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Photodisintegration of A127
Nuclear Physics 22 (1961) 207-215;©North-Holland Publishing Co ., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
PHOTODISINTEGRATION OF A127 (I) Photoneutron Cross Section J . E. E. BAGLIN, M. N. THOMPSON and B. M. SPICER Physics Department, University of Aielbourne Received 3 October 1960 Abstract : The AI 27(y, n) cross section has been observed up to 18 .5 MeV with ge?d resolution . It contains a plateau at 14 MeV and a hump at 16 MeV, and is consistent with recent high resolution measurements of the total absorption cross section .
l . Introduction The photonuclear giant resonance in heavy non-spherical nuclei was observed to contain two peaks (see Fuller and Weiss 1 ), Spicer et al. ?.), Pars- ns 3 ), Thies and Spicer 4)) . This splitting was initially interpreted in terms of the collective model as applied to deformed nuclei by Danos s) and Oka*1oto s) . It has, however, been pointed out by Litherland et al . 7 ) and Bromley s) that a collective model can also be applied to nuclei as light as A125 and Mg2--5 , and Spicer 9) has applied this model to the photodisintegration of several light nuclei. é A search has been made for a similar resonance splitting in A1 27 by o )serving the (y, n) cross section. This nucleus was chosen because of its, positive deformation, and its availability as a 100 % pure isotope. Prior to 1959, determinations of the (y, n) and (y, p) cross sections lacked the resolution required to test adequately the applicability of the DanosOkamoto theory to the aluminium nucleus (Diven and Almy 1°), Katz and Cameron 11), Montalbetti et al . 12), Ferrero et al. 13), Halpern and Mann 14)) . The purpose of the present experiment was to determine the (y, n) cross section with sufficient resolution to make this test. 2. Neutron Yield Determination The aluminium target was spectroscopically pure (largest impurity 0.01 Cu) and was in the form of a cylinder 3.85 cm in diameter, 13 .5 crn long, and weighing 421 g. Using the Melbourne 19 MeV synchrotron, the yield of photoneutrons from the target was determined for bremsstrahlung with peak energies ranging from 11 .6 MeV to 18.6 MeV, the measurements being made at 0.25 MeV intervals. 207
208
).
B. B . BAGLIN
et ai.
The radiation dose was measured with a 25r Victoreen thimble used as a transmission ionization chamber. This chamber had been previously calibrated relative to the "lucite r6ntgen" (see Johns et al . 1b)) in this laboratory . Neutrons were detected with Bl°-enriched BF3 counters in a paraffin house similar to that described by Spicer et al. 2) . However, some modification of the counter-target separation was found to be necessary. Tests carried out using sources of neutrons of different energies indicated that a separation of target and counter centres of 10 cm rather than the previous 13 cm gave a detection efficiency more nearly insensitive to initial neutron energy. Because of the relatively low yield of neutrons, and the accuracy required, every care was taken to reduce personal and random errors to a minimum. Counter efficie-acy was checked before and after each run, using a Ra-Be neutron source. The Victoreen thimble was periodically checked, and showed no detectable leakage. A chart record of transient fluctuations in the peak synchrotron magnetic field was used to apply an effective energy correction to each run. Measurements at each energy were performed several times, in order to reduce the effects of small variations in the machine energy, and statistical fluctuations. Each point was the result of from 12 to 18 measurements, involving a count from 4 X 104 to 106 neutrons. 3. Background Determination The yield of background neutrons was determined in the same way as the total yield, but with the Al target removed, and at 16 MeV it amounted to 20 of the total yield. This decreased to 0.7 % at 18 MeV, and increased to 50 at 14 MeV. At energies below the A127(y, n) threshold, the yield with the target in position was observed to be 30 % higher than the background yield with no target. This increase in yield was attributed to neutrons produced in the lead collimator and platinum target, which travelled down the beam hole and were scattered by the Al target. To test this assumption the Al target was replaced by one of spectroscopically pure carbon, which was designed to have the same scattering characteristics for an incident evaporation spectrum of neutrons. With .this target in position, the yield Yc was determined . The photoneutron yield Y13 from the C13 present was calculated from the results of Cook 16) , and subtracted from Yc , leaving a nett yield which was assumed to be the sum of the background with no target YBG, and the scattered yield Y., Below the A127 (y, n) threshold, Yse was found to be identical with the increment in background due to the presence of the aluminium target, and was proportional to YBG at all energies . This confirms the hypothesis that the increment in the case of aluminium was, in fact, also a scattering effect. The results are summarized in table 1, where YBG+Yec is to be compared with YBG X 1 .31 .
