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Preparation of radioactive samples by isotope separation
NUCLEAR
INSTRUMENTS
AND METHODS
38 (t965) 3 1 2 - 3 1 3 ; © N O R T H - H O L L A N D
PUBLISHING
CO.
P R E P A R A T I O N OF R A D I O A C T I V E S A M P L E S BY I S O T O P E S E P A R A T I O N O. B. NIELSEN The Niels Bohr Institute, University of Copenhagen, Denmark
This survey is based on information received from a large number of isotope separator laboratories in response to our request to supply data on their activities with respect to the preparation of
radioactive source material for nuclear investigations since the Paris conference in 1962. Emphasis is put on the techniques used during the preparations.
This survey was hoped to serve the purpose of giving the Conference an impression of the present use of isotope separators as a tool for the preparation of radioactive sources for nuclear spectroscopy and also of communicating such new technical developments which might be of general interest without being suited for presentation as a contribution. The material received from numerous separator groups in response to our request forms an extremely valuable source of information. It also reveals one very encouraging fact: 12 to 15 separators are now run in some kind of cooperation with nuclear spectroscopy groups, against 5 to 6 at the time of the Paris conference in 1962. However, it turned out also to be somewhat discouraging: only few technical improvements seem to have been developed since the last conference, or even since the conference in Vienna in 1960. As is well known, the problem in connection with the separation of radioactive nuclides are mainly related to three aspects: 1. The efficiency must be high, both in view of scarcity of the active charge material, or for reasons of health hazards and contamination. 2. The chemical preparation of the charge compound must be performed on small amounts of material. 3. When dealing with short lived nuclides, only limited time is available for chemical preparation and isotope separation. The elements which are volatile, or easily form volatile compounds, of course, need not cause great trouble. Only the chlorides or fluorides of many elements, however, are sufficiently volatile to allow easy evaporation in the ion sources. It was mainly with the aim to overcome the difficulties in preparing small amounts of waterfree chlorides that Skilbreid and Sidenius introduced the CCl,~-method which is characterized by the formation of the chlorides inside the ion source proper through the reaction of CC14 vapour on oxide compounds at reasonably high temperatures. Since this technique was presented at the Vienna conference, it has been adopted by most separator groups. It is effective with a surprisingly large number
of elements, such as the rare earths and the actinides. In addition to removing the necessity of preparing chlorides prior to separation, the procedure usually is advantageous also in view of the fact that it allows a stable operation of the ion source and thus also improves the efficiency. Some groups report efficiencies of the order of 10% in separations of sources for nuclear spectroscopy; efficiency here is defined as the ratio of the separated activity to the activity introduced into the ion source. It is interesting to note that now this method has been adopted generally also for large scale separations, such as for calutron work in Harwell and Oak Ridge, often raising the efficiency by a factor of two. With respect to the third point, the method is not especially suited for short lived activities, since the highest yields are only obtained in separations lasting hours. A realistic figure for efficiencies obtained in shorter periods is about 4%/h; activities with half-lives of the order of minutes can thus only be separated with a considerably reduced yield. A method especially designed for solving the problem of handling very short lived nuclides was presented at the Vienna conference by Alv~.ger and Uhler. They suggested the use of two ovens in the ion source, the first one for establishing good operational conditions by having a charge of stable isotopes of the element to be separated, whereas a carrierfree activity is later introduced into the ion source via the second oven, from which it is evaporated without disturbing the operation very much. In this way the activity is separated under the most favourable conditions in a minimum of time. During the last few years also this method has been adopted as a standard technique in most laboratories performing separations of radioactive nuclides. From the contribution of Dr. Camplan it appeared that the Saclay group has even developed an ion source utilizing altogether five different ovens, thereby securing a maximum of flexibility. It should be noted that the carbon-tetrachloride and the multiple oven techniques are intended to solve essentially different problems in ion source operation. 312
PREPARATION
OF R A D I O A C T I V E
