than the embodiment of fig2 because probe 44 39 provides a more even less edge effects source of particles for finite other plate 46 and because there is less capacitance between probe 44 39 and plate 46 than between plates 44 and 46 in designing the system it is important to remember that the number of ion pairs collected from the resistor 40 must equal the number of ion pairs formed by the radioactive material sampled in chamber 30 as stated above as the voltage across resistor 40 increases the number of ion pairs collected increases if an insufficient source of beta is contained in resistor 40 the circuit will never be able to generate the number of pairs necessary to equal the current from chamber 30 if too active a source of beta is contained in resistor 40 the circuit will stabilize at too low a voltage to provide an accurate measurement a third embodiment of the invention is shown in fig5 wherein the resistor of fig3 has added thereto a beta absorbing disk 50 made of paper or plastic or any other solid material known to block betas and having a centered hole by reducing the size of.