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Special ORNL isotopic preparations
N U C L E A R I N S T R U M E N T S AND METHODS
38 (I965) 87-90;
© N O R T H - H O L L A N D P U B L I S H I N G CO.
SPECIAL ORNL ISOTOPIC PREPARATIONS* L. O. LOVE, H. R. GWINN, G. D. ALTON, F. R. O'DONNELL, E. E. McCOMBS, R. L. BAILEY, J. O. YOUNGHANSE and E. W. McDANIEL Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA
The electromagnetic separations group at Oak Ridge National Laboratory (ORNL) has collected and purified isotopes for special purposes, including 46Ca, 48Ca, 182W, 184W, 186W, 335, 29Si and 30Si. Certain contaminants have been reduced to the ppb-range in some preparations. Experiences in preparing ~ 10-g crystals of 48CAF2 and > 100-g cylinders of tungsten isotopes are given.
The results of experimental ion collections on single crystals of copper and on various other substrates are presented. These collection techniques are applicable for preparation of specimens for diffusion studies, the formation of semiconductors, etc. Singly or doubly charged ions can be deposited with a considerable choice of ion velocity. In other preparations, actinide isotopes have been electrodeposited uniformly on a variety of metal forms.
1. Introduction
showed the purity to be slightly but consistently higher when collected on (110) oriented crystals than on other substrates. In collections where the isotopic purity is already adequate, improvements o f this magnitude do not warrant the use o f such an approach. However, in cases involving marginal purity o f isotopes o f very low natural abundance, collection on single crystals could be used to improve the purity o f very small samples.
O R N L is often asked to enrich to high isotopic purities significant a m o u n t s of isotopes having low natural abundances and to purify small quantities of isotopic material for conversion or fabrication into usable forms with as little loss, dilution, or chemical contamination as possible. Because o f the availability o f unique equipment and technical personnel proficient in the specialized techniques required, the Electromagnetic Isotope Separations Department at O R N L has prepared targets by calutron ion implantation and by electrodeposition o f radioactive materials.
TABLE 1 Comparative collections of 46Ca Target material
Estimated wt (,ug)
Recovered wt (~ug)
Assay 46Ca (~)
Ratio 46Ca/44Ca
2. Isotope separations 2.1. CALCIUM 2.1.1. Processing o f 46Ca Experimental data received from the L a b o r a t o r y for Mass Separation at A m s t e r d a m in 1960 suggested the influence o f crystal orientation on the sputtering ratio and indicated that the (110) copper orientation gave a lower sputtering ratio than others. This indicated that ion penetration and absorption in the more open (110) direction might be higher than in other orientations, a difference which could be significant when collecting isotopes having low natural abundances. The increase in ion penetration could improve the ratio o f energetic to neutral particle retention which would increase isotopic purity o f collected samples. The concept was investigated by collecting 46Ca on single crystals o f copper using both (110) and ( l l 2 ) orientations, on polycrystalline copper, on graphite and on pyrolytic graphite (table 1). Results pertaining to retention were inconclusive, but experiments conducted during regular collections over a two-year period (total 46Ca, collected mainly by conventional methods, equaled ,,~ 500 mg having an isotopic purity > 40%)
Cu Cu (110) Cu (112) C*
960 960 960 960
210 350 375 525
45.6 47.0 45.6 33.0
15.7 18.2 14.8 12.7
Cu Cu (110) Cu (112) Ct
1100 1100 1100 1100
355 413 255 450
47.7 50.5 49.3 41.8
18.8 20.1 19.4 17.6
Cu Cu (110) Ag C*
1850 1850 1850 1850
360 785 475 863
49.1 51.0 50.1 44.6
17.0 19.5 17.2 17.1
* Type C-18 graphite. t Pyrolytic graphite. 2.1.2. Data on sputtering In connection with the calcium separations, some data have been accumulated on sputtering. The average n u m b e r o f carbon atoms sputtered by each 35-keV 4°Ca ion at an average current density of 1.5 m A / c m 2 was approximately three (table 2). Since the temperature o f the graphite block varied over its surface, the numbers are presented as average values and no attempt has been made to correct for the lack o f uni-
* Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation. 87
II. ION OPTICS, ETC.
L.O. LOVE et al.
