The (nth, α) reaction on 233U, 235U and 238U and determination of the 238U(nth, α) 235Th reaction characteristics

Nuclear Physics A362 (1981) 1-7 @ North-Holland Publishing Company

THE (nt,,, a) REACTION ON z33U, “‘U AND 238U AND DETERMINATION OF THE 238U(n~,,,a)235Th REACTION CHARACTERISTICS C. WAGEMANS* Nuclear

and

E. ALLAERT**

Physics Laboratory, B-9000 Gent, Belgium and SCKICEN, B-2400 Mel, Belgium

A. DE CLERQ,

P. D’HONDT***

Nuclear Physics Laboratory,

and B-9000

A. DERUYTTER Gent, Belgium

G. BARREAU Institut Laue-Langevin,

F-38042

Grenoble, France

and A. EMSALLEM Znstitut de Physique Nu&aire, Received

F-69622

11 November

Villeurbanne,

France

1980

Abstract: The thermal neutron induced (n, a) reaction cross sections of 233U, 235U and 238U were measured using a highly pure thermal neutron beam from the Grenoble High Flux Reactor. In the case of the 23*U(n,h, a)*‘%~ reaction, a Q, value of 8.7OkO.05 MeV, a *“Th mass of 235.04751 f 0.00006 amu and a Q, value of 1.92* 0.16 MeV (for the P-decay of r3?h) were determined. Also statistical model calculations of the (n, 0) cross section were performed which were in poor agreement with the measured value.

E

NUCLEAR

REACTIONS distribution;

23sU, 235U(n, F), 233.235.238U(n, a), E = thermal; deduced u(n, a), 235Th deduced mass and Q,.

measured

E,

1. Introduction For heavy nuclei, very little information is available on the thermal neutron induced (n, CY)reactions. Especially for the uranium isotopes these reaction cross sections should be known since they contribute to the production of He gas in nuclear reactors. From a nuclear physics point of view their study is of interest for nuclear spectroscopy and reaction systematics purposes. Since these (n, cy) particles are emitted in the presence of an intense background e.g. due to the neutron induced fission process (the presence of !ong-range a-particles (LRA) causes special * NFWO ** TWONL *** IIKW June

1981

2

C. Wagemans

et al. / (nrhr a)

difficulties) and radioactive decay a-particles, a strong background reduction and a proper identification of the particles is needed. Both requirements are fulfilled when using a AE-E telescope detector. Combined with the outstanding neutron beam characteristics available at the Grenoble High Flux Reactor, excellent experimental conditions for this type of measurement can be realised, which enabled the study of the 233U, 235U, 238U(nti,, (Y)reactions. 2. Experimental

conditions

A detection chamber was installed at the end of the 87 m curved neutron guide of the High Flux Reactor at the ILL (Grenoble). The flux at this point is 6 x 10’ neutrons/cm’ * s and the ratio of slow neutrons to epithermal and fast neutrons is about 106. Also the direct -y-ray flux from the reactor is reduced by a factor of about 106. So the background due to fast neutron and y-induced reactions will be very small. The target, inclined at an angle of 30” with respect to the incident neutron beam, was viewed by a telescope assembly consisting of two gold-silicon surface barrier detectors. The fully depleted AE-detector had a thickness of 26.7 km, an active area of 300 mm2 and an energy resolution of 35 keV for 5.5 MeV alphas. The E-detector had a thickness of 700 pm, an active area of 450 mm2 and an energy resolution of 25 keV. After amplification, the coincident AE and E detector signals were coded and stored event by event on a digital cassette recorder; 4096 channels were allocated to either of the two parameters. The data reduction of the coincident AE-E spectra was done with a PDP 15 computer. The uranium targets were prepared at the Central Bureau for Nuclear Measurements (Geel) by the evaporation under vacuum of UF, on a thin backing foil. For all targets highly isotopic enriched materials were used. Especially, in the 23*U target only 0.0014 atom % of 235U was present which strongly reduces the background due to the 235U(n,h, f) reaction. Detailed target characteristics are given in table 1. TABLE 1 Target characteristics Isotope 233 234 235 236 238

U U U U U

thickness (wg U/cm’)

Atom %

Atom %

99.7617 0.0182 0.0091 0.0001 0.2109

0.1699 99.4759 0.0273 0.3269

4.6 U

233

150 U

235

Atom %

<0.0007 0.0014 <0.00007 99.9986 94.9 238U

C. Wagemans et al. / (nrh, a)

3. Measurements The thickness

of the AE detector

3

and results

(26.7 pm) was chosen

in such a way that all the

fission fragments were completely stopped. Also the a-particles due to the spontaneous radioactive decay of the various uranium isotopes were stopped to a large extent; the remaining fraction was eliminated by a suitable discriminator setting in the E-detector chain. Thus by realizing a coincidence between the AE and E detectors, only the light particles emitted during the fission process (mainly longrange (Y-particles) and the (nth, (Y)reaction particles passing through the AE detector were recorded. one to identify

