A measurement of the fission cross sections of 233U, 235U and 239Pu

Journal of Nuclear Energy. Vol. 22. pp. 477 to 486. Pergamon Press 1968. Printedin NorthernIreland

A MEASUREMENT OF THE FISSION CROSS SECTIONS OF 233U, 235U AND 23gPu R. L. G. KEITH, A. MCNAIR* and A. L. RODGERS Atomic Weapons Research Establishment, Aldermaston, Berks. (First received 18 December

1967 and infinalform

5 April 1968)

Abstract--Precise values for the fission cross sections of PaaU, nssU and z38Pu relative to the neutron capture cross section of 68Co have been obtained by irradiating thin foils in a well-thermal&d flux of reactor neutrons. The 2200 m/s values have been derived using the Westcott convention. The cross sections for BsVJ and as5U are based on a new measurement of the half-life of nssU. 1. INTRODUCTION

As PART of another experiment the ratio of the number of fissions produced in very thin layers of various fissile materials was measured relative to the amount of 6oCo produced in layers of cobalt held in close proximity to the fissile material. The materials were irradiated in a well-thermalized flux region in the graphite thermal column of the HERALDresearch reactor (MCENHILL et al., 1965). Recent improvement in the accuracy with which the capture cross section of ssCo is known (VANINBROUKX, 1966) has since made it possible to deduce precise values for the 2200 m/s fission cross sections of 233U, 235Uand 23sPu from these measurements. This paper also discusses differences between the presently accepted values for the fission cross sections of 233Uand 235U(WESTCOTTet al., 1965) and some previous measurements. It considers the significance of a new measurement (KEITH, 1968) of the half-life of 233U in reducing these differences. 2. EXPERIMENTAL METHOD A thin layer of cobalt deposited on a polystyrene film was placed close behind a thin layer of fissile material in a fission chamber. The number of fissions produced during irradiation was measured by counting the fission fragment pulses and the integrated neutron flux was obtained from the 6oCo activity induced in the cobalt. The 2200 m/s neutron capture cross section of 5sCo was assumed to be 37.4 f 0.15 barns (VANINBROUKX, 1966). This cross section is based on a value of 98.8 barns for the neutron capture cross section of ls7Au. Vaninbroukx quotes an error of f 0.3 barns but states that this is a 20 error. 2.1 The j&ion chamber The construction of the parallel plate chamber is shown in Fig. 1. The anode disk (1.375 in. diameter) has a central hole (O-0625 in. diameter) through which the counting gas (90 % argon + 10 % methane) flowed continuously during the experiment. The cathode was an aluminium foil (l-75 in. diameter, 0403 in. thick) which was clamped in intimate contact to the body of the counter by screwing the Perspex cap onto the counter. The fissile source, on a polished aluminium disk, was held on the cathode at its centre by a thin coating of nitrocellulose adhesive. Immediately behind * Present address: RCC, Amersham, Bucks. 477

478

R. L. G. KEITH, A. MCNAIR and A. L. RODGERS Anode

Perspex cap

Gas inlet

Output

terminal

Gas out let

P.T.F.E.

Aluminium FIG.

P.TEE.

‘9 aluminium spacers

1

the cathode was placed a cobalt monitor source on thin (0~0005 in.) polystyrene film supported on a Perspex annulus, in such a way that the cobalt was separated from the cathode only by the polystyrene. Between the cobalt source and the Perspex cap of the chamber was placed another Perspex annulus and polystyrene f3m to monitor the neutron-induced activity in the cobalt source support. Reactor grade aluminium (less than @02 per cent copper) was used almost exclusively in the construction of the chamber in order to keep the induced radioactivity low. On the simplest assumption of a fissile source with plane parallel surfaces the fraction of fissions from which fragments enter the counting volume from the source is 1 - t/2R, where R is the range of fission fragments in the fissile material and t is the thickness of the fissile material. The thickness of the electroplated fissile sources was less than 4 ,ug/cm2 so that more than 99.95 per cent of the fission fragments were expected to enter the counting volume. A typical EHT plateau and scaler bias curve (Tables 1 and 2) show the excellent TABLE l.-FISSION CHAMBER PLATEAU-VOLTSvs. COUNTS kV counts

