e —— a With the Hornyak, this occurs at about 400 KEV and with the Hurst this occurs at about 200 KEV, For many purposes these values are entirely too hich and a large proportion of an energy degraded neutron spectrum cannot be measured, particularly this is the cace when the maximum original energy of a neutron occurs at about 15 MEV. There exists a high energy basis in which the hydrogen cross section on which all these counter instruments that we use depends to a greater or lesser extent varies from its maximum zero energy values of about 80 down to about one barn in 10 MEV and 2/10th of ‘« barn in 40 MEV, Now, since most counters either raise a count when a pulse ie higher than the precent peak voltage, this mesns that due to the emall size of the instrument and the decreasing cross section it is difficult to get reliable results when neutron being measured has energies higher than 15 MEV, the long counter recponse ia down to about 85 per cent at 5 MEV due to the increase in the rean free path of the instant neutron in the paraf-~ fine Jacket. a, 107