IL23674A - Neutron activation of substances - Google Patents

Description

23674/2 Neutron activation of substances COMMISSARIAT A L'EHEROIB ATOMIQUE C. 22736 23674/2 This invention relates to neutron activation of substanees y irradiation, in particular substances which have a high neutron capture cross-section* She invention further extends to the novel industrial products consisting of activated substances whioh are obtained as a result of the application of said method* A common method of activation of substances, especially for forming sources, consists in irradiating said substances in a nuclear reactor channel* However, this method cannot readily be applied when the substance has a high neutron capture cross-section for the first reason that, in this instance, only the peripheral portion of the substance is irradiated (owing to the low value of the mean free path of the neutrons within said substance)* the second reason being that the neutron absorption produces a substantial local depression of the neutron flux : consequently, it has not hitherto proved possible to achieve a high level of specific activity when irradiating an appreciable quantity of such a substance* The invention is directed to the basic concept of a method for the preparation of activated substances which meets practical requirements more effectively than has been the case in methods of the prior art, especially insofar as it permits of activation by irradiation of substances having a high capture cross-section.

With this object in mind, the invention proposes a method which consists in putting the substance to be activated in a dispersed form, in mixing said substance with an additive which has a lower neutron capture oross-section and different substance .

In a preferred mode of operation whereby a specific activity which is close to the maximum theoretical activity can be achieved, said substance is endowed with a particle-size distribution such that the mean diameter of the grains is smaller than the mean free path of the neutrons within said substance.

One particularly simple solution to the problem of separation of the substance from the additive after irradiation consists in selecting a substance to be irradiated and an additive such that one is soluble and the other is insoluble in water ; said substance can be constituted by the element which is intended to undergo activation but, in certain cases, it will be necessary to employ a chemical compound of said element whose properties are a cause of hindrance either in the irradiation process or during handling. This will be the case in particular of elements which have a low melting point and the activation of which in reactor channels at high temperature will be carried out by making use of a refractory oxide of said element. Since these oxides are usually insoluble, they will accordingly be mixed with a water soluble salt (alkali carbonate, for example). If, on the contrary, the substance to be irradiated is soluble, the additive chosen will be an insoluble substance (silica powder, for example).

The proportion of substance to be irradiated and additive in the mixture will be determined in each particular case as a function of the different parameters and in particular of the capture cross-section of the substance to be irradiated, the proportion of additive' higher„ The substance and additive, which are both in powdered form, are intimately mixed ; prior to irradiation of the mixture, it is preferable to endow this latter with a certain degree of cohesion, for example by compacting it into pellets ; this cohesion will prevent the segregation which might otherwise result, for example, from vibrations or packing processes during exposure to radiation. After irradiation, washing with water carries the additive away (if said additive is soluble) and the activated substance is retained ; the reverse operation is carried out if the activated substance is soluble, in which case said substance passes into solution.

The arrangements contemplated by the invention will become more readily apparent from a perusal of the following description of certain modes of practical application of the invention which are given without implied limitation and which relate to the preparation of activated europium.

Natural europium is a mixture in proportions of the same order of two isotopes having the atomic weights 151 and 155 j the first of which is activated by neutron is trans ormed into irradiation and«tbppiiee- the isotope 152 which is a β and γ emitter, The neutron capture cross-section of natural europium is very high, namely of the order of 3 , 500 barns. Consequently, it is not possible as a rule to effect direct irradiation either of europium or of its oxide O^ within a reactor channel. According to a first mode of application of the invention, the powdered europium oxide is admixed with alkali carbonate, the proportion by weight free path of neutrons within the europium is of the order of 80 microns, the maximum dimension of the oxide particles is chosen so as to have a lower value, preferably of the order of 50 microns.

Once the oxide and carbonate powders are intimately mixed, pellets are formed by compaction with sufficient cohesion to prevent segregation of the oxide (which is heavier and therefore would otherwise be liable to concentrate at the bottom of the mixture) during handling operations and exposure to radiation.

The pellets which are withdrawn from the reactor channel are washed in a stream of water and disintegrate therein. The soluble carbonate is carried away and the europium oxide is alone held back and recovered by centrifugation. with ^ ' J¾-^&e--eaao o£/a sufficient time of exposure to ^ ' radiation, the oxide thus obtained uphi-eh- has been ' irradiated right through?has a specific activity which can attain practically the maximum theoretical value, namely 20 34- curies per gram.

By way of example which illustrates a second 3 mode of application, a sintered mass of silica of 40 cm is impregnated with a solution of europium chloride containing 1 gram of europium. After drying, the aggregate is exposed to radiation. The eu^ropium chloride is then extracted from the sintered mass of silica by means of a dilute hydrochloric acid solution, whereupon the europium is precipitated from an oxalic acid solution. The activity obtained is the same as in the example which 0 illustrates the first mode of application.

When the substance to be Irradiated consists of an element which is stable at high temperature or has an oxide which is stable at high temperature, there can in that case be employed as additive a combustible substance whioh it will be possible to eliminate by combustion after irradiation, the irradiated element being recovered in the first case as the element itself and in the second case as an oxide* By way of example, the preparation of activated iridium can be carried out by irradiation of pellets prepared by compacting a mixture of powdered carbon and Iridium powder* The size of the iridium particles is advantageously of the order of 100 microns, the mean path of thermal neutrons within the iridium being 325 microns* The proportion of the mixture is, for example, such that 1 gram of iridium is 3 dispersed in a volume of 5 cm , After irradiation, the pellets are calcined until the carbon is completely eliminated and is given off in the form of COg. The Iridium whioh is recovered can attain a specific activity whioh is higher than 900 curies per gram when irradiated in a 14 flux of 10 ^ neutrons per second.

The method according to the invention makes it possible to irradiate gold, thallium and so forth and is applicable to substances which are enriched with one of their isotopes such asf for example, Bu151t Ir191t Re185, It will be readily apparent that the invention is not limited in any sense to the mode of application which has been described herein and it must be understood that the scope of this patent extends to alternative forms of either all or a part of the method whioh remain within the definition of equivalent means*

Claims (1)

1. 23674/2 A process for neutron activation of a matter having a high neutron absorption cross section, comprising the steps o reducing said matter to powder form* mixing said powder with powder of an additive having at least one of its chemical and physical properties notably different from the corresponding property of said matter and having also a much lower neutron absorption cross section, irradiating said mixture in a neutron flux and separating said matter from said additive by taking advantage of said one property* 2« A process for neutron activation of a matter having a high neutron absorption cross section, comprising the steps of reducing said matter to powder form, mixing said powder with an additive having a lower neutron cross section, said matter and said additive one being soluble and the other being insoluble in a predetermined solvent, irradiating said mixture in a neutron flux and using said solvent for separating said matter and additive* 3* A process according to Claim 1 or 2, wherein said matter is reduced to a powder having a mea grain size smaller than the mean free path of neutrons in said matter* 4· A process according to Claim 1 or 2, wherein said mixture is formed into pellets having a compactness sufficient to prevent settling out of said matter during irradiation* 5» A process for neutron activation substantially as hereinbefore described. For the Applicants SR. REINHQLD COH ft PARTSERS IS/DL