material but it is preferably held in a matrix material because the matrix phosphor is readily available and the phosphor itself is difficult to form in very thin layers the matrix material should be inert in that it should not adversely affect the functioning of the phosphor it should therefore be transparent to light which is later released if the stored energy is later released by heat of course the matrix material should be able to withstand the heat without damage so that the dosimeter can be re used it should also be inert to the conditions found in the field e g moisture various organic resins can be used but polytetrafluoroethylene is preferred because it is inert and can withstand a 300 u00b0 c readout temperature a dosimeter can be made by mixing the phosphor and matrix material using about 2 to about 30 by weight phosphor preferably about 5 to 15 by weight phosphor is used if too much phosphor is present the matrix material will lack strength and if too little is present the dosimeter will lack sensitivity the mixture is solidified and cut or otherwise formed to the necessary thickness whether the phosphor is used alone as a polycrystalline or single crystal material or in a matrix the thickness of the phosphor is critical to the success of this invention if the thickness is greater than about 18 mg cm 2 the dosimeter will lack alpha sensitivity because it will also significantly register beta and gamma ray background radiation on the other hand if it is thinner than about 5 mg cm 2 it will lose the signal from alpha particles.