0.2 49840

0.3 50690

0.4 50950

TABLE2.-FISSION CHAMBER

0.5 50580

0.6 50460

PLATEAU-SCALER

0.7 50590

0.8 50360

0.9 50590

BIASVS. COUNTS

Bias V counts

(x 10-y

characteristics of the fission chamber and fissile sources. The inter-electrode gap was normally set at 0.22 in. but changing it by a factor of two altered the length of the EHT plateau only slightly. Linear extrapolation of numerous bias curves to zero bias showed that 0.05 per cent of the pulses were lost below the scaler bias level. Therefore the counter was assumed to have an efficiency of 99.9 per cent for fission fragments. The EHT was set at 500 V at which setting it was certain that the chamber was operating on the plateau and yet the background counts were low. The scaler bias

A measurementof the fission cross sections of assU,asaUand esoPu

479

was set 4 V above the first bias setting that gave a count that was well on the bias plateau (e.g. at 13 V in Table 2). Alpha particle pulse ‘pile-up’ was first observed at 9 V bias but at 7 V bias still amounted to only 0.1 per cent of the usual fission fragment count rate. 2.2 Preparation of the cobalt monitor sources Spectrographically pure cobalt sponge (99.95 per cent) was dried in an electric furnace at 150°C and a portion (O-2402 g) was dissolved in the minimum quantity of warm concentrated nitric acid (A.R. quality). Dilution with conductivity water gave the flux monitor solution. Besides a correction for the 0.05 per cent impurity in the cobalt an additional correction of 0.49 & 0.01% had to be made for surface oxidation. This was measured by irradiating the dried sponge with 14 MeV neutrons and counting the lsN formed (COLEMAN,1962). Polystyrene film (0*0005 in.) was placed across the aperture of a Perspex annulus (1.9 in. o.d., 1.0 in. i.d., O-036 in. thick) and held in place with a thin smear of nitrocellulose adhesive. Care was taken to ensure that the source mounts were kept clean to reduce any radioactive contamination after irradiation. An aliquot of the cobalt solution (approx. 0.2 g), measured by difference weighing from a small Polythene ampoule, was deposited on to the centre of a polystyrene support which had first been wetted with insulin solution and rinsed with conductivity water. The annulus was rotated to spread the solution over an area of approximately 2 cm2 and dried in a vacuum desiccator over phosphorus pentoxide to give an average source thickness of about 0.4 mg/cm2. After a final drying under a heat lamp the source was sprayed with a little alcohol-soluble lacquer to ensure that the cobalt nitrate deposit adhered firmly to the support. The thin lacquer coating also reduced the hygroscopic activity of the cobalt salt during irradiation. Before and after irradiation the sources were stored in a desiccator. Background sources were prepared in the same way except that dilute nitric acid was used instead of cobalt nitrate solution. 2.3 Preparation of theJissile sources Extremely pure materials were used for these sources. The 233Uand 235U(isotopic purity 99.995 and 99.93 per cent respectively) were obtained from Oak Ridge National Laboratory. The 23gPu (isotopic purity 99.996 per cent) was produced in the AWRE Isotope Separator. Details of the mass analyses, made on the AWRE MSY mass spectrometer, are given in Table 3. This spectrometer (RIDLEYet al., 1965) is an allmetal tandem machine with 25 in. magnetic stage radius giving high abundance sensitivity. In the uranium mass region the contribution from a beam at one mass displacement is lo-* parts. Very thin uniform sources were prepared by electrodeposition from ammonium oxalate solution (RULFSet al., 1957) on to polished aluminium disks (1 in. diameter, 0.018 in. thick) using a rotating platinum anode. Plating was continued for about 4 hr at 0.5 A. The Polythene-lined, stainless-steel electroplating cylinder was operated in a water bath maintained at 85°C during the deposition. To ensure that good quality sources were obtained only about 60 per cent of the active material was deposited. With an aluminium substrate, increasing the plating time to increase the efficiency

480

R. L. G. KEITH, A. MCNAIR

and A.