PHOTODISINTEGRATION OF A1
27
209
(I)
Thus the effective background with the Al target in position could be derived, above or below threshold, by multiplying YBG by a constant factor (1.31 in this case) . TABLE 1
Background determination Synchrotron energy (MeV) 17 .35 15 .35 13 .35 12 .35
YsG
147±3 97±2 49±3 29±6
YC 255±4 182±6 92±1 60±4
Y13
YsG -f- Y190 = YC - Y13
YSC
YsG X 1 .31
66±2 53±2 35±2 27±2
189±6 139±6 57±5 33±4
42±5 43±6 8±4 4±4
193±4 133±3 64±4 38±1
To show in another way that the increase in yield below (y, n) threshold was due to scattered neutrons, a 10 cm length of pure graphite was placed between the lead collimator and the paraffin counting block. This was sufficient to scatter all neutrons which would normally travel down the beam hole. With the graphite in position the yields below the A127 (y, n) threshold, with and without the Al target in position were identical. Above the threshold the agreement between the yields, as determined, fully supported the constant background multiplying factor (1 .31) referred to above. 4. Absolute Yield The yield has been standardized relative to the yield from natural copper 17) . The photoneutron spectra from copper and aluminium are predominantly evaporation spectra, and both differ markedly from the spectrum of neutrons given by a radium-oe-beryllium source. Therefore a comparison of aluminium photoneutron yield with that from copper was used to set an absolute yield scale, rather than a comparison with the Ra-Be source. This course of action has the further advantage that other laboratories follow this same procedure of standardization. A target was constructed from discs of electrolytic copper. The total thickness of copper was calculated to have the same atomic absorption of y-rays above 10 MeV, and the separation of the discs was such as to give the same total length as the Al target, thus ensuring identical conditions for neutron production and detection. The copper yield was measured at 17 NIeV, 15 MeV, and 13 MeV, and normalized as above. 5. Cross Section Yield points were derived by subtracting the modified background points from the total yield values, and allowing for counting geometry and atomic
210
J. E. E. BAGLIN 8a
al .
are-plotted in fig. 1 . absorption of the y-rays in the aluminium . The yield points statistics. The errors shown are standard errors, and not due only to counting They therefore include the effects of synchrotron energy variations. The yield points shown in fig. 1 were analyzed in 0.5 MeV steps, using the method of Penfold and Leiss 115) . No smoothing of any kind was used. This gave two independent sets of cross section points interlaced at 0.25 MeV intervals, each point having its own standard error computed from the standard errors of the unsmoothed yield data. The cross section points are shown in fig. 2, together with a smooth curve fitted by eye. A curve of best fit was also computed using Fourier synthesis up to the 11th order harmonics, and this agreed almost exactly with that drawn by eye . To test the statistical validity of the cross section curve drawn, a MonteCarlo technique was used, similar to that described by Thies 19) . A set of six cross section curves was synthesized by adding random statistical variations to each point on the cross section curve drawn through the experimental points. Each of these six exhibited the structure shown by the curve in fig. 2, and all were within the limits of error shown by the dotted lines. Considering the experimental errors shown in fig. 2, and using the theory of Thies, the resolution was calculated to be (0.7 MeV) -1. This indicates that the plateau region between 13.7 MeV and 14.7 MeV, the hump at 16.25 MeV, and the increasing cross section up to 18.5 MeV are all real. The integrated cross section of the A127 (y, n) reaction from threshold up to 18.5 MeV was found to be 28 MeV-mbarn. 