Tedmical Notes New Methods L'or Introducing 75se into Radlopharmaceuticals

by aluminlum shows the presence of a very short range component issuing from the platinum surface. This component makes a noticeable contribution to the count-rate up to an absorber thickness of 80 mg Al/crn s. I t is observed with approximately the same relative intensity if the needle is surrounded by platinum, but it is absent if the needle is surrounded by aluminium. I t is reasonable to assume that this radiation is produced by the photoelectric effect in the platinum. Conversion electrons due to Compton absorption must also be present, but these would not be recognized as a difference in absorption of radiation outside a platinum and outside an aluminium surface, the Compton mass attenuation coefficient being even a little higher in aluminium than in platinum. Compton electrons will certainly contribute to the countrates measured above the gamma contribution through absorbers between 80 and 250 mg/Al/cm s. However, this part of the absorption curve is very difficult to interpret. As the thickness of the aluminium absorber foil is raised the efficiency of the counter for gammarays is also increased. This in part compensates for the reduction of the count-rate from the softer radiation emitted by the needle. Evidently, if one would take a platinum needle surrounded by a sufficiently thick aluminium cylinder and if the aluminium absorber foils would then be fitted snugly around this cylinder, one would observe only a reduction of the count-rate due to gamma-absorption in the various layers of aluminium. We have actually observed a strong dependence of the shape of the absorption curve measured with aluminium covered needles on the geometry. Thus we may conclude that there is a contribution from a low penetration component to the absorbed dose in the first one or two millimeters of tissue around a radium needle, but that this component cannot be noticeably reduced by increasing the thickness of the platinum wall. In as far as this soft component consists of Compton dcctrons it only serves to establish the normal radiation cquilibrlum between photons and Compton electrons as it normally cxists in tissue. (A slight diffcrcncc will rcmain due to the transition from a high Z to low Z matcrlal). The photo-electrons, on thc other hand, provide an extra contribution to the radiation field, which will penetrate for scvcral dccimctcrs the air around an uncovercd needle. In therapy this short range radiation will hardly be noticed becausse of the 1/r-dependence of the doserate. Under certain other circumstances, however, one might possibly wish to provide a shielding against this part of the radiation emitted by radium needles. This can easily be done by using 1 mm thickness of plastic or some similar light material. Acknowledgement--This work has been performed in the Instituut voor Kemphysisch Onderzoek as part of the programme of the Foundation for Fundamental Research on Matter (FOM) with financial support of the Organization for Pure Research.