Heertje, Nagel, Aten
W. Jr., A. H. W.
REACTIONS by I. HEERTJE,
(a, fi) AND
and A. H. W. ATEN
Synopsis With deuterons
in the external
by bombarding beam
of the Amsterdam
of Be, Co and Ta with 26 MeV
of (n, 9) and
(n. 2%) cross sections has been investigated. The following reactions have been studied a*Ni(n, p)5*Co, 24Mg(n, p)%_Xa, 14N((n, 2n)13N, lGO(n, 2n)150, IgF(?t, 2n)lsF, 23Na(n,
Introduction. Obviously the only exact way to obtain information about fast neutron activation cross sections is by means of measurements with mono-energetic neutrons .Nevertheless useful information about excitation curves may be obtained by irradiation with neutrons of a known energy distribution. Different neutron energy spectra can be used to determine very roughly the shape of cross section vs. energy curves. To this purpose we used the neutron spectra obtained by bombarding thick targets of Be, Co and Ta with 26 MeV deuterons from the external beam of the Amsterdam cyclotron 1) 2). The procedure does not, of course, give more than an approximation, and any fine structure will remain unobserved. However, for purposes of neutron dosimetry this is not a disadvantage. Furthermore in some cases the cross section vs. energy curve is already partly known, then a control of these published data is of interest. In other cases a 14 MeV point may be known which facilitates an extrapolation. For several of the reactions studied here, an average cross section has already been determined for fission neutrons. In some cases agreement is quite bad with the value calculated from the curves suggested by us. This may, of course, be due to the fact that the fission energy spectrum falls off much more rapidly towards high energies than the spectrum of cyclotron neutrons. This would mean that the beginning of our cross section curve (the low energy end) is incorrect, which in our measurements would influence the average cross section only to a minor extent. (In any case this part of our *) Most data I. Heertje
in this paprr
in the thesis
curves should not be expected hand such a disagreement
to have a very high accuracy). neutrons
Experiments. In table I some data are given concerning investigated and the measurement techniques used. TABLE
Data on the reactions Reaction
On the other
should not be given too much weight, as measure-
ments of average cross sections for fission often tend to show very bad disagreement.
1 c~~~~~~ 1 ?~~~f Y 47+--Y
Tb 71 d 15.0 hr
carbonate 14N(n, 2n)‘3N
comparison 23Na(n, 2n)zWa
ssNa source Y
68.0 100 1.40
36 hr 13.2d 52 hr
The experimental conditions are the same as described in earlier publications 1) 2). Calibrations were performed with a NaI(T1) crystal of known efficiency or by means of 47cj3-y coincidence measurements. As already mentioned three different neutron spectra were used in these experiments. At lower energies the spectral differences - which are very appreciable - are of great importance in determining the shape of the excitation curves. Above about 12 MeV, on the other hand, the shape of the different neutron energy distributions is nearly the same. This means that for reactions with a threshold of about 12 MeV or higher (i.e. for most n, 2n reactions), measurements with one cyclotron target will furnish the same information as measurements with another cyclotron target. In the irradiations the reaction alP(~z, p)siSi or the reaction ssS(~,, p)s2P was used as flux monitor. It is then possible to compare the calculated values of the activities for a certain cross section curve with the activities observed in our experiments (Table II). Results and discussion. ssNi(rt, p)ssCo. This reaction has often been discussed as suitable for fast neutron measurements in reactors. A number of measured cross section values is available as indicated in figure 1. We have drawn a curve almost coinciding with the figures of Barry 3) and those of
2”Na(w, 21~)=Na **)
2rt)57Ni 2n)‘ZGI 2rC)==Pb
and Weigold (Table II).
