, Nucl . Phys . A132 (1969) 512. [3] A. Wolf, R. Moreh and O. Shahal, Nucl . Phys . A227 (1974) 373. [4] R. Moreh, Y. Birenbaum and Z. Berant, Nucl . Instr. and Meth . 155 (1978) 429. [5] A.O . Hanson, in : Fast neutron Physics, part I, eds. J.B . Marion and J.L. Fowler (Interscience, New York, 1960) p. 1. [6] S.E. Binney et al ., Nucl . Instr. and Meth 97 (1971) 139. [7] R. Moreh, Nucl . Instr. and Meth . 166 (1979) 69 . [8] R. Moreh, S. Shlomo and A. Wolf, Phys . Rev. C2 (1970) 1144 . G.A . Bartholowmew et al ., Nucl . Data A3 (1967) 367
Nuclear Instruments and Methods in Physics Research A309 (1991) 503-507 North-Holland
Section A
A strong dynamic neutron source based on the R. Moreh
a,
R. Fedorowicz
b,
W.V. Prestwich
b,
T.J. Kennett
b
209Bi(y,
n) reaction
and M.A. Islam
b
Physics Department, Ben-Gurion University, and Nuclear Research Center-Neger, Beer-Shera, Israel n Department of Physics, McMaster University, Hamilton, Ontario, Canada
Received 6 May 1991 and in revised form 8 July 1991
An intense dynamic n-source, I = 10 5 n/s having two n-groups En = 177 keV and 114 keV is reported . It is based on the Bi(y, n) reaction where a chance overlap occurs between a -y-line of the Cu(n, y) reaction and a nuclear level at 7637 keV in 209 13i. The thermal neutrons which induces the Cu(n, y) reaction are obtained from a reactor. The spin, parity and total and radiative widths of the 7637 keV level in Bi were determined . The resonance cross sections for n-production are: o'yn=275±40 nib and 93±14 nib for the 177 keV and 114 keV groups respectively . The resulting dynamic n-source is the most intense which can be produced by this method . 1. Introduction A fast n-source based on the Bi(y, n) reaction is obtained using a photon source generated by the Cu(n, y) reaction using thermal neutrons . It has been known [1] that one of the y-lines at 7637 keV emitted by the Cu(n, y) reaction photoexcites by chance an unbound resonance level in 2°9Bi . This resonance process was first reported by Ben-David et al . [1] who also measured the absolute scattering cross section of the elastically scattered photons, o-yy = 4 nib. The same resonance event was later studied using a Ge(Li) detector by Cesareo et al . [2] and an attempt to determine the spin of this level by measuring the angular distribution of the elastically scattered radiation was made . A more careful study of the spectroscopic properties of this level in 2o9Bi was carried out by Wolf et al . [3] who made an attempt to determine F0 , Fy, and F (being the ground state radiative width, the total radiative width and the neutron width of the level) . This was carried out by measuring: (1) The absolute photon elastic resonance scattering cross section yielding ayy = 4.0 ± 0.7 nib with F,/F, = 1.0 . (2) The angular distribution of the 7637 keV elastically scattered photons which yielded an uncertain determination, J= z, of the spin of the 7637 keV resonance level in 2°9Bi; (3) The ratio of the scattering cross section at T = 297 K and T= 78 K (liquid nitrogen temperature) . (4) The nuclear self-absorption ratio, and (5) the neutrons emitted by the 2°9 Bi(y, n) reaction were also detected, but the energy resolution of the 3He detector used did not permit a good energy separation between the thermal peak and the neutron groups emitted by the effect
of the 7637 keV -y-line on the Bi target . From the above measurements only lower limits on the values of the ground state radiative width Ft and the total radiative width F have been deduced. In the present work, a commercial high resolution 3He neutron detector produced by Seforad was used and the (y, n) cross section was measured using a transmission method and also by comparing the neutron yield emitted in the Bi(y, n) reaction with that of another resonance reaction, 2° 'Pb(y, n) of known cross section [4]. In this latter reaction, the incident y-line, at 7632 keV is generated by a photon source based on the Fe(n, ^l) reaction ; the cross section was reported to be o-y = 0.37 b, with a neutron energy of E = 86 keV. It should be remarked that the (y, n) reaction is a well-known method for generating fast neutron sources. In particular the 'Be(-y, n) reaction was used [5,6] for producing neutrons in the keV range where the photons inducing the reaction were obtained from the 124Sb radioactive source . 2. Experimental method Fig. 1 shows a schematic diagram of the experimental system . The y-source was generated by the (n, y) reaction on 3 kg of metallic Cu filings contained in an aluminum box 5.4 cm square by 18 .4 cm high placed within the core of the McMaster Nuclear Reactor. The -y-beam was neutron filtered by passing it through 56 cm of borated wax, and collimated to a 2 .5 cm diameter at the target position, where the intensity of the 7637 keV -y-line was about 10 5 photons/s. A 2.5 cm