6. Other All' Photodisintegration Experiments Other measurements of the A127 (y, n) cross section by direct neutron detection have been made by Montalbetti, Katz and Goldemberg 12), Ferrero et al. 13) and Gusakow, 20) . To compare the resolutions of these experiments with the present one, some estimate of the standard errors in yield determination must be made. In the case of Montalbetti et al., a standard` error of ±2 % was estimated from the distribution of the measured points about the smooth curve used in their analysis . Ferrero has indicated that the statistical errors in his yield determination rknged from 1 % to 1j %. According to Thies 19) standard errors of this magnitude would allow peaks separated by 3 MeV or more to be resolved with 85 % assurance. However, this is insufficient resolution to observe the structure at 14 and 16 MeV. The other cross section mentioned, is that determined by Gusakow . This reveals some structure near 17 MeV and shows evidence of the plateau region at about 14 MeV. It is difficult to assign an experimental resolution to this determination, since arbitrary yield curve smoothing was employed in the cross
PHOTODISINTEGRATION OF A127
Ô r 2
(I)
211
8
r Π7 W W
86 W a5
0
04
W z v W
3 2
0
Wz
1
i
14
0 13
15 16 17 SYNCHROTRON ENERGY
18
Mè4
Fig . 1 . Measured yield of photo-neutrons from aluminium, plotted as a function of synchrotron energy.
14 12 E z 0
10
w e 0 0 6
U
13
14
15
115 PHOTON ENERGY
17
18 MeV
Fig. 2. Cross section for the A127 (y, n) A1 26 reaction plotted as a function of photon energy The solid line is the curve of best fit consistent with the experimental points . The dotted lines define an estimated error band (see text) .
1 . E. E. BAGLIN Ci al.
12
section analysis . The structure drawn would imply a resolution better than (2.5 McV)`l. In light nuclei the (y, n) cross section does not necessarily reflect the behaviour of the total photo-absorption cross section, as it does for heavy nuclei. Thus to compare experimental results with the relevant theoretical predictions either a measurement of the À127 (y, p) cross section (the other major photo-process for A12?), or a direct determination of the absorption cross section is required. The only determination of the A127 (y, p) cross section is that of Halpern and Mann 14), which is not sufficiently accurate to resolve the structure believed to be present. However, several measurements have recently been made of the 260
0tr z F
50
w VI
(n 40 N
O u z 0
30
20 0 N m a 10
J
Q O
J
PHOTON ENERGY
24
26 MeV
Fig. 3 . Cross section for photo-absorption in aluminium obtained from an average of the determinations of Mihailovic and Ziegler weighted according to their resolutions.
total absorption cross section . The most detailed of these, by Mihailovic 21 ), is estimated to have a resolution of (1 McV)-1, and that of Ziegler 22 ), (3 McV)-1. The third determination by Kockum and Starfelt 2s) has poor resolution, but the experimental points are consistent with the other two determinations . The peak absorption cross section indicated by Mihailovic is 80 mb and that by Ziegler 35 mb ; yet the sum of experimental (y, n) and (y, p) peak cross sections is about 40 mb. Reconciliation can be brought about if it is remembered that the determination of the nuclear absorption cross section involves an allowance for atomic absorption. This is very large, and it is possible for the zero of the nuclear absorption cross section scale to be wrongly placed. For this reason the zero of Mihailovic's scale has been shifted up by 20 mb and that of Ziegler's down by 15 mb. When this is done a reasonable over-all agreement between the curves can be obtained. An average absorption cross section for A127 is shown in fig. 3, this being the mean of determinations by Mihailovic and Ziegler, weighted according to the experimental resolution. The structure evident in this absorption cross section consists of peaks at 16 McV,18.3 MeV and
PHOTODISINTEGRATION OF A127 (I)
21 3
20.3 MeV, and it is more complex than that expected from the Danos-Okamoto theory. There is a definite correspondence between the 16 MeV hump, and the rise up to 18 MeV occurring in this average cross section, and the same features in the (y, n) cross section reported here. This (y, n) cross section is also consistent with the recent absorption cross section results of Ziegler 24) . 7. The Ratio Q(y, p)/a(7, n) The plateau in the (y, n) cross section about 14 MeV is not evident in the total absorption cross section. It is postulated that this is a result of suppression of neutron emission relative to proton emission just above the (y, n) threshold.