88 TABLE 2
Carbon sputtering with 35 keV 40Ca ions 4oCa ion
Loss of C (g)
Monitored 40Ca (g)
Sputtering ratio C/Ca
current (mA/cmZ)
102.850 97.150 99.949 129.031 96.129
95.508 95.973 103.672 128.468 96.501
3.59 3.38 3.22 3.35 3.32
1.45
1.51 1.56 1.62 1.86
formity in intensity of ion distribution at the collector. An experiment has also been designed to yield further data on the saturation value as well as the sputtering ratio for 35-keV 4°Ca ions bombarding copper crystals and to establish the isotopic purity of the calcium trapped in the target lattice. The project is being carried out in the 180 °, 24"-radius calutron. Angular divergence is limited to ___2 ° by a baffling assembly located at the 90 ° position and by the geometry of the receiver entrance slit. The data will be applied to a calculation of the mean ion penetration depth to test the validity of the "ion sorption" approach to ion penetration studies. 2.1.3. Reprocessing o/48Ca F r o m recent calcium separations ~ 14 g of 96.5% 4SCa was allocated to Brookhaven National Laboratory (BNL) for use in an experiment designed to determine a lower limit for the half-life of 48Ca, which decays by neutrinoless double-beta emission. The isotopically enriched calcium sample was transferred to Harshaw Chemical Company where it was converted to CaF2, 0.5% europium was added as a scintillator and a single crystal was grown and mounted in a stainless steel sleeve to form an optical device (fig. 1) for use at BNL. During initial phases of the experiment at BNL, an alpha activity of 4000 cph was detected. This was an intolerable interference, and the work had to be discontinued. B N L concluded that the alpha-emitting contaminant was uranium and the crystal was returned to Oak Ridge for repurification. Chemical separation of uranium from the CaF z and determination of the quantity of uranium by a mass spectrometric isotope dilution method at O R N L established the presence of 27 ppb uranium. After very careful reprocessing, analysis of the final sample showed < 0.5 ppb uranium contamination. The reprocessed material was returned to Harshaw where another crystal was grown and forwarded to
Fig. 1. 48CAF2crystal. B N U The alpha activity had been reduced by a factor of 5 below the level shown in the original crystal and the experiments were continued. 2.2. TUNGSTEN As a service to another government agency, O R N L has recently provided kilogram quantities of enriched tungsten isotopes. These materials are to be used in making detailed neutron cross-section and reactivity measurements to evaluate the feasibility of using separated tungsten isotopes in high-temperature nuclear reactors. Three cylinders of equal height and diameter and containing ~ 125 g of 182W, 184W and ~86W, respectively, were formed (at 95% theoretical density) by isostatic pressing and sintering. Two other tungsten cylinders containing a total of ~ 150 g of 94°/~ 184W were prepared (at 90% theoretical density) for use in making neutron-scattering measurements. These also were isostatically pressed and sintered to particular specifications without drilling or machining. 2.3. SULFUR The O R N L Biology Division requires 33p of high specific activity and comparatively free of 32p to use in
SPECIAL ORNL ISOTOPIC PREPARATIONS
studying the effects of radiation on various viruses and micro-organisms. One approach to the production of 33p involves irradiation of high-purity 338. After a considerable amount of development workl), the electromagnetic separations group supplied ,,~ 100 mg of ~ 70% 335, part of which is now being irradiated to provide 33p for use in these biochemistry studies. 2.4. S1LICON The separations group has also made available 100-g quantities of 29Si and 3°Si (natural abundances are 4.68 and 3.05% respectively) achieving > 95% isotopic purity with both isotopes2). Quantities of these two isotopes had been requested by the nuclear crosssection group. The isotopes were to be used for growing single crystals needed in fabricating lithium-coated, silicon barrier-layer detectors for improving the counting efficiency of neutron spectrometers. It has also been suggested that neutron detectors of enriched silicon isotopes could be used for determining the flux and spectra of neutrons in space.