Such a procedure considerably reduces the particles via the relation ‘) T/a = (E + AE)‘.73 -E’.73

the background

and allows

,

where T is the thickness of the AE detector, E and AE the energy losses in the E and AE detectors respectively and a a constant specific for each particle. In this way the (Y-particles can easily be separated from all other particles. In the present measurements on 233U and 238U no aluminium absorber was placed between the target and the detectors. This has the advantage that the long-range a -particles and the (nth, a) reaction particles do not loose energy; hence their energy distribution is not distorted initially and the detection level is lowered. The disadvantage of such a procedure is that the AE detector is continuously exposed to the fission fragments and to the (Y-particles due to the spontaneous radioactive decay of the target. To avoid this, a 20 pm thick aluminium foil was placed between target and detector in the 235U case. In a previous measurement *) on this isotope, results were obtained using an unshielded telescope detector. In the present experiments an absorber was needed since we wanted a substantial improvement of the statistical accuracy. The energy calibration of the telescope was done based on the well-known energies of the a-particles emitted during the spontaneous decay of the various uranium isotopes. The 6Li(n,h, a)t (Ea = 2.055 MeV) and the 143Nd(n,r,, (u) reactions (Em = 9.45 MeV) were used as well. The neutron flux calibration was done via the thermal neutron induced fission cross section of 235U [& = 587.6 b, ref. ‘)I. The present results can be subdivided in two groups. In the case of 233U and 235U, the fission cross section, which results in a LRA production cross section of about 1 b is very important. The (nth, (Y) particles are then superposed on an intense LRA emission. For 238U, on the contrary, ai is very small [2.7 +b, ref. 4)] which allows a very clean detection of the (nth, a) particles. The energy distributions of the identified a-particles in the case of 235U (corrected for energy loss in the aluminium) and 233U are shown in fig. 1. About 65 000 events were recorded in the 233U and 700 000 in the 235U case. The present shape of the (Y-particle energy distribution for 233U + nth clearly confirms previous results 2’5) distribution is found, with, obtained for 235U. In both cases a quasi-gaussian

4

Fig. 1. Energy distribution and 235U+nth.

C. Wagemans

et al. / (n,h, ff)

of the identified LRA [and eventual (nth, (Y)] particles produced by 233U + nth The full line is a gaussian fit to the data points above 12.5 MeV.

however, a‘clear deviation from a gaussian shape at lower energies. An explanation for such a deviation has been presented elsewhere 2,5). We have also determined the parameters of the gaussian distribution adapted to the energy distribution of the (Y-particles for 233U + nth (for CY -particle energies above 12.5 MeV). The peak energy of 16 f 0.2 MeV and the FWHM = 9.7 f 0.4 MeV agree with the currently accepted values. From the measured a-particle distributions we evaluated the 233U(nth, a)230Th and the 23sU(n,h, a)232Th reaction cross section. To do this we determined the energy

5

C. Wagemans et al. / (nth, a)

resolution of the telescope with the 143Nd(nth, CX)reaction, which emits a 9.45 MeV a-line close to the 235U(nth, (Y) and 233U(n,h, (u) lines expected at about 11 MeV according to the Q, values. The 143Nd(n,h, a) line is detected with the telescope as a very sharp a-line (FWHM = 0.12 MeV). Such a sharp a-line is not present in the spectra shown in fig. 1. So the statistical accuracy of the present data allowed us to calculate an upper limit of 100 kb for the 235U(nti,, (Y) and of 300 kb for the 233U(nti,, a) reaction cross section. Fig. 2 shows the identified a-particle energy distributions obtained for the 238U(nth, a)235Th reaction. The upper part shows a 460 h real measurement; besides, a 370 h background run was performed (lower part) with the beam open and the target removed. In both cases the 212Po and 216Po a-lines (daughter products of 232Th impurities present in the vacuum chamber) are clearly observed. An additional background run with the target mounted but the neutron beam closed showed these same (Y-lines. So fig. 2 clearly indicates the presence of an (Y-line at 8.55 MeV, which we ascribe to the 238U(nth, c~)~~‘Threaction. The intensity of this line corresponds to a c(nth, (u) value of lS=tOS kb, and from the value E, = 8.55kO.05 MeV a Q, value of 8.70* 0.05 MeV can be calculated for the 238U(nti,, a)235Th reaction. With this Q, value and taking into account the well-known 6, masses of 238U, n and a, the

NE)2’elJ( n,,,al

I

I

I

I

3

kz m

lo-

. 5. .. .I

..

..

5-

. .... .

..

11. .

. .

.

.

ID

. . .n

.. . ....

"

.

..A

. .