12
'210 E Ô 8 t= U W N
6
O U c
4
v
13
14
1
15
1
16 ENERGY
0
17
MeV
Fig. 4. Points show the A121 (y, n) cross section deduced from the total absorption cross-section assuming neutrons to be emitted with l = 3 near threshold. The line indicates the observed pAotonuclear cross section.
This results from the low penetrability of the centrifugal barrier by the neutrons emitted with large orbital angular momentum. The ground state of A127 has spin and parity 2+. The fact that the known energy levels of Mg26 and A126 below 6 MeV have even parity indicates that for electric dipole excitations of the A1 27 nucleus up to 15 MeV the photo-nucleons must be emitted with odd orbital angular momentum. The ratio of photoneutron to photoproton cross-sections was calculated taking into account penetration probabilities of the centrifugal barrier (see Blatt and Weisskopf 25)) for both particles, and of the Coulomb barrier for protons 26) .
1.
B. IL BAGLIN
et al.
Assuming that neutron and proton emission are the only decay modes of the photo-excited nucleus, a theoretical (y, n) cross section was deduced .-om the experimental absorption cross section using the calculated ratio o(y, p) /a (y, n), (Y
= abs.
1
1+
Q (Y, n)
The assumption that all nucleons were emitted with orbital angular momentum t = 3 gave the curve shown in fig. 4. The agreement with the measured (y, n) cross section is good. No such agreement was obtained for any other value of l. 8. Summary Both the present experiment and the total nuclear absorption cross-section obtained by Mihailovic indicate that the photon absorption cross-section of aluminium contains structure which is more complex than can be allowed on the Danos-Okamoto theory. It has been shown that the results of this experiment agree well with those of Mihailovic . A general discussion of the photodisintegration of aluminium in terms of the model of single particle motion in a non-spherical potential is given in the accompanying paper (see page 216) . References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)
E. G. Fuller and M. S. Weiss, Phys. Rev . 112 (1958) 560 Spicer, Thies, Baglin and Allum, Aust. J . Phys. 11 (1958) 298 R. W. Parsons and L. Katz, Can . J. Phys. 37 (1959) 809 H . H. Thies and B. M. Spicer, Aust. J. Phys. (1960) (to be published) M. Danos, Nuclear Physics 5 (1958) 23 K. Okamoto, Phys. Rev. 110 (1958) 143 Litherland, Paul, Bromley and Gove, Can . J. Phys. 36 (1958) 378 Bromley, Gove and Litherland, Can. J . Phys. 35 (1957) 1057 B. M. Spicer, Aust. J. Phys. 11 (1958) 490 B. C. Diven and G. M. Almy, Phys . Rev . 80 (1950) 407 L. Katz and A. G. W. Cameron, Phys. Rev. 84 (1951) 115 Montalbetti, Katz and Goldemberg, Phys. Rev . 91 (1953) 659 rerrero, Malvano, Mendari and Terracini, Nuclear Physics 9 (1958-59) 32 J . Halpern and A. K. Mann, Phys. Rev. 83 (1951) 370 Johns, Katz, Douglas and Haslam, Phys. Rev. 80 (1950) 1062 B. C. Cook, Phys. Rev. 106 (1957) 300 King, Haslam and Parsons, Can . J. Phys. 38 (1960) 231 A. S. Penfold and J . E. Leiss, Analysis of Photo Cross Sections (University of Illinois, 1958) H. H . Thies, Aust. J . Phys. (1960) (to e published) M. Gusakow, Ph. D. Thesis, Faculty of Science, University of Paris (1958) M. V. Mihailovic (1959), private communication ; see also: Dular, Kernel, Kregar, Mihailovic, Pregl, Rosina and Zupancic, Nuclear Physics 14 (1959) 131