0168-9002/91/$03.50 © 1991 - Elsevier Science Publishers B .V . All rights reserved
504
R Moreh et al . / A strong dynamic neutron source
RNS
wnM1 .G
;ZVI=
0
Reactor Wall Reactor Pool
Cast Iron
Concrete
He-3
Counter
VZOO lead
Paraffin
Plywood
Fig. 1. Schematic diagram (not to scale) of the experimental system showing the photon source, the n-detector, the reactor core, and the shielding. The target which lies below the n-detector is not shown. diameter, 4 cm long cylindrical target of metallic 2°9Bi was used . It was positioned with its axis perpendicular to that of the -y-beam. The angular distribution of the neutrons was measured at three laboratory angles of s
4"lO i
3
CHANNEL NUMBER Fig. 2. Photoneutron spectrum of the 2'Bi(y, n) reaction . The photon source is obtained from the Cu(n, y) reaction . The n-groups at 177 keV and 114 keV are produced by the 7637 keV y-line and correspond to transitions to the ground state and the 63 keV level in 2°8Bi . The n-group at 455 keV is produced by the effect of the 7915 keV -y-line of the Cu -y-source. The channel number of the thermal peak is 270.
57 ° , 90 ° , and 123 ° , by using a cylindrical 3He neutron detector placed with its axis in a horizontal direction and parallel to that of the 2°9Bi target ; it could be rotated around the target at a constant distance of 13 cm . The detector was shielded by a sheet of Cd and 5 mm Pb against the effect of thermal neutrons and soft -y-rays. The energy resolution (FWHM) of the n-detector (manufactured by Seforad Applied Radiation Ltd., of Emek Hayarden, Israel) was around 15 keV for thermal neutrons increasing to around 17 keV at a neutron energy of 177 keV. Fig. 2 shows the energy spectrum of neutrons emitted by the 2°9Bi target where the incident photons are produced by the Cu(n, y) reaction . It shows two strong intensity n-groups at E = 177 keV and 114 keV, identified as being produced by the 7637 keV y-line and leading to the ground and first excited states of 2°sBi . Another n-group at 455 keV was observed which is produced by the intense -y-line at 7915 keV of the Cu(n, y) reaction . 3. Spin and parity of 7637 keV level in 209 Bi Since the 2°9Bi ground state has JO' = 9 -, then the spin of the 7637 keV resonance level, being very likely
R. Moreh et al. / A strong dynamic neutron source
50 5
4. The (y, n) cross section
U') 1 .1 z
w 1-
1 .0
w Q LLJ
9/2~~7637
Po
1 .2 1 .1 - 9/2+
0.9
50
209 Bi
70
En=177
keV
5+" -0 0 208Bi
90
110
130
150
ANGLE[DEG) Fig. 3. Angular distribution of the 177 keV and the 114 keV n-groups as measured using the ; He n-spectrometer . The generation process of the two n-groups is indicated. The solid line passing through the experimental points corresponds to 1 =0.
photoexcited by dipole transition, is expected to have one of the spin and parity values : J~ = z t, z }, or t. The results of the angular distribution of the elastically scattered photons of ref. [3] made an uncertain assignment of J = 9 to this level. This assignment seems to be correct because the resonance level was found to proceed by an isotropic angular distribution, I n = 0, to both the ground, 5 +, and the 63 keV, 4 +, excited level in 2°9 Bi as shown in fig. 3. The fact that I n = 0 also implies that the parity of the resonance level is even and hence J' = z +. It should be emphasized that any other spin assignment, say J = z or ~z would require the neutrons to proceed by 1n > 0, leading to a drastic reduction in the intensity of one of the emitted neutron groups and to a possible non-isotropic angular distribution . The generation process of the two neutron groups is shown in fig. 3.