L. RODGERS

TABLE 3.-IKITOPIC COMPOSITION OF FISSILE MATERLU

232 233 234 235 236 237 238 239 240

OX@08 99.995 omO5 0.0022 00001

(ATOMS %)

+ 0.0003 0.9541 ;t om47 0.025 f 0.003 98.980

f 0+002 0.022 f 0.003 & 0+0002 99.93 f 0~00001 OmOl f 0*00001

0+)014 f OwOl

0.048

f 0.003

04001 99.996 0*003 & owl

0.051

f 0.003

of deposition was found to result in sources giving a poor fission pulse plateau. Variation of the plating conditions might have overcome this problem but this was not investigated. 2.4 Assay of the&wile sources As the half-lives of B3U (l-620 & OWO5 x IO5 yr) and 2sgPu (24,360 f 50 yr) 1960; POPPLEWELL, were apparently known very accurately (HYDE, 1957; DOKUCHAEV, 1961; STROMINGER, 1958) a-counting appeared to provide the best way of determining the small quantity of f%ssilematerial on the 233Uand 23gPu sources. The a-counting was done in two low-geometry proportional counters (HURST et al., 1951), having geometry factors of about 700. The accuracy of the counters was estimated to be 0.25 per cent from metrology of the dimensions, from an International Intercomparison (WATT et al., 1963) and from calibration with 241Am standardized by coincidence counting. The results of the mass analyses and of a-spectrometry using a gridded ion chamber showed that a small correction was needed to allow for a-particles from 232U and daughters (0.2 per cent), and trace a-emitting impurities in the plutonium (0.2 per cent). The trace amount of 234Uin the !WJ contributed a large proportion of the total a-count from the 236Umaterial and this, coupled with the long and rather uncertain half-life of 235U, meant that assay of the 235Uby direct a-counting would have been rather inaccurate. Instead, about 1 per cent of pure 238U was added to the =W. In this mixed solution about 95.9 per cent of the a-particles were produced in the decay of the added mu so that only the half-life of 233Uwas of significance in estimating the amount of 235Uin the solution. The mass analysis of the mixed 2W and 233Uis given in Table 3. A correction was made for the a-activity of the 232U content of the 233U. The disintegration rate of the mixed 233Uand au sources was too low for counting in the low-geometry counter, but was large enough to give a good count rate on an a-scintillation counter whose efficiency was about 35 per cent. Each source was compared with two 2W standards calibrated by low-geometry counting. A large number of short counts were made on each mixed 2W and 2W source and the standards alternately, over two days, in order to reduce the effect of any possible change of efficiency with time. An additional uncertainty of O-2 per cent was allowed to take account of the scintillation counter measurements. The average thickness of the sources over 2.5 cm2 active area was approximately 2.0, 2.6 and O-7 ,ug/cm2 for the 233U, 235Uand 23gPusources respectively.

