The (n, 2n) cross sections of 85Rb, 87Rb and 144Sm

The (n, 2n) cross sections of 85Rb, 87Rb and 144Sm

Nuclear Physics A223 (1974) IiS- 124; @ North-Holland Pcrblishing Co., Amsterdam Not to be reproduced by photopr~t or microfilm without written perm...

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Nuclear Physics A223 (1974) IiS-

124; @ North-Holland Pcrblishing Co., Amsterdam

Not to be reproduced by photopr~t or microfilm without written permission from the publisher

THE (n, 2n) CROSS SECTIONS OF 85Rb, *‘Rb AND r44Sm S. K. GHORAI Alabama State University, Montgomery, Alabama 36101

R. VOS Tuskegee Institute, Tuskegee, Alabama 36088 and J. R. COOPER and W. L. ALFORD Auburn Uniuersity, Auburn, A~bama 36830 Received 21 January 1974 Abstract: The (II, 2n) cross sections at neutron energies between 14.9 and 17.0 MeV have been measured for a5Rb, *‘Rb and “44Sm by the mixed-powder method and y-ray detection by a Ge(Li) spectrometer. Using the a’Al(n, a)24Na reaction for mo~tor~g, the measured cross sections were (in mb): *“Rb(n, 2n)84(m**)Rb, 1125&141, 1177&148 and 1235&162 at 15.O,tO.4 MeV, 16.2kO.7 MeV and 17.0fO.Q MeV, respectively; 8sRb(n, 2n)84mRb, 662&83, 688&87 and 765&99 at 15.0&0.4 MeV, 16.2kO.7 MeV and 17.0&0.9 MeV, respectively; a’Rb(n, 2n)86(m+*)Rb, 1336&168 and 1301f162 at 15.0&0.4 MeV and 16.2f0.7 MeV respectively; 144Sm(n, 2n)143(m+s)Sm, 1202f130,1300&141, 1516&-179 and 1514+179 at 14.9+0.3 MeV, 15.5&0.3 MeV, 16.4105 MeV and 16.7f0.2 MeV, respectively. The measured values are compared with the statistical model calculations of Pearlstein. E



85**‘Rb, 144Sm(n, 2n) E = 14.9 to 17.0 MeV; measured a(E). Natural targets.

1. Introduction

In a recent paper Rao et al. ‘) have made a survey of the (n, 2n) cross sections of *‘Rb over the neutron energy range 13 to 15 MeV. Whenever measurements were available at several neutron energies by one group, the trend of the excitation function was indicated. Extending the neutron energy range to 18 MeV, the (n, 2n) excitation function for 8‘Rb is shown in fig. 1. A rather wide discrepancy exists between the measurements of Prestwood and Bayhurst 2), who counted P-particles, and Bormann et al. ‘), who counted y-rays. Rao observed that, in general, the measurements from j&counting tended to give higher vaIues for cross sections than those determined from y-counting. The (n, 2n) cross sections of “Rb as measured by Prestwood were higher than those of Bormann by as much as 49 %. Measurements by other workers near 14 MeV are presented in table 1. The wide variations in these measured cross sections motivated the present experiment on 85Rb. Included in the results presented are measurements of the 87Rb (n, 2n)86Rb and 85Rb(n, 2n)84”Rb cross sections which were determined along with the study of the 85Rb(n, 2n)84Rb reaction. 118


(n, 24 I800



13 0

I4 0


I5 0



17 0

16 0

I8 0

E, &ieV) Fig. 1. The *SRb(n, 2n)s4(=+*‘Rb cross section as a function of incident neutron energy. TABLE 1

s5Rb(n, 2n)84(m*g)Rb reaction cross section (mb)

Prestwood “)

13.69*o.I0 14 14.09rfo.10 14.4 14.5 hO.20 14.68*0.26 14.7 14.70&0.1 14.81-40.31 14.88 15 15.0 hO.4 15.62 16.2 ho.7 16.31 17.0 10.9 17.23 17.95CO.32 19.76f0.431 “) Ref. 2).