The (y, n) cross section was measured by a transmission experiment through a Bi absorber and composed to that of a Pb absorber . The result was: o, (y, n,,) = 275 f 40 mb and o-(y, n,) = 93 ± 14 mb . This neutron production cross section is high and leads to the most intense source produced hitherto using neutron capture -y-rays. The high neutron yield is caused by the high intensity of the 7637 keV -y-line producing the n-groups and also by the high natural abundance of the 2°9 Bi isotope emitting the neutrons, being 100% . For comparison, table 1 lists the parameters of another resonance (y, n) reaction which produces another intense source of neutrons. The table shows that the present source is more intense by a factor of 3.4 relative to that of the 207Pb(y, n) reaction which provides a nearly monoenergetic neutron source with an energy of 86 keV . The present source is distinct in the sense that it has a higher energy, higher intensity and has a much lower background of high energy photons relative to that of the 2°7 Pb(1', n) source. This point is discussed in more detail below. It is of interest to note that the ratio of experimental cross sections o-yn,, and o,,n of the two n-groups leading to the ground and first excited state in 2°s Bi may be understood by considering the n-energies and the spins of the final states . This is because the nuclear structure of the ground and the first excited states in 2°sBi are essentially identical, hence the neutron decay intensity of the 7637 keV resonance level in 209 13i is governed by the volume in phase space and by the statistical factor . Thus, the cross-section ratio may be written as : o-Yno
R =-= 0',n
(2Jo±1)El.5 (2Jt ± 1)Elt ~
=2 .3 6
where the subscripts 0 and 1 refer to the ground and first excited states in 2°s Bi and E, refer to the corresponding n-energies . This value of R overlaps, within one standard deviation, the experimental ratio 2.96 ±
Table 1 Comparison between the values of the thermal n-cross section Q, Y , the yield Y of the -y-line (expressed in units of number of photons per 100 neutron captures [9]), the natural abundance a of the emitting isotope in the (y, n) reaction cross section and the neutron yield In . The n-yield was normalized in such a way that In was taken as 100 for the yield from the 2°9 Bi(y, n) reaction . anY
Y [no ./100]
3.8
14 .5
a
2.6
27 .2
a
[b]
2°9 Bi(y,
n)
207pb(y n) Ref. [9]. n Ref. [4]. a
a [%]
100 22.6
QYn,,
QYnI
In
275±40
93±16
100
[mb] 370±50 b
[mb]
29
506
R. Moreh et al. / A strong dynamic neutron source
0.67 obtained from table 1 . This is regarded as a nice agreement in view of all uncertainties involved in the above comparison . 5. Width of the 7637 keV level in
2°9Bî
From the knowledge of the total (y, n) cross section, being 368 mb, and that of o,,,, = 4 mb, the branching ratio TOIT for the decay of the 7637 keV level was deduced using the relation : 0-,n/o-, =T /T = 0.0109, (2) where o-yy and o-, are the elastic photon scattering cross section and the total cross section for the emission of both photons and neutrons . In the present case o-, = Q because cr, = o, + o-yy and o-yy « o", When this value of To/h = 0.0109 is combined with the results of the measurements of the elastic scattering cross section o-yy, and the self absorption ration RT (reported in ref. [3]) it was possible to determine the level width. The details of this method of determination of the level width were described in detail elsewhere [3,5]; here we shall only give a short outline of the method . To do so we first note that in cases where the natural width of the level is larger than its Doppler width A,, namely I' >> D y, the variation of the temperature of the scatterer has no influence on the scattering cross section. Indeed this was found to be true for the present case because the measured ratio RT of the scattered intensities I(78 K)/I(279 K) at 78 K and 297 K was found [3] to be RT = 1 .00 ± 0.02. For such a case the value of S (being the energy distance between the peaks of the 7637 keV line shapes of the Cu(n, y) reaction and the nuclear level in 2°9Bî) can not be determined from a measurement of the temperature effect [3]. We may thus simplify matters by assuming S = 0. It can be shown that the choice of any other value below 3 = 20 eV does not change significantly the present conclusions . To determine the level width we draw a line in the (T , I') plane which contains all points for which the