A measurement of the fissioncross sections of assU,2s5Uand 288Pu

481

2.5 Electronic equipment

A wide-band linear amplifier was used with a transistorized differenti head amplifier to eliminate spurious pulses from electrical interference. The fission chamber was operated with 500 V negative on the body of the chamber and the anode at earth potential. The chamber was contained within an earthed aluminium container and insulated from it by Perspex spacers. Interference pulses occurred simultaneously on the anode and screen circuits of the head amplifier and gave no output to the main amplifier. Two ~-MC/S transistorized scalers were used in parallel and the outputs were fed into register units. On the rare occasions when the results from the two scalers differed by more than O-2 per cent the measurement was rejected. An uncertainty of only O-1 per cent has been allowed in the fission count because at least 4 x lo9 fissions were recorded from each source. The dead-time of the whole counting system was found by comparing the change in counting rate from the fission chamber with that from a monitor fission chamber at various reactor operating power levels from 0.5 to 5 MW. The monitor fission chamber was placed in a low flux region of the same irradiation facility where the counting rate was low and the dead-time corrections very small. The fission chamber used for the cross-section measurements was operated in a higher flux region than normal to give an increased dead-time loss. The measured dead-time was (0.85 -& O-15) psec. The fission rate obtained during the cross-section measurements was never more than lo4 fissions/set so an error of ho.15 % was allowed in the final results for the uncertainty in the measured dead-time. 2.6 Irradiations The fission chamber in its electrostatic screen was installed in the thermal column and the time taken to measure the scaler bias plateau noted. The count rate observed after the bias level had been set was used to correct for the fissions unrecorded during the setting-up procedure. At the end of each irradiation similar measurements were made to check that the equipment was still functioning correctly. The time taken for these checks was about 1 hr in an irradiation period of about 120 hr. The uncertainty in estimating the number of fissions during the checking periods was negligible in comparison with the total number of fissions recorded. To check that day-to-day variations in the count rate from the fission chamber were caused by changes in the neutron flux in the thermal column rather than by mal-functioning of the counting equipment a flux monitor was placed in the same irradiation facility. The flux monitor system was identical to the one used for the cross-section measurements. Any significant variation in fission count rate from one system relative to the other was used as a criterion for rejecting that particular measurement. At the beginning of the series of irradiations the background of the fission chamber was less than 0.1 cps under the normal operating conditions, but by the end of the experiment had risen to 5 cps. The maximum correction for background in a fission count was less than 0.1 per cent and the error in this correction was negligible. Traces of material lost from the fissile sources could explain the increased background. Alpha counting was used to check each source before and after each irradiation, and these

482

R. L. G. KEITH,A. MCNAIRand A. L. ROWERS

measurements proved that a negligible amount of fissile material was lost during each irradiation. 2.7 Measurement of the @‘Cocontent of the cobalt monitors The 6oCo was determined by 47$-y coincidence counting. The @-detector was an argon + methane (90% + 10%) gas-flow proportional counter with two straight tungsten wire anodes (O%ll in. diameter). Two 3 x 3 in. thallium-activated sodium iodide crystals were used as the y-detectors and were placed in contact with thin aluminium windows in the walls of the /?-counter. Because of the rather low counting rates from the 6oCo sources, background in the y-detectors was the largest single source of possible error in the measurements. The effective background was kept to a minimum by using a single-channel pulse-height analyser to accept only the 1.17 MeV and I.33 MeV full-energy absorption peaks in the y-ray spectrum. The counting of the sources was done inside a low-background room (WILSON et al., 1961) and a further reduction in background was obtained by using a 2-in. thick lead shield round the counters. It was originally intended that the 6oCo sources would be counted shortly after the irradiation and that the activity of the background mounts would be used to correct the activity of the Yo sources. Although the activity on the background trays was only about 1 per cent of the 6oCo activity after a few days’ decay it was found to be not quite proportional to the 6oCo activity produced during the irradiation. The 6oCo sources were therefore counted after nearly a year’s decay when the contaminating activity was negligible, yet the uncertainty in the decay correction was still only 0.03 per cent. A half-life of 5.27 & 0.01 yr (Nuclear Data Sheets, 1964) was used for the decay of 6oCo. No trace of 6oCo was found inside the fission chamber after the experiment was over and this was taken to imply that no cobalt was lost during the irradiations. 2.8 Correctionsfor neutron&x gradient Absorption of neutrons in the aluminium and polystyrene film separating the fissile source from the cobalt monitor was estimated to be less than 0.1 per cent. The flux gradient inside the fission chamber itself was measured using gold, cobalt and aluminium foils. These measurements detected no difference in flux between the two positions, but as the precision of these measurements was no better than O-2 per cent an indirect measurement was made. The neutron flux gradient in the irradiation facility was measured by placing O@Ol-in. thick gold foils at 2 cm intervals in the void. These measurements suggested that the difference between the neutron flux at the position of the fissile foil and that at the position of the cobalt monitor might be as much as 0.35 per cent. The measurements were repeated, but a piece of Perspex similar to the cap of the fission chamber was placed in front of the array of gold foils to simulate the cross-section measuring conditions as closely as possible. The Perspex reduces the flux gradient (Table 4) and the difference in neutron flux at the cobalt and fissile source positions was calculated to be less than 0.05 per cent. The flux depression caused by the cobalt and the fissile material was calculated but was less than 0.05 per cent. A standard deviation of 0.1 per cent has been included in the possible errors in