It is possible, however, to combine the two methods by introducing a carrierfree activity into an ion source which operates on a stable isotope reacting with carbontetrachloride; some progress has hereby been made in the handling of very short lived isotopes of non-volatile elements. The results obtainable by present techniques in the separation of radioactive isotopes could be optimistically summarized as follows: 1. All elements can be separated, maybe with the exception of the platinum metals. 2. Efficiencies of the order of 10% can be obtained, with a few more exceptions than under 1. 3. If necessary, separations can be performed in a few minutes. 4. Enrichment factors of the order of 1000, or more than usually needed, are obtainable. 5. The method is unique with respect to the quality of the sources. From such a survey one could be tempted to conclude that new methods have not been developed in recent years because the techniques at present are satisfactory. I think, however, that all workers in this field will agree that the points listed above do not outline a realistic picture of the present state of affairs. Many elements are separated with great difficulties in most laboratories and the average efficiency actually obtained is probably closer to 1% than to 10%. More
S A M P L E S BY I S O T O P E S E P A R A T I O N
313
reliable and reproducible methods of ion source operation are urgently needed. In addition, the very high efficiencies, up to 50%, which are reported at intervals, remind us of the fact that the limitations in the usual performance are not dictated by any law of physics. When considering the separation of isotopes with half-lives in the region of minutes, one has to realize that the present methods are only of limited value for the nuclear physicist. Valuable information can be obtained, of course, from measurements of short duration, but detailed spectroscopic investigations of a nuclide demand tens or even hundreds of hours of measurements. The progress of the on-line projects is an exciting development in nuclear physics, not only because it opens possibilities for the study of activities with still shorter half-lives. The continuous deposition of activity characterizing the on-line separation will be of decisive importance also for the measurements of nuclides with half-lives of the order of ½ - 1 h. When discussing the need for such time consuming experiments, we have to take into account the possibility of intensive utilization of the activities separated on-line. Usually beams of several active isotopes will be formed simultaneously and, by applying conventional beam handling methods, these isotopes can be studied separately in different spectroscopic set-ups.
VI. A P P L I C A T I O N S
TO N U C L E A R
PHYSICS
INSTRUMENTS
AND METHODS
38 (t965) 3 1 2 - 3 1 3 ; © N O R T H - H O L L A N D
PUBLISHING
CO.
P R E P A R A T I O N OF R A D I O A C T I V E S A M P L E S BY I S O T O P E S E P A R A T I O N O. B. NIELSEN The Niels Bohr Institute, University of Copenhagen, Denmark
This survey is based on information received from a large number of isotope separator laboratories in response to our request to supply data on their activities with respect to the preparation of
radioactive source material for nuclear investigations since the Paris conference in 1962. Emphasis is put on the techniques used during the preparations.
This survey was hoped to serve the purpose of giving the Conference an impression of the present use of isotope separators as a tool for the preparation of radioactive sources for nuclear spectroscopy and also of communicating such new technical developments which might be of general interest without being suited for presentation as a contribution. The material received from numerous separator groups in response to our request forms an extremely valuable source of information. It also reveals one very encouraging fact: 12 to 15 separators are now run in some kind of cooperation with nuclear spectroscopy groups, against 5 to 6 at the time of the Paris conference in 1962. However, it turned out also to be somewhat discouraging: only few technical improvements seem to have been developed since the last conference, or even since the conference in Vienna in 1960. As is well known, the problem in connection with the separation of radioactive nuclides are mainly related to three aspects: 1. The efficiency must be high, both in view of scarcity of the active charge material, or for reasons of health hazards and contamination. 2. The chemical preparation of the charge compound must be performed on small amounts of material. 3. When dealing with short lived nuclides, only limited time is available for chemical preparation and isotope separation. The elements which are volatile, or easily form volatile compounds, of course, need not cause great trouble. Only the chlorides or fluorides of many elements, however, are sufficiently volatile to allow easy evaporation in the ion sources. It was mainly with the aim to overcome the difficulties in preparing small amounts of waterfree chlorides that Skilbreid and Sidenius introduced the CCl,~-method which is characterized by the formation of the chlorides inside the ion source proper through the reaction of CC14 vapour on oxide compounds at reasonably high temperatures. Since this technique was presented at the Vienna conference, it has been adopted by most separator groups. It is effective with a surprisingly large number