PHOTODISINTEGRATION OF A127 (I)
215
B. Ziegler, Zeits . f. Phys. 152 (1958) 566 J . Kockum and N. Starfelt, Nuclear Instruments and Methods 5 (1959) 37 B. Ziegler, Nuclear Physics 17 (1960) 238 J . M. :Blatt and V. F. Weisskopf, Theoretical Nuclear Physics, (J. Wiley and Sons, New York, 1952) 26) Feshbach, Shapiro and Weisskopf, Tables of penetrabilities for charged particle reactions, NYO-3077 (1953) 22) 23) 24) 25)
PHOTODISINTEGRATION OF A127 (I) Photoneutron Cross Section J . E. E. BAGLIN, M. N. THOMPSON and B. M. SPICER Physics Department, University of Aielbourne Received 3 October 1960 Abstract : The AI 27(y, n) cross section has been observed up to 18 .5 MeV with ge?d resolution . It contains a plateau at 14 MeV and a hump at 16 MeV, and is consistent with recent high resolution measurements of the total absorption cross section .
l . Introduction The photonuclear giant resonance in heavy non-spherical nuclei was observed to contain two peaks (see Fuller and Weiss 1 ), Spicer et al. ?.), Pars- ns 3 ), Thies and Spicer 4)) . This splitting was initially interpreted in terms of the collective model as applied to deformed nuclei by Danos s) and Oka*1oto s) . It has, however, been pointed out by Litherland et al . 7 ) and Bromley s) that a collective model can also be applied to nuclei as light as A125 and Mg2--5 , and Spicer 9) has applied this model to the photodisintegration of several light nuclei. é A search has been made for a similar resonance splitting in A1 27 by o )serving the (y, n) cross section. This nucleus was chosen because of its, positive deformation, and its availability as a 100 % pure isotope. Prior to 1959, determinations of the (y, n) and (y, p) cross sections lacked the resolution required to test adequately the applicability of the DanosOkamoto theory to the aluminium nucleus (Diven and Almy 1°), Katz and Cameron 11), Montalbetti et al . 12), Ferrero et al. 13), Halpern and Mann 14)) . The purpose of the present experiment was to determine the (y, n) cross section with sufficient resolution to make this test. 2. Neutron Yield Determination The aluminium target was spectroscopically pure (largest impurity 0.01 Cu) and was in the form of a cylinder 3.85 cm in diameter, 13 .5 crn long, and weighing 421 g. Using the Melbourne 19 MeV synchrotron, the yield of photoneutrons from the target was determined for bremsstrahlung with peak energies ranging from 11 .6 MeV to 18.6 MeV, the measurements being made at 0.25 MeV intervals. 207
208
).
B. B . BAGLIN
et ai.