A measurement of the fission cross sections of l=U, **YJand *Y’n

483

the measurements to cover uncertainties in the flux gradient, neutron absorption in the foils and flux depression by the sources. 2.9 Correction for neutron velocity spectrum

The irradiations were done in a void in the thermal column of the water-moderated reactor HERALD. Between the pool and the void lay 100 cm of graphite and surrounding the void in all other directions was at least 60 cm of graphite except for a 11*5-cm diameter bent tube through which the fission chamber was inserted. TABLE &-NEUTRON

FLUX GRADIENT IN THE IRRADIATION FACILITY

% Flux gradient/O.001 in. Normal With Perspex

Flux gradient position Measured over a distance of 2 cm in the highest flux region Measured over a distance of 2 cm behind the first position Measured over a distance of 2 cm behind the second position

0.016

0.001

0.017

@006

0.014

0.009

All of the irradiations were done at a reactor operating power of 5 MW and in a thermodynamic temperature of (35 f 5)“C, determined by thermocouple measurements. The WESTCOTT (1960,1964) epithermal index Ywas calculated to be (8.5 f 0.4) x lo4 at the irradiation position, from cadmium ratio measurements on gold foils inside the fission chamber. The values for the 2200 m/s cross sections were calculated from the results using the Westcott convention and based on the measured epithermal index and thermodynamic temperature. The results have not been further corrected for any divergence of the neutron energy spectrum from Maxwellian shape and no estimate of error has been included in the results to cover this possibility. A direct investigation of the energy spectrum was not made but results of WHITE et al. (1967) using the same irradiation facility, but working nearer the reactor core, showed that the spectrum is a good approximation to a Maxwellian distribution. Their thermal column crosssection measurements agreed within experimental error with their mono-kinetic measurements at O-0253 eV using the same detector and foils. Cross-section values have also been calculated, using a time-of-flight measurement of the thermal column neutron spectrum (REICHELT, 1965) which are well within the errors quoted in this paper. As this spectrum originated a few feet away from our irradiation facility the Westcott convention using the relevant cadmium ratio and thermodynamic temperature is preferred. A computer analysis has shown that there are negligible changes in cross-section values due to spectrum changes arising from the materials of the fission chamber. 3. RESULTS The experimental results from form of a ratio R defined by

each irradiation

R=-

F AxC

are tabulated

in Table

5 in the

R. L. G. Kerrr-r, A. MCNAIRand A. L. RODGERS

484

TABLE5.--EXPERIMENTAL VALUESOF R *sqJ

2aqJ

=*SPU

Source

R

Source

R

Source

R

A B

2.397 2.369

A A

254.5 254.5

A A

0.5549 0.5401

:

2.364 2,364

B C

255.4 2565

B C C

0.5438 0.5504 0.5432 0.5485

Mean

2.373 f O+IO8

255.2 i 0.5

O-5468 & 0.0022

TABLE6.-CALCULATEDSTANDARD DEVIATION POR EACHR-VALUE Standard deviation (“4 !&XU z*sPuand *ssU

Source of error Low geometry counting Fission counting Weight of cobalt on sources B°Cocounting Alpha scintillation counting

0-l 0.1 0.1 0.3

0.1 0.1 0.1 0.3 0.2

Total

0.35

0.4

TABLEI.-2200

m/s FISSIONCROSSSECTIONS

Nuclide

Cross section (barns)

e*qJ 815U =oPu

5346 f 5.3 5829 f 6.4 742.0 f 6.7

All results are relative to ra7Au o,,, = 98.8 b. ON THE PISSIONCROSSSECTIONS TABLEK-STANDARDDEVIATION Source of error