Es (MeV)

Bormann b,



this work

Pearlstein’s predicted values “)

Pearlstein’s values with Lu’s parameter d,

1336& 68 687f 14471



72 872


1498% 15 1520& 76 1682&222 1430& 71

3, 6)



1530% 77 I174194 90










1170*93 1160193 1196176 16591166 1767&177 “) Ref. j).

‘) Ref. *).

d, Ref. ‘).

Also measured was the (n, 2n) cross section of l 44Sm. Previous measurements by Bormann et al. “) and by Rayburn lo) over a range of neutron energy are presented


S. K. GHORAI et al. TABLE 2

144Sm(n, 2n)‘43(m+*)Sm reaction cross section (mb) u(experimenta1)

E. (MW

Bormann “) Rayburn b,


a(theoretical) ref.

14.1 14.2 AO.02

1470f 1315


11) 12

14.5 14.7 14.8 &IO.8 14.8 14.87 14.9 *0.3 15 15.5 AO.3 15.52 16.18 16.4 kO.5 16.7 kO.2 16.85 17.78

1452&166 1600&240 1200&300 923* 99

i3 ; i4) -) 16)



*) Ref. 3).


1637i160 1703&168 1703*168 1856&181 “) Ref. lo).

this work Pearlstein’s Pearlstein’s predicted values values “) with Lu’s parameter d,

1560 1202&130






15,16&179 1514*179

1750 1771

1540 1543

1600 1600 1640 2350 ‘) Ref. a).

d, Ref. p).

in table 2. The measurements attributed to Rayburn were actually read from the cross section curve presented in ref. lo). The results of other workers near 14 MeV are summarized in table 2.

2. Experimental procedure Neutrons were produced by the Auburn University 3 MV dynamitron accelerator using the 3H(d, n)4He reaction. The mixed-powder method, first developed by Rao and Fink r8) and later used extensively by others, was used to measure the cross sections. The samples were homogeneous mixtures of known proportions of natural Rb&03 and natural A1,03 in the case of the 85Rb and “Rb measurements, and of natural Sm,O, and natural A1,03 in the case of the 144Sm measurements. In all measurements aluminum served as the neutron flux monitor through the “Al(n, a) 24Na reaction. The samples were enclosed between two pieces of thin plastic supported by concentric plastic rings. The samples were about 2.86 cm2 in area. Each rubidium sample was bombarded by neutrons emitted from the tritium target in a cone with half-angle of 63” and with axis along the direction of the incident deuteron beam. The corresponding cone for each samarium sample had a half-angle of 50”. The irradiated samples were counted for their y-activities by a 20 cm3 Ge(I,i) de-


(n, 2n)

tector in conjunction with a 2048 multichannel analyzer. The y-rays of interest were identified by energy and half-life. The detector resolution was about 3 keV full-width at half-maximum at 1.33 MeV. The photopeak efficiency curve for the detector was determined by using standard sources obtained from National Bureau of Standards, Washington, D.C. In determining the photopeak efficiency curve for the detector the procedure outlined in ref. I’) was followed. The target current was monitored and kept constant during each irradiation. Also the neutron flux was monitored by a long counter. At each neutron energy at least two irradiations were made and the results were averaged. 3. Results and discussion Cross sections were determined from photopeak counts by using the half-lives and y-ray energies and intensities given in table 3. The information in table 3 and the Reaction


TABLE3 half-lives and y-ray energies and intensities


Product half-life

85Rb(n, 2n)84mRb

20.0 min

8 5Rb(n, 2n)84(m+*)Rb *‘Rb(n, 2n)8a(m+l)Rb 144Sm(n, 2n)143(m+*YJm *‘Al(n, a)24Na

33.0 d 18.66 d 9.0 min 14.96 h

used in the present experiment Er (kev) 250 464 880 1078 511 1396

q%) 65 32 74 8.8 80 100

natural abundances of the isotopes studied were taken from Lederer et al. ‘“) except for the 143Sm y-intensity which came from ref. “I). The cross section of the monitor reaction, *‘Al(n, a)24Na, was taken from the work of Paulsen and Liskien ‘“). The cross section values for the monitor reaction were taken to be 116.0 mb at 14.9 MeV, 115.0 mb at 15.0 MeV, 106.5 mb at 15.5 MeV, 92.0 mb at 16.2 MeV, 90.5 mb at 16.4 MeV, 84.5 mb at 16.7 MeV and 79.8 mb at 17.0 MeV. The errors quoted for the present measurements are estimates of standard errors and include contributions due to uncertainties in photopeak efficiencies, monitor cross section, decay scheme parameters, counting statistics, absorption corrections, y-ray summing effects, timing, and sample weight. 3.1. THE *‘Rb(n, 2n)84(m+s)Rb REACTION