The radiation dose was measured with a 25r Victoreen thimble used as a transmission ionization chamber. This chamber had been previously calibrated relative to the "lucite r6ntgen" (see Johns et al . 1b)) in this laboratory . Neutrons were detected with Bl°-enriched BF3 counters in a paraffin house similar to that described by Spicer et al. 2) . However, some modification of the counter-target separation was found to be necessary. Tests carried out using sources of neutrons of different energies indicated that a separation of target and counter centres of 10 cm rather than the previous 13 cm gave a detection efficiency more nearly insensitive to initial neutron energy. Because of the relatively low yield of neutrons, and the accuracy required, every care was taken to reduce personal and random errors to a minimum. Counter efficie-acy was checked before and after each run, using a Ra-Be neutron source. The Victoreen thimble was periodically checked, and showed no detectable leakage. A chart record of transient fluctuations in the peak synchrotron magnetic field was used to apply an effective energy correction to each run. Measurements at each energy were performed several times, in order to reduce the effects of small variations in the machine energy, and statistical fluctuations. Each point was the result of from 12 to 18 measurements, involving a count from 4 X 104 to 106 neutrons. 3. Background Determination The yield of background neutrons was determined in the same way as the total yield, but with the Al target removed, and at 16 MeV it amounted to 20 of the total yield. This decreased to 0.7 % at 18 MeV, and increased to 50 at 14 MeV. At energies below the A127(y, n) threshold, the yield with the target in position was observed to be 30 % higher than the background yield with no target. This increase in yield was attributed to neutrons produced in the lead collimator and platinum target, which travelled down the beam hole and were scattered by the Al target. To test this assumption the Al target was replaced by one of spectroscopically pure carbon, which was designed to have the same scattering characteristics for an incident evaporation spectrum of neutrons. With .this target in position, the yield Yc was determined . The photoneutron yield Y13 from the C13 present was calculated from the results of Cook 16) , and subtracted from Yc , leaving a nett yield which was assumed to be the sum of the background with no target YBG, and the scattered yield Y., Below the A127 (y, n) threshold, Yse was found to be identical with the increment in background due to the presence of the aluminium target, and was proportional to YBG at all energies . This confirms the hypothesis that the increment in the case of aluminium was, in fact, also a scattering effect. The results are summarized in table 1, where YBG+Yec is to be compared with YBG X 1 .31 .
PHOTODISINTEGRATION OF A1
27
209
(I)
Thus the effective background with the Al target in position could be derived, above or below threshold, by multiplying YBG by a constant factor (1.31 in this case) . TABLE 1
Background determination Synchrotron energy (MeV) 17 .35 15 .35 13 .35 12 .35
YsG
147±3 97±2 49±3 29±6
YC 255±4 182±6 92±1 60±4
Y13
YsG -f- Y190 = YC - Y13
YSC
YsG X 1 .31
66±2 53±2 35±2 27±2
189±6 139±6 57±5 33±4
42±5 43±6 8±4 4±4
193±4 133±3 64±4 38±1
To show in another way that the increase in yield below (y, n) threshold was due to scattered neutrons, a 10 cm length of pure graphite was placed between the lead collimator and the paraffin counting block. This was sufficient to scatter all neutrons which would normally travel down the beam hole. With the graphite in position the yields below the A127 (y, n) threshold, with and without the Al target in position were identical. Above the threshold the agreement between the yields, as determined, fully supported the constant background multiplying factor (1 .31) referred to above. 4. Absolute Yield The yield has been standardized relative to the yield from natural copper 17) . The photoneutron spectra from copper and aluminium are predominantly evaporation spectra, and both differ markedly from the spectrum of neutrons given by a radium-oe-beryllium source. Therefore a comparison of aluminium photoneutron yield with that from copper was used to set an absolute yield scale, rather than a comparison with the Ra-Be source. This course of action has the further advantage that other laboratories follow this same procedure of standardization. A target was constructed from discs of electrolytic copper. The total thickness of copper was calculated to have the same atomic absorption of y-rays above 10 MeV, and the separation of the discs was such as to give the same total length as the Al target, thus ensuring identical conditions for neutron production and detection. The copper yield was measured at 17 NIeV, 15 MeV, and 13 MeV, and normalized as above. 5. Cross Section Yield points were derived by subtracting the modified background points from the total yield values, and allowing for counting geometry and atomic
210
J. E. E. BAGLIN 8a
al .