3. Target preparations Recently the electromagnetic separations group has been developing and perfecting skills and equipment for use in preparing target specimens by direct ion deposition in the calutron and by electrodeposition of several actinides onto a variety of metallic forms. 3.1. ION-IMPLANTATIONTECHNIQUE The ion-implantation technique affords a means of obtaining mass separation and implantation in a onestep process using high-vacuum conditions which minimize chemical contamination of the samples. The exact amount of material striking the target can be monitored accurately, and rotation of substrates by the use of specially designed collectors (fig. 2) results in uniform deposition and provides economical formation of multiple targets in short periods of time. One type of rotating collector accommodates 100 substrates, each having an area of ~ 0.5 cm 2. The design permits each row of ten specimens to be bombarded with a different quantity dosage or energy level. A second holder, also capable of rotation, has three circumferentially located rings with 18 substrates per ring. A third collector is smaller and can be used for bombardments using mildly radioactive materials. This target holder assembly can be withdrawn into an air lock for target removal, and the holder itself can be disengaged from the gear mechanism and removed by simply lifting it out of the assembly. With these special collectors and by manipulation of
89 5 / zJ ~
Fig. 2. Apparatus for ion implantation. 1. Slide type indexing shutter; 2. Rotating substrate holder; 3. Beam entrance slots to substrates; 4. Beam entrance slots to probe; 5. Single crystalline material to be bombarded.
the magnetic field and accelerating voltages, donor or acceptor ions can be embedded into the host crystal at discrete energies ranging from 10 to 160 keV. The energy is spanned by a combination of technique and equipment modifications. Implantation of 10- to 40keV ions requires only adjustment of the applied positive accelerating potential; ions of 40 to 80 keV can be provided either by collection of doubly ionized particles or by the application of negative potentials of 0 to 40 kV to the target holder; ions of 80 to 160 keV require the combined use of doubly ionized particles and negative receiver potentials. Samples prepared by the ion-implantation technique have been used by the O R N L Reactor Chemistry Division to measure diffusion rates. Specific examples of these targets include 232Th and 235U implanted in pyrolytic carbon and graphite at energies of 40 keV. 3.2. ELECTRODEPOSITION METHOD
A number of targets have been prepared by the electrodeposition of separated isotopes of several actinides onto a variety of metallic forms. The following examples illustrate three variations in materials and methods used to meet definite specifications. A particular fission chamber assembly required the electrodeposition of 233U on both the inside and outside of a nest of eight concentric titanium cylinders (height, 2"; dia, 1"-2.5"). One nest was plated at ~ 0.1 mg/cm 2 and the other at ~ 0.9 mg/cm/. After plating, each set was assembled and sealed into an airtight chamber using an argon atmosphere. 235U was deposited on six 8" dia. zirconium foils for II. ION OPTICS~ ETC.
90
L.O. LOVE et al.
accelerator work. An active area 6" in dia. was plated with 90 mg of uranium. Uniformity of the target layer produced was verified by preparing an experimental foil, removing ~3 t! dia. disks and determining thickness of the deposit by alpha counting. Several hundred sources of 241Am plated on platinum disks were fabricated for use in instrument calibration, particularly solid state detectors, to meet these specifications: good adherence, high resolution of
alpha energy, activity of ~0.10/~Cur and an active area slightly less than 3 mm in dia.
References 1) L. T. Newman, R. M. Ennis and W. A. Bell et al., these proceedings, p. 82. z) L. O. Love and W. A. Bell, Review of ORNL Electromagnetic Separation Program (1963) USAEC Rpt. ORNL-3606 (May 1964).