computed value of the nuclear self-absorption ratio R is equal to the experimental value (reported in ref. [3] to be R = 0.02 + 0.01 obtained for a 10 g/cm 2 thick absorber placed at an angle of 60° and a detector angle of 135 ° with respect to the incident photon beam). Another line in the (To , I') plane is drawn which contains all points for which the computed value of o-yy is equal to the experimental value. It turns out that these two lines approach each other at large values of T and T but do not have a clear intersection region and thus do not yield a unique solution for the values of ro and T. It is only by including a third line in the (To, T) plane corresponding to the value of T,/I' measured in the present work that it was possible to obtain an intersection region representing a unique solution to T and I' with rather large errors : r=40+400
eV, T =0 .44+0
. .20 eV
Details of the calculations necessary for drawing the lines in the (I' , I') plane are described in detail in ref. In this connection it should be remarked that for bound levels, it is usually sufficient to carry out the experiments mentioned in section 1 for an unambiguous determination of To and I' . For unbound levels it is usually necessary to carry out another measurement namely of the absolute (y, n) cross section for the determination of the I', and T. 6. Photon background This dynamic n-source is not free of photon background . Low and high energy photons contribute to the background . The low energy photons arise from the Compton effect, pair production and the other interaction processes of the photon beam on the Bi target . At 90 ° to the photon beam direction, the energy of the scattered photons is in the 500 keV range and their intensity can be drastically reduced using a 5 mm thick Pb absorber, while keeping the n-yield relatively unaffected . The high energy photons have an energy of
Table 2 Factors governing the intensity of the "background" photons emitted by the effect of n-capture gamma rays on Bi and Pb where cr., denotes the resonance photon scattering cross section and L, the resulting intensity of the resonantly scattered 'Y-lines. The other listed parameters were defined in the caption to table 1 . y-Source 209 Bi(y, y) 208 Pb(y, y) a Ref. [9]. b Ref. [3]. ` Ref. [8].
Cu(n, y) Fe(n, y)
EY [keV] 7637 7279
~ny [b]
3.8 2.6
Y [no./100] 14 .5 4 .6
a
a
a [%]
100 52 .4
ayy [mb]
4.0 b 5560 `
I 0.64 100
R. Moreh et al. / A strong dynamic neutron source
7637 keV and are contributed by elastic resonance scattering on z° . Bi . The corresponding photon intensity is much lower than that of the 86 keV n-source based on the Pb(y, n) reaction [4]. In this latter source, the high energy background photons occur at 7279 keV and are generated by a chance overlap of the 7279 keV line of the Fe(n, y) source with a nuclear level in 208 Pb, which just happens to have the highest known resonance scattering intensity [6]. Table 2 lists the important parameters governing the intensity of the high energy photon background emitted by the above two neutron sources; the figures show that the intensity of the 7637 keV photons emitted by the Bi source is a factor 160 lower than that of the 7279 keV photons. In this respect the present n-source has a much better quality than that of the 86 keV source.
50 7
References [1] B. Ben-David, B. Arad, J. Balderman and Y. Schlesinger, Phys . Rev. 146 (1966) 852. [2] R. Cesareo, M. Giannini, P. Oliva, D. Prosperi and M.C . Ramorino, Nucl . Phys . A132 (1969) 512. [3] A. Wolf, R. Moreh and O. Shahal, Nucl . Phys . A227 (1974) 373. [4] R. Moreh, Y. Birenbaum and Z. Berant, Nucl . Instr. and Meth . 155 (1978) 429. [5] A.O . Hanson, in : Fast neutron Physics, part I, eds. J.B . Marion and J.L. Fowler (Interscience, New York, 1960) p. 1. [6] S.E. Binney et al ., Nucl . Instr. and Meth 97 (1971) 139. [7] R. Moreh, Nucl . Instr. and Meth . 166 (1979) 69 . [8] R. Moreh, S. Shlomo and A. Wolf, Phys . Rev. C2 (1970) 1144 . G.A . Bartholowmew et al ., Nucl . Data A3 (1967) 367