are-plotted in fig. 1 . absorption of the y-rays in the aluminium . The yield points statistics. The errors shown are standard errors, and not due only to counting They therefore include the effects of synchrotron energy variations. The yield points shown in fig. 1 were analyzed in 0.5 MeV steps, using the method of Penfold and Leiss 115) . No smoothing of any kind was used. This gave two independent sets of cross section points interlaced at 0.25 MeV intervals, each point having its own standard error computed from the standard errors of the unsmoothed yield data. The cross section points are shown in fig. 2, together with a smooth curve fitted by eye. A curve of best fit was also computed using Fourier synthesis up to the 11th order harmonics, and this agreed almost exactly with that drawn by eye . To test the statistical validity of the cross section curve drawn, a MonteCarlo technique was used, similar to that described by Thies 19) . A set of six cross section curves was synthesized by adding random statistical variations to each point on the cross section curve drawn through the experimental points. Each of these six exhibited the structure shown by the curve in fig. 2, and all were within the limits of error shown by the dotted lines. Considering the experimental errors shown in fig. 2, and using the theory of Thies, the resolution was calculated to be (0.7 MeV) -1. This indicates that the plateau region between 13.7 MeV and 14.7 MeV, the hump at 16.25 MeV, and the increasing cross section up to 18.5 MeV are all real. The integrated cross section of the A127 (y, n) reaction from threshold up to 18.5 MeV was found to be 28 MeV-mbarn. 6. Other All' Photodisintegration Experiments Other measurements of the A127 (y, n) cross section by direct neutron detection have been made by Montalbetti, Katz and Goldemberg 12), Ferrero et al. 13) and Gusakow, 20) . To compare the resolutions of these experiments with the present one, some estimate of the standard errors in yield determination must be made. In the case of Montalbetti et al., a standard` error of ±2 % was estimated from the distribution of the measured points about the smooth curve used in their analysis . Ferrero has indicated that the statistical errors in his yield determination rknged from 1 % to 1j %. According to Thies 19) standard errors of this magnitude would allow peaks separated by 3 MeV or more to be resolved with 85 % assurance. However, this is insufficient resolution to observe the structure at 14 and 16 MeV. The other cross section mentioned, is that determined by Gusakow . This reveals some structure near 17 MeV and shows evidence of the plateau region at about 14 MeV. It is difficult to assign an experimental resolution to this determination, since arbitrary yield curve smoothing was employed in the cross
PHOTODISINTEGRATION OF A127
Ô r 2
(I)
211
8
r Π7 W W
86 W a5
0
04
W z v W
3 2
0
Wz
1
i
14
0 13
15 16 17 SYNCHROTRON ENERGY
18
Mè4
Fig . 1 . Measured yield of photo-neutrons from aluminium, plotted as a function of synchrotron energy.
14 12 E z 0
10
w e 0 0 6
U
13
14
15
115 PHOTON ENERGY
17
18 MeV
Fig. 2. Cross section for the A127 (y, n) A1 26 reaction plotted as a function of photon energy The solid line is the curve of best fit consistent with the experimental points . The dotted lines define an estimated error band (see text) .
1 . E. E. BAGLIN Ci al.
12
section analysis . The structure drawn would imply a resolution better than (2.5 McV)`l. In light nuclei the (y, n) cross section does not necessarily reflect the behaviour of the total photo-absorption cross section, as it does for heavy nuclei. Thus to compare experimental results with the relevant theoretical predictions either a measurement of the À127 (y, p) cross section (the other major photo-process for A12?), or a direct determination of the absorption cross section is required. The only determination of the A127 (y, p) cross section is that of Halpern and Mann 14), which is not sufficiently accurate to resolve the structure believed to be present. However, several measurements have recently been made of the 260
0tr z F
50
w VI
(n 40 N
O u z 0
30
20 0 N m a 10
J
Q O
J
PHOTON ENERGY
24
26 MeV
Fig. 3 . Cross section for photo-absorption in aluminium obtained from an average of the determinations of Mihailovic and Ziegler weighted according to their resolutions.