38 (I965) 87-90;
© N O R T H - H O L L A N D P U B L I S H I N G CO.
SPECIAL ORNL ISOTOPIC PREPARATIONS* L. O. LOVE, H. R. GWINN, G. D. ALTON, F. R. O'DONNELL, E. E. McCOMBS, R. L. BAILEY, J. O. YOUNGHANSE and E. W. McDANIEL Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA
The electromagnetic separations group at Oak Ridge National Laboratory (ORNL) has collected and purified isotopes for special purposes, including 46Ca, 48Ca, 182W, 184W, 186W, 335, 29Si and 30Si. Certain contaminants have been reduced to the ppb-range in some preparations. Experiences in preparing ~ 10-g crystals of 48CAF2 and > 100-g cylinders of tungsten isotopes are given.
The results of experimental ion collections on single crystals of copper and on various other substrates are presented. These collection techniques are applicable for preparation of specimens for diffusion studies, the formation of semiconductors, etc. Singly or doubly charged ions can be deposited with a considerable choice of ion velocity. In other preparations, actinide isotopes have been electrodeposited uniformly on a variety of metal forms.
1. Introduction
showed the purity to be slightly but consistently higher when collected on (110) oriented crystals than on other substrates. In collections where the isotopic purity is already adequate, improvements o f this magnitude do not warrant the use o f such an approach. However, in cases involving marginal purity o f isotopes o f very low natural abundance, collection on single crystals could be used to improve the purity o f very small samples.
O R N L is often asked to enrich to high isotopic purities significant a m o u n t s of isotopes having low natural abundances and to purify small quantities of isotopic material for conversion or fabrication into usable forms with as little loss, dilution, or chemical contamination as possible. Because o f the availability o f unique equipment and technical personnel proficient in the specialized techniques required, the Electromagnetic Isotope Separations Department at O R N L has prepared targets by calutron ion implantation and by electrodeposition o f radioactive materials.
TABLE 1 Comparative collections of 46Ca Target material
Estimated wt (,ug)
Recovered wt (~ug)
Assay 46Ca (~)
Ratio 46Ca/44Ca
2. Isotope separations 2.1. CALCIUM 2.1.1. Processing o f 46Ca Experimental data received from the L a b o r a t o r y for Mass Separation at A m s t e r d a m in 1960 suggested the influence o f crystal orientation on the sputtering ratio and indicated that the (110) copper orientation gave a lower sputtering ratio than others. This indicated that ion penetration and absorption in the more open (110) direction might be higher than in other orientations, a difference which could be significant when collecting isotopes having low natural abundances. The increase in ion penetration could improve the ratio o f energetic to neutral particle retention which would increase isotopic purity o f collected samples. The concept was investigated by collecting 46Ca on single crystals o f copper using both (110) and ( l l 2 ) orientations, on polycrystalline copper, on graphite and on pyrolytic graphite (table 1). Results pertaining to retention were inconclusive, but experiments conducted during regular collections over a two-year period (total 46Ca, collected mainly by conventional methods, equaled ,,~ 500 mg having an isotopic purity > 40%)
Cu Cu (110) Cu (112) C*
960 960 960 960
210 350 375 525
45.6 47.0 45.6 33.0
15.7 18.2 14.8 12.7
Cu Cu (110) Cu (112) Ct
1100 1100 1100 1100
355 413 255 450
47.7 50.5 49.3 41.8
18.8 20.1 19.4 17.6
Cu Cu (110) Ag C*
1850 1850 1850 1850
360 785 475 863
49.1 51.0 50.1 44.6
17.0 19.5 17.2 17.1
* Type C-18 graphite. t Pyrolytic graphite. 2.1.2. Data on sputtering In connection with the calcium separations, some data have been accumulated on sputtering. The average n u m b e r o f carbon atoms sputtered by each 35-keV 4°Ca ion at an average current density of 1.5 m A / c m 2 was approximately three (table 2). Since the temperature o f the graphite block varied over its surface, the numbers are presented as average values and no attempt has been made to correct for the lack o f uni-
* Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation. 87
II. ION OPTICS, ETC.
L.O. LOVE et al.