1. Cross section vs. energy curve for the reaction : jsISi(n, G161 4) References : Ba 61 3) Na 62 27) Pa
Pr 60 35) Tr 62 26)
Ko 63 5, Me 63 6)
The recent results of K o nij n and Lauber 5) - and by Meadows and Whalen 6) in the low energy region have been indicated by a few points only in fig. 1. Actually these authors have observed a fine structure which
CROSS SECTIONS OF SOME (n, $) AND (92, 24
we have neglected
in our calculations.
(As we have already pointed out, the
steepest part of the curve at the lowest neutron energies which provides the main contribution to the average cross section for fission neutrons is of only minor importance in our experiments). Passe 17) has reviewed a number of direct determinations of average cross sections for fission neutrons. A reasonable
average would be 102 mb, which should be compared
from our curve.
to 121 mb
sJMg(n, P)sdNa. This reaction too is frequently used for fast neutron measurements in reactors. A complete cross section vs. energy curve has been measured by Butler and Santrys). The activities calculated for their curve differ by about 10% from our observed values, as is indicated in table II. Taking into account the experimental error, we consider this to be a reasonable agreement. (n, 2%) reactions. We will now discuss a number of (n, 2%) reactions. Something which helps to interpret these measurements, is the observation that in these cases the experimental threshold energy in general lies one half to one MeV above the theoretical threshold energy (which is equal to the binding energy of the last neutron in the target nucleus). 20
V(m barn) . [ Ref.
Fe 60 I?0
* 58 x
Du 54 q Pa 5:o Cc 62 l As
Fig. 2. Cross section vs. energy curve for the reaction: 14N(n, 2n)13N. The curve of Brill e.a. (14) is drawn in full. References : Fe 60 13) Du 54 11) Ce 62 13) Ra 58 3) Pa 53 lo) As 58 39
14N(n, 2n)isN. This reaction has a theoretical threshold of 11.3 MeV. A number of 14 MeV points are available a) 10) 11) is). Ferguson and Thompson 13) have measured a set of cross section values between 12 and 18 MeV. These values are somewhat higher than the recent values of Brill e.u.14). All known published data are represented in fig. 2. The curve
AND A. H. W. ATEN
drawn by Brill e.a. was used for the calculation in table II. The agreement between calculated and observed activities is good. (The part of the cur-\-e above about 20 MeV contributes little to the total activity observed by us. Therefore this part of the curve is hardly supported by our experiments. It should also be kept in mind that for this reaction agreement between calculated and observed activities, with one deuteron target means that the other case Brill least
two must show a similar agreement. The same situation exists in the of the other (n, 2%) reactions). Our results indicate that the values of e.a., in particular a value of about 5 mbarns at 14.0 MeV, must be at nearly correct. This curve now seems to be rather well established.
Fig. 3. Cross section Brill
e.u.14) is drawn
for the reaction:
in full. The dashed
160(m, 2>t)150. ‘I‘he curve of be in agreement with our ob-
160(n, 2n)r5O. The theoretical threshold energy of this reaction is 16.7 MeV. There is only one series of measurements in the region from 18-37 MeT: by Brill e.a.14). Their curve is represented in fig. 3. As may be concluded from table II their measurements are in serious disagreement with our results. The observed activities are about a factor three higher than the activities calculated from the curve of Brill e.a., which means that a cross section curve of the same shape, but with ordinates about three times the value of Brill e.a. would agree with our results. It should, however, be stressed that in this case our experiments do not have the same weight as for the other reactions, because the reliability of our neutron spectra above the threshold of the r6O(n, 2n)r50 reaction has not been established with the same certainty 1) 2) as it has been at lower energies.