Standard deviation (%) a*qJ asrqj esopu

Experimental error on R Alpha purity of fissile material “OPu and *% half-lives Mass analysis 2ssU/g8sU Weight of cobalt metal Purity of cobalt metal Preparation of cobalt solution eoCo decay correction Scaler accuracy Low geometry factor Alpha scintillation counting Neutron flux gradient Fission chamber efficiency Fission chamber background Fission chamber dead-time Capture cross section 6gCo Neutron temperature and cadium ratio Systematic error in YZo counting

o-35 O-1 0.7 0.1 0.1 0.1 0.03 0.1 0.25 0.1 0.1 0.01 0.15 0.4 0.01 0.1

0.20 0.1 0.7 0.5

8:b 0.1 0.25 0.2 0.1 0.1 o-01 0.15 0.4 o-1 0.1

Total

1.0

1.1

8::

O-40 0.1 0.2 0.1 o-1 0.1 0.03 0-l 0.25 0: 0.1 0.01 o-15 0.4 0.4 0.1 0.9

485

A measurementof the fissioncross sections of essU,VJ and sssPu

where F is the total number of fissions produced in the fissile source during the irradiation, A the cc-disintegrations per second in the fissile source and C the %o disintegrations per second induced per gramme of cobalt in the monitors during the irradiation. This ratio should be constant for any one fissile isotope. The mean value and standard deviation on the mean are also given for each isotope. The 2200 m/s cross sections calculated from the experimental R-values are given in Table 7. The uncertainty in the various items required in the calculations are set out in Table 8, which also gives the overall standard deviation on the results. Although some of the errors are non-random in nature the individual errors are all relatively small and have been combined as a variance. 4. DISCUSSION When the fission cross sections were first evaluated from the measurements described in this report taking (1.620 &0*006) x lo5 yr (HYDE, 1957; DOKUCHAEV, 1960; POPPLEWELL,1961) as the half-life of 233Uit was found that the 23gPu fission cross section and the 233U/235Uratio of cross sections were in good agreement with the present recommended values (WESTCOTT,1965). The individual 233U and 235U cross-section values were, however, both about 3 per cent below the recommended values. This was a larger discrepancy than might have been expected from the estimated standard deviation on the present measurements. These low values, however, agreed with precise results reported by BIGHAMet al. (1958). The common factor in these investigations is that the half-life of 233Uis used to evaluate the fission cross sections of 233Uand 235U. The fact that the fission cross-section ratio B3U/235Uand the fission cross section of 23gPu agrees with the recommended values within experimental error is regarded as evidence to support the experimental reliability of the present work. Thus it appears that the low values for the fission cross sections of 233Uand 235Uinitially recorded by the authors and by other workers (BIGHAMet al., 1958) might be attributed to a correspondingly high value for the 233Uhalf-life. This quantity was therefore measured by one of us (KEITH, 1968) using in part the same 233U sources as were used for the 233Ufission cross-section measurements. TABLE 9.-RATIOS OF 220 m/s FISSIONCROSSSECTIONS Nuclide

2asu/*ssu zs9Pu/aasU zssPu/zasU

Cross-section

ratio

0.917 f 0006 1.388 f 0.014 1.273 & 0.014

TABLE lo.--STANDARD DEVIATIONON THE FISSIONCROSS-SECTION RATIOS Source of error

3

Standard deviation (%) assfi,assU 99spU,2asU z3sU/255U

Experimental error on R ratios Mass analysis essU/23SU Purity of fissile material aasPu and 2asU half-lives Alpha scintillation counting Neutron temperature and cadium ratio