01

I

1

I

I

6

7

8

9

10

I

E,IMeVl

Fig. 2. Energy

distribution

of the identified background

238U(n,h, CY)particles (upper spectrum (lower part).

part) and the corresponding

6

C. Wagemans et al. / (nrhr a)

unknown mass of 235Th was determined to be 235.04751 *to.00006 amu. From this mass a value Qp = 1.92 f 0.16 MeV was obtained for the p-disintegration energy of 235Th. 4. Discussion Our experimental values and the previously reported values of the (nth, cu) cross sections are summarized in table 2. Statistical model calculations were performed in the same way as described elsewhere 7), using the resonance parameters r,,, D and uY recommended in ref. “). The results are given in table 2. The reported values are a sum of the ground-state transitions and of the transitions to the two lowest excited levels (at about 50 and 150 keV), so as to have the same energy resolution as for the experimental results. From table 2 it is clear that the calculated c(nth, a) values are lower than all the experimental data. This could be due to the fact that the present calculations do not take into account the deformation of the nuclei, which might enhance the (Y-particle emission. TABLE

Reported

233

Ref. Sowinsky et al. 9, Chwaszczewska et al. lo) Wagemans and Deruytter Almodovar et al. I’) Asghar et al. 7, D’hondt et al. *) this work: exp talc “) 65% confidence

2

values of the (Q,,, a) cross sections U

235

(pb) U

238

U

3x104 3x103 S(5*2)x104

11)

GlOO 1.3i0.6 S300 0.02

S330 “) Cl00 0.005

1.5*0.5 1o-6

limit (1~).

The present results for 235U clearly contradict the high c+(nth,a) values reported in the literature 9-11). This discrepancy can be explained by the complex nature of the background in this type of measurement and by the fact that the previous results were obtained under the hypothesis of a gaussian shape for the LRA energy distribution. Since it is well established now that the lower part of this distribution deviates from a gaussian shape, important errors may have been introduced by such a procedure. The present results for 238U were obtained under considerably better conditions compared to previous work. A very substantial improvement was the depletion in 235U of the target material. In these measurements, the amount of 23sU in the target was only 0.0014 atom % compared to 0.01755 atom% in the case of Asghar et al. ‘) and 0.71 atom % in the case of Almodovar et al. ‘*). This resulted in a strong reduction of the main background source, i.e. the 235U(n, f) reaction, as illustrated by the extremely low background shown in fig. 2. A second major improvement is the

7

C. Wagemans et al. / (Q, a)

identification of the a-particles in the present measurement, which removes the ambiguity on, the nature of the peaks observed. Nevertheless, the determination of pb cross-section values is at the limit of our present experimental possibilities. TABLET Q, values

for the 238U(n,h, a)235Th reaction P-decay of 235Th

Ref. Zeldes et al. 13) Garvey et al. 14) Asghar et al. ‘) Wapstra and Bos 6) Koiesnikov and Demin this work

and 0,

Q, (MeV)

9.20 8.80 is) 8.70

values

0,

for the

(MeV) 2.14 2.04 1.44 1.84 1.83 1.92

Table 3 summarises the Q, values for the 238U(nti,, a)235Th reaction and the disintegration energy values for the p-decay of 235Th. The present Q-values are in good agreement with the evaluated values of Wapstra et&. 6). This is also the case for our experimentally determined mass of 235Th (235.04751 amu) for which Wapstra et al. 6, obtained a value of 235.04740 amu. Furthermore, there is good agreement between the present Qp value and those of refs. 6*13914*15), which increases the confidence in the presently determined 238U(n,h, a)235Th reaction characteristics. The authors wish to thank Dr. M. N&e de Mevergnies for a critical reading of the manuscript and the CBNM Sample Preparation Group for the preparation of the targets. The Belgian NFWO/IIKW is acknowledged for its financial support. References 1) F. Goulding, D. Landis, J. Cerny and R. Pehl, Nucl. Instr. 31 (1964) 1 2) P. D’hondt, A. De Clercq, A. Deruytter, C. Wagemans, M. Asghar and A. Emsallem, 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)

Nucl. Phys. A303 (1978) 275 A. Deruytter, J. Spaepen and P. Pelfer, J. Nucl. Energ. 27 (1973) 645 C. Lederer and V. Shirley, Table of isotopes (7th Ed.) 1978 C. Wagemans, P. D’hondt, A: De Clercq, E. Aiiaert, G. Barreau and A. Deruytter, Report BLG 539 (1980); and references therein A. Wapstra and K. Bos, At. Data and Nucl. Data Tabl. 19 (1977) 175 M. Asghar, A. Emsallem, R. Chtry, C. Wagemans, P. D’hondt and A. Deruytter, Nucl. Phys. A259 (1976) 429 S. Mughabghab and D. Garber, BNL 325,3rd Ed. (1973) M. Sowinsky, M. Dakowski and H. Piekarz, Phys. Lett. 6 (1963) 321 J. Chwaszczewska, M. Dackowski, T. Krogulski, E. Piasecki, M. Sowinski, A. Stegner and J. Tys, Acta Phys. Pal. 35 (1969) 187 C. Wagemans and A. Deruytter, Z. Phys. A275 (1975) 149 I. Almodovar, J. Cantareil and H. Bielen, Z. Phys. 177 (1964) 451 N. Zeldes, A. Grill and A. Simievic, Mat. Fys. Skr. Dan. Vid. Selsk. 3 (1967) No. 5 G. Garvey, W. Gerace, R. Jaffe, I. Talmi and I. Kelson, Rev. Mod. Phys. 41 (1969) Sl N. Koiesnikov and A. Demin, Report JINR-P6-9420 (1975)