106 instrument. It uses a nuclear inter-action to favor neutron pulses in a cirsuit to accept only those pulees greater than a precept pesk voltage fo discriminate neutron produced pulses from pulses pro- duced from gamma radiation, The Hornyak and the Hurst Proportional Counter are both survey type Anstrumente, The former uces the light pulses produced in @ zino sulphide lucite sointillator and the latter uses -= well, in the first -éase they are produced either by recoiled protone with the lucite or the reacbion protons from the sulphur reaction and the latter uses the recoil from the ethylene filling. Eince’ both of these = since the pulse heights in both of these cases depend con the kinetic energy of the instant neufron, there are fluxes with sufficiently low ener~ gies that will not be registered by the counte ing instruments. | In other words there are low energy cutorfs below which these inetruments will not work.