88 TABLE 2
Carbon sputtering with 35 keV 40Ca ions 4oCa ion
Loss of C (g)
Monitored 40Ca (g)
Sputtering ratio C/Ca
current (mA/cmZ)
102.850 97.150 99.949 129.031 96.129
95.508 95.973 103.672 128.468 96.501
3.59 3.38 3.22 3.35 3.32
1.45
1.51 1.56 1.62 1.86
formity in intensity of ion distribution at the collector. An experiment has also been designed to yield further data on the saturation value as well as the sputtering ratio for 35-keV 4°Ca ions bombarding copper crystals and to establish the isotopic purity of the calcium trapped in the target lattice. The project is being carried out in the 180 °, 24"-radius calutron. Angular divergence is limited to ___2 ° by a baffling assembly located at the 90 ° position and by the geometry of the receiver entrance slit. The data will be applied to a calculation of the mean ion penetration depth to test the validity of the "ion sorption" approach to ion penetration studies. 2.1.3. Reprocessing o/48Ca F r o m recent calcium separations ~ 14 g of 96.5% 4SCa was allocated to Brookhaven National Laboratory (BNL) for use in an experiment designed to determine a lower limit for the half-life of 48Ca, which decays by neutrinoless double-beta emission. The isotopically enriched calcium sample was transferred to Harshaw Chemical Company where it was converted to CaF2, 0.5% europium was added as a scintillator and a single crystal was grown and mounted in a stainless steel sleeve to form an optical device (fig. 1) for use at BNL. During initial phases of the experiment at BNL, an alpha activity of 4000 cph was detected. This was an intolerable interference, and the work had to be discontinued. B N L concluded that the alpha-emitting contaminant was uranium and the crystal was returned to Oak Ridge for repurification. Chemical separation of uranium from the CaF z and determination of the quantity of uranium by a mass spectrometric isotope dilution method at O R N L established the presence of 27 ppb uranium. After very careful reprocessing, analysis of the final sample showed < 0.5 ppb uranium contamination. The reprocessed material was returned to Harshaw where another crystal was grown and forwarded to
Fig. 1. 48CAF2crystal. B N U The alpha activity had been reduced by a factor of 5 below the level shown in the original crystal and the experiments were continued. 2.2. TUNGSTEN As a service to another government agency, O R N L has recently provided kilogram quantities of enriched tungsten isotopes. These materials are to be used in making detailed neutron cross-section and reactivity measurements to evaluate the feasibility of using separated tungsten isotopes in high-temperature nuclear reactors. Three cylinders of equal height and diameter and containing ~ 125 g of 182W, 184W and ~86W, respectively, were formed (at 95% theoretical density) by isostatic pressing and sintering. Two other tungsten cylinders containing a total of ~ 150 g of 94°/~ 184W were prepared (at 90% theoretical density) for use in making neutron-scattering measurements. These also were isostatically pressed and sintered to particular specifications without drilling or machining. 2.3. SULFUR The O R N L Biology Division requires 33p of high specific activity and comparatively free of 32p to use in
SPECIAL ORNL ISOTOPIC PREPARATIONS
studying the effects of radiation on various viruses and micro-organisms. One approach to the production of 33p involves irradiation of high-purity 338. After a considerable amount of development workl), the electromagnetic separations group supplied ,,~ 100 mg of ~ 70% 335, part of which is now being irradiated to provide 33p for use in these biochemistry studies. 2.4. S1LICON The separations group has also made available 100-g quantities of 29Si and 3°Si (natural abundances are 4.68 and 3.05% respectively) achieving > 95% isotopic purity with both isotopes2). Quantities of these two isotopes had been requested by the nuclear crosssection group. The isotopes were to be used for growing single crystals needed in fabricating lithium-coated, silicon barrier-layer detectors for improving the counting efficiency of neutron spectrometers. It has also been suggested that neutron detectors of enriched silicon isotopes could be used for determining the flux and spectra of neutrons in space.