of elements, such as the rare earths and the actinides. In addition to removing the necessity of preparing chlorides prior to separation, the procedure usually is advantageous also in view of the fact that it allows a stable operation of the ion source and thus also improves the efficiency. Some groups report efficiencies of the order of 10% in separations of sources for nuclear spectroscopy; efficiency here is defined as the ratio of the separated activity to the activity introduced into the ion source. It is interesting to note that now this method has been adopted generally also for large scale separations, such as for calutron work in Harwell and Oak Ridge, often raising the efficiency by a factor of two. With respect to the third point, the method is not especially suited for short lived activities, since the highest yields are only obtained in separations lasting hours. A realistic figure for efficiencies obtained in shorter periods is about 4%/h; activities with half-lives of the order of minutes can thus only be separated with a considerably reduced yield. A method especially designed for solving the problem of handling very short lived nuclides was presented at the Vienna conference by Alv~.ger and Uhler. They suggested the use of two ovens in the ion source, the first one for establishing good operational conditions by having a charge of stable isotopes of the element to be separated, whereas a carrierfree activity is later introduced into the ion source via the second oven, from which it is evaporated without disturbing the operation very much. In this way the activity is separated under the most favourable conditions in a minimum of time. During the last few years also this method has been adopted as a standard technique in most laboratories performing separations of radioactive nuclides. From the contribution of Dr. Camplan it appeared that the Saclay group has even developed an ion source utilizing altogether five different ovens, thereby securing a maximum of flexibility. It should be noted that the carbon-tetrachloride and the multiple oven techniques are intended to solve essentially different problems in ion source operation. 312
PREPARATION
OF R A D I O A C T I V E
It is possible, however, to combine the two methods by introducing a carrierfree activity into an ion source which operates on a stable isotope reacting with carbontetrachloride; some progress has hereby been made in the handling of very short lived isotopes of non-volatile elements. The results obtainable by present techniques in the separation of radioactive isotopes could be optimistically summarized as follows: 1. All elements can be separated, maybe with the exception of the platinum metals. 2. Efficiencies of the order of 10% can be obtained, with a few more exceptions than under 1. 3. If necessary, separations can be performed in a few minutes. 4. Enrichment factors of the order of 1000, or more than usually needed, are obtainable. 5. The method is unique with respect to the quality of the sources. From such a survey one could be tempted to conclude that new methods have not been developed in recent years because the techniques at present are satisfactory. I think, however, that all workers in this field will agree that the points listed above do not outline a realistic picture of the present state of affairs. Many elements are separated with great difficulties in most laboratories and the average efficiency actually obtained is probably closer to 1% than to 10%. More
S A M P L E S BY I S O T O P E S E P A R A T I O N
313
reliable and reproducible methods of ion source operation are urgently needed. In addition, the very high efficiencies, up to 50%, which are reported at intervals, remind us of the fact that the limitations in the usual performance are not dictated by any law of physics. When considering the separation of isotopes with half-lives in the region of minutes, one has to realize that the present methods are only of limited value for the nuclear physicist. Valuable information can be obtained, of course, from measurements of short duration, but detailed spectroscopic investigations of a nuclide demand tens or even hundreds of hours of measurements. The progress of the on-line projects is an exciting development in nuclear physics, not only because it opens possibilities for the study of activities with still shorter half-lives. The continuous deposition of activity characterizing the on-line separation will be of decisive importance also for the measurements of nuclides with half-lives of the order of ½ - 1 h. When discussing the need for such time consuming experiments, we have to take into account the possibility of intensive utilization of the activities separated on-line. Usually beams of several active isotopes will be formed simultaneously and, by applying conventional beam handling methods, these isotopes can be studied separately in different spectroscopic set-ups.
VI. A P P L I C A T I O N S
TO N U C L E A R
PHYSICS