0.4 0.5 0.1 0.0 0.2 0.1

0.5 0.0 0.1 0.7 0.0 0.4

0.5 0.5 0.1 0.7 0.2 0.4

Total

0.7

1.0

1.1

486

R. L. G. KEITH, A. MCNAIR and A. L. RODGERS TABLE ll.--‘IkE

EFFECTOF THE CHANGEOF ‘*‘U HALF-LIFEON THE FISSIONCROSS SECTIONSOF ==‘U AND ‘=‘U Half-life used

(yr> BIGHAM et al. (1958) KIE’l’H (1968) World average values BIGHAM et al. (1958) KEITH (1968)

1.620 1.620 1.553 1.553

x lo6 x lo6 x 106 x 106

All results are relative to lo7Au a,,,

=au a, r (barns)

*ssI_Jun * (barns)

514 512 528 536 534

565 558 580 589 583

;t 4 i 5 f f

4 5

f f

6 6

f f

6 6

740 f 5 741 & 7 742 -

= 98-7 b.

Two determinations of the233U half-lifegaveameanvalue of (1553 & 0.010) x 106 yr which is approximately 4 per cent below previously reported values. This new half-life value when used to evaluate the 233U and 235Ufission cross section results of BIGHAM et al. (1958) and the present work brings them into good agreement with the recommended values (Table 11). Given in Table 11 are the %OPu cross-section values, which are independent of the 2W half-life, to indicate the probable reliability of the input data. It would appear therefore that someof the discrepancies which have been observed in the 2200 m/set fission cross sections might be explained by a corrected mu halflife. In view of the success it has in producing a self-consistent set of data we have adopted the new 2W half-life value in these fission cross-section evaluations. Acknowledgments-We should like to thank Mr. A. C. T~RRELL for the mass analyses of the fissile samples and the staff of the HERALDreactor for their help and co-operation. REFERENCES BIGHAM C. B., HANNA G. C., TUNNICLIFFEP. R., CAMPIONP. J., LOUNSBURYM. and MACKENZIE D. R. (1958) Proceedings of the Second International Conference on the Peaceful Uses of Atomic Energy, Geneva, Vol. 16, p. 125. United Nations N.Y. COLEMANR. F. (1962) Analyst 87,590. DOKUCHAEVI. P. and OSIPOV I. S. (1960) Reactor Sci. (J. nucl. Energy Part A) 11, 194. HURST R., HALL G. R., GLOVER K. M. (1951) UKAEA Report AERE C/R 647. HYDE E. K. (1957) USAEC Report TID 5223. KEITH R. L. G. (1968) This issue, p. 471. MCENHILL J. J., RODGERSA. L. and TODD M. C. J. (1965) J. Br. Nucl. Energy Sot. 4,344. Nuclear Data Sheets (1960) N.R.C. National Academy of Science, Washington D.C. POPPLEWELLD. S. (1961) Reactor Sci. Technol. (J. nucl. Energy Parts A/B) 14,50. REICHELTJ. M. A. (1965) Report AWRE R4/65. RIDLEY R. G., HARDY R. W. D., HAYES R., MUNRO R., WILSON H. W. and YOUNG W. A. P. (1965) Advances in Mass Spectrometry, Vol. 3, p. 553, Pergamon Press. RULPS C. L., DE A. K. and ELVING P. J. (1957) J. electrochem. Sot. 104,80. STROMINGERD., HOLLANDERJ. M. and SEABORGG. T. (1958) Rev. mod. Phys. 30,2. VANINBROUKXR. (1966) Nucl. Sci. Engng 24, 87. WATT D. E., KEITH R. L. G. and BROWN F. (1963) UKAEA Report AWRE O-59/63. WESTCOTTC. H. (1960) Report AECL 1101; (1964) Addendum. WESTCOTTC. H., EKBERGK., HANNA G. C., PA~~FNDENN. J., SANATANIS. and ATI-REEP. M. (1965)

Atom. Energy Rev. 3 (2) 3.

WHITE P. H., RFICHELTJ. M. A. and WARNER G. P. (1957) Nuclear Datafor Reactors, Vol. 2, p. 29, IAEA, Vienna. WILSON H. W., WATT D. E. and RAMSDEND. (1961) Znt.J. appl. Rudiat. Isotopes 10, 156.