To measure the total (n, 2n) cross section of s5Rb the 33.0 d activity was followed by counting the 880 keV y-ray. Since natural rubidium was used, the *‘Rb(n, 01)84gBr reaction produced a 32.0 min activity resulting in the same 880 keV y-ray. Thus a period of at least one day was allowed to elapse between irradiation of the sample and counting of the 33.0 d activity. Table 1 shows the results of the present experiment



et al.

in measuring the cross section of the 85Rb(n, 2n) 84(m+b)Rb reaction. The results of others are shown for comparison. The present experimental results agree very well with those of Bormann et al. “). The results of the Pearlstein statistical model calculations “) are given. These calculations were also made using the empirical fit for the ratio of neutron emission to all modes of compound-nucleus decay as given by Lu et al. ‘) and are presented in the last column of table 1. In making the statistical model calculations required separation energies were taken from Mattauch et al. ‘“). The results of the present experiment are plotted in fig. 1 for clear comparison with the earlier excitation functions. 3.2. THE

85Rb(n, 2n)84mRb




In the decay of 84mRb there are cascade y-rays of 216 keV and 250 keV and a crossover transition of 464 keV. To measure the cross section to the isomeric state in the present experiment the 250 keV and 464 keV y-rays were counted. Summing effects, which were significant, were considered in determining y-intensities. The average TABLE 4 85Rb(n, 2n)84mRb reaction cross section (mb) o(experimenta1)

En (MeV) Bormann 3, 14.410.3 14.7 14.8 14.88 15 15.0f0.4 15.62 16.2&0.7 16.31 17.OhO.9 17.23



478&48 926&61 714&50

‘) :;





this work


777*89 688 *87 742*88 765199 773+90

isomeric cross section based on the 250 keV and 464 keV y-rays is presented in table 4 along with the results of others. Agreement between the present experiment and that of Bormann appears good. Near 14 MeV the discrepancy between the various measurements is rather large and outside the estimated errors. 3.3. THE

87Rb(n, 2n)86Rb




Results of the present and previous experiments are presented in table 5 for the *‘Rb(n 32n)86Rb reaction. Again the discrepancies are rather large. However, the agreement is quite good for a number of measurements. Theoretical values are also included in the table.


(n, 2n) TABLE 5

87Rb(n, 2n)!j6Rb reaction cross section (mb)



14 14.05 *to.5 14.4 *0.3 14.5 14.70&0.1 14.7 14.8 14.81 15.0 kO.4 16.2 f0.7 “) Ref. a). 3.4. THE ‘%jm(n,



En (MW

this work

Pearlstein’s predicted values “)

Pearlstein’s values with Lu’s parameter


838&136 1394&139 995& 99 1211+ 61 1560&-156 2551&350 1417& 72 1191f 60 1336&168 1301+162

1322 1412

1245 1266

“) Ref. 9). 2n)?Sm



The present experimental cross sections for ‘44Sm(n, 2n)‘43Sm together with those of other experiments and the Pearlstein predicted values have been presented in table 2. Present measurements are slightly lower than those of Bormann et al. “) and of Rayburn lo) and fit the theoretical cross sections somewhat better when parameters from Lu et al. ‘) are utilized. Although time did not permit a present measurement of the cross section near 18 MeV, the divergence of the results of Bormann and Rayburn suggests a check of the cross section at this energy. The authors wish to thank the personnel of the Auburn University Computer Center for assistance in processing the experimental data. The experimental work was made possible by the staff and facilities of the Edmund C. Leach Nuclear Science Center at Auburn. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12)

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et al.

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