total absorption cross section . The most detailed of these, by Mihailovic 21 ), is estimated to have a resolution of (1 McV)-1, and that of Ziegler 22 ), (3 McV)-1. The third determination by Kockum and Starfelt 2s) has poor resolution, but the experimental points are consistent with the other two determinations . The peak absorption cross section indicated by Mihailovic is 80 mb and that by Ziegler 35 mb ; yet the sum of experimental (y, n) and (y, p) peak cross sections is about 40 mb. Reconciliation can be brought about if it is remembered that the determination of the nuclear absorption cross section involves an allowance for atomic absorption. This is very large, and it is possible for the zero of the nuclear absorption cross section scale to be wrongly placed. For this reason the zero of Mihailovic's scale has been shifted up by 20 mb and that of Ziegler's down by 15 mb. When this is done a reasonable over-all agreement between the curves can be obtained. An average absorption cross section for A127 is shown in fig. 3, this being the mean of determinations by Mihailovic and Ziegler, weighted according to the experimental resolution. The structure evident in this absorption cross section consists of peaks at 16 McV,18.3 MeV and
PHOTODISINTEGRATION OF A127 (I)
21 3
20.3 MeV, and it is more complex than that expected from the Danos-Okamoto theory. There is a definite correspondence between the 16 MeV hump, and the rise up to 18 MeV occurring in this average cross section, and the same features in the (y, n) cross section reported here. This (y, n) cross section is also consistent with the recent absorption cross section results of Ziegler 24) . 7. The Ratio Q(y, p)/a(7, n) The plateau in the (y, n) cross section about 14 MeV is not evident in the total absorption cross section. It is postulated that this is a result of suppression of neutron emission relative to proton emission just above the (y, n) threshold.
12
'210 E Ô 8 t= U W N
6
O U c
4
v
13
14
1
15
1
16 ENERGY
0
17
MeV
Fig. 4. Points show the A121 (y, n) cross section deduced from the total absorption cross-section assuming neutrons to be emitted with l = 3 near threshold. The line indicates the observed pAotonuclear cross section.
This results from the low penetrability of the centrifugal barrier by the neutrons emitted with large orbital angular momentum. The ground state of A127 has spin and parity 2+. The fact that the known energy levels of Mg26 and A126 below 6 MeV have even parity indicates that for electric dipole excitations of the A1 27 nucleus up to 15 MeV the photo-nucleons must be emitted with odd orbital angular momentum. The ratio of photoneutron to photoproton cross-sections was calculated taking into account penetration probabilities of the centrifugal barrier (see Blatt and Weisskopf 25)) for both particles, and of the Coulomb barrier for protons 26) .
1.
B. IL BAGLIN
et al.
Assuming that neutron and proton emission are the only decay modes of the photo-excited nucleus, a theoretical (y, n) cross section was deduced .-om the experimental absorption cross section using the calculated ratio o(y, p) /a (y, n), (Y
= abs.
1
1+
Q (Y, n)
The assumption that all nucleons were emitted with orbital angular momentum t = 3 gave the curve shown in fig. 4. The agreement with the measured (y, n) cross section is good. No such agreement was obtained for any other value of l. 8. Summary Both the present experiment and the total nuclear absorption cross-section obtained by Mihailovic indicate that the photon absorption cross-section of aluminium contains structure which is more complex than can be allowed on the Danos-Okamoto theory. It has been shown that the results of this experiment agree well with those of Mihailovic . A general discussion of the photodisintegration of aluminium in terms of the model of single particle motion in a non-spherical potential is given in the accompanying paper (see page 216) . References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)
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215
B. Ziegler, Zeits . f. Phys. 152 (1958) 566 J . Kockum and N. Starfelt, Nuclear Instruments and Methods 5 (1959) 37 B. Ziegler, Nuclear Physics 17 (1960) 238 J . M. :Blatt and V. F. Weisskopf, Theoretical Nuclear Physics, (J. Wiley and Sons, New York, 1952) 26) Feshbach, Shapiro and Weisskopf, Tables of penetrabilities for charged particle reactions, NYO-3077 (1953) 22) 23) 24) 25)