3. Target preparations Recently the electromagnetic separations group has been developing and perfecting skills and equipment for use in preparing target specimens by direct ion deposition in the calutron and by electrodeposition of several actinides onto a variety of metallic forms. 3.1. ION-IMPLANTATIONTECHNIQUE The ion-implantation technique affords a means of obtaining mass separation and implantation in a onestep process using high-vacuum conditions which minimize chemical contamination of the samples. The exact amount of material striking the target can be monitored accurately, and rotation of substrates by the use of specially designed collectors (fig. 2) results in uniform deposition and provides economical formation of multiple targets in short periods of time. One type of rotating collector accommodates 100 substrates, each having an area of ~ 0.5 cm 2. The design permits each row of ten specimens to be bombarded with a different quantity dosage or energy level. A second holder, also capable of rotation, has three circumferentially located rings with 18 substrates per ring. A third collector is smaller and can be used for bombardments using mildly radioactive materials. This target holder assembly can be withdrawn into an air lock for target removal, and the holder itself can be disengaged from the gear mechanism and removed by simply lifting it out of the assembly. With these special collectors and by manipulation of
89 5 / zJ ~
Fig. 2. Apparatus for ion implantation. 1. Slide type indexing shutter; 2. Rotating substrate holder; 3. Beam entrance slots to substrates; 4. Beam entrance slots to probe; 5. Single crystalline material to be bombarded.
the magnetic field and accelerating voltages, donor or acceptor ions can be embedded into the host crystal at discrete energies ranging from 10 to 160 keV. The energy is spanned by a combination of technique and equipment modifications. Implantation of 10- to 40keV ions requires only adjustment of the applied positive accelerating potential; ions of 40 to 80 keV can be provided either by collection of doubly ionized particles or by the application of negative potentials of 0 to 40 kV to the target holder; ions of 80 to 160 keV require the combined use of doubly ionized particles and negative receiver potentials. Samples prepared by the ion-implantation technique have been used by the O R N L Reactor Chemistry Division to measure diffusion rates. Specific examples of these targets include 232Th and 235U implanted in pyrolytic carbon and graphite at energies of 40 keV. 3.2. ELECTRODEPOSITION METHOD
A number of targets have been prepared by the electrodeposition of separated isotopes of several actinides onto a variety of metallic forms. The following examples illustrate three variations in materials and methods used to meet definite specifications. A particular fission chamber assembly required the electrodeposition of 233U on both the inside and outside of a nest of eight concentric titanium cylinders (height, 2"; dia, 1"-2.5"). One nest was plated at ~ 0.1 mg/cm 2 and the other at ~ 0.9 mg/cm/. After plating, each set was assembled and sealed into an airtight chamber using an argon atmosphere. 235U was deposited on six 8" dia. zirconium foils for II. ION OPTICS~ ETC.
90
L.O. LOVE et al.
accelerator work. An active area 6" in dia. was plated with 90 mg of uranium. Uniformity of the target layer produced was verified by preparing an experimental foil, removing ~3 t! dia. disks and determining thickness of the deposit by alpha counting. Several hundred sources of 241Am plated on platinum disks were fabricated for use in instrument calibration, particularly solid state detectors, to meet these specifications: good adherence, high resolution of
alpha energy, activity of ~0.10/~Cur and an active area slightly less than 3 mm in dia.
References 1) L. T. Newman, R. M. Ennis and W. A. Bell et al., these proceedings, p. 82. z) L. O. Love and W. A. Bell, Review of ORNL Electromagnetic Separation Program (1963) USAEC Rpt. ORNL-3606 (May 1964).