CROSS SECTIONS OF SOME (92, $) AND (n, 2n) REACTIONS
energy for this reaction
MeV. A number of 14 MeV points and two series of measurements at other energies are available. The different measurements are not in good agreement with each other as is shown in fig. 4. Our observed activities disagree both with Brill’s curve and with the points of Rayburn, A curve of the same shape - as the one suggested ordinates quite
as is shown in table II. by Brill e.a. - but with
about one half of their values would reproduce
(fig. 4). Furthermore
an independent As
our observations measurement
Fig. 4. Cross section vs. energy curve for the reaction: isF(n, 2n)rsF. The curve of Brill e.a.14) is drawn in full. The dashed curve would be in agreement with our observations. + indicates our measurement at 14.3 MeV. References : As 58 2s) Ra 62 29) Ce 62 1s) Pa 53 lo) us *) at
14.3 MeV gives a value of 43 f 4 mbarn, which also suggests a as given in the dashed curve of fig. 4. This means that only the
measurement of Cevolani and Petraliais) agreement with our observations.
at 14.1 MeV is in reasonable
Ref. Pr 55
Fig. 5. Suggested cross section vs. energy curve for the reaction: References: Pr 55 17)
*) These irradiations were performed with a Philips neutron generator tube, type number 1860015)
AND A. H. W. ATEN
aaNa(n, 2n)ssNa. This detector may be of some value as a fast flux integrator for the high energy part of the fission spectrum **). The reaction has a theoretical threshold energy of 12.95 MeV. Activation experiments have been performed with the Be target only. There is a 14.1 MeV point determined this point,
by Pr es t woo d 17). We have drawn a curve (fig. 5) through for which the calculated activity is the same as the observed
activity. (Table II). Evidently this curve is largely hypothetical, the part above about 20 MeV does not represent more than a guess. Hughes, Spatz and Goldstein founcl the cross section for fission neutrons to be 0.006 mb Is), from our curve a value 0.0022 mb is calculated. saNi(n, 2n)sTNi. The theoretical threshold energy is here 12.4 MejT. A large amount of published data is available. They are all represented in fig. 6. To bring the observed activities into agreement with the calculated V(m 100
barn) Ref. Pa53 Pr 60 GI 61 cr.52 Pr61 Br 58
Fig. 6. Cross section vs. energy Keferences:
+ x A P
curve for the reaction:
Pa 53 lo)
Cr 62 30)
Pr 61 lg)
Iia 61 =)
Br 58 20)
activities (Table II), a curve was drawn a little below the values of P r e s twood and Bayhurst at higher energies la), and almost coinciding with these values and those of Glover and Weigold at lower energies 4). (The figures taken from the Brookhaven compilation 20) are much too high). The part of the curve above 20 MeV is very uncertain. It contributes little to the activity in our experiments. **)
cross section for thermal neutron use as a fast flux integrator.
Farinelli 16), which mentions an extrrmely high capture in SzNa, suggests that this detector will have only R limited
Schumann cross section
21) have found 0.0012
for fission neutrons,
mb for the average
from our curve a value of 0.0023
calculated. 127I(n, 2n)iW. This reaction has a theoretical threshold at 9.2 MeV. There are a few results from Martin and Taschek 22) and a 14.5 MeV point from Paul
10). A curve (fig. 7) for which the calculated
, , , , , , , , , . . , IO
Cross section vs. energy curve for the reaction: lW(n, References : Pa 53 10) Ma 53 22)
activity agrees well with the above the points of Martin above about 16 MeV draws mainly drawn to give a slope Gjm
observed activity (Table II) is drawn a little and Taschek. Again the part of the curve little support from our measurements. It is similar to that of the earlier data.
Fig. 8. Suggested cross section vs. energy curve for the reaction:
994Pb(n, 2n)999Pb. The theoretical threshold for this reaction is 8.4 MeV. This is the only information, no other data are known. In fig. 8 a curve is drawn for which calculated and observed activities (Table II) are in agreement with each other. Its shape has been chosen to be similar to that of other (n, 2%) reactions, e.g. the reaction 197Au(,, 2n)i99Au. Only the rising part of the curve should be taken seriously, the part little influence on the activities observed by us.
16 ?uleV has
Acknowledgement. This work is part of the research program of the Institute for Nuclear Physics Research (IKO), made possible by financial support from the Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organization for Pure Scientific Research (ZWO). Received
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