Fermilab neutron therapy facility neutron spectrum determination by threshold foils

The Fermilab Neutron Therapy Facility produces neutrons by bombarding a 2.21-cm-thick beryllium target with 66-MeV protons. The neutron beam is collimated to produce a desired field size and is monitored using dual parallel-plate ionization chambers. The neutron spectrum of this beam was determined by unfolding bare and moderated foil activation data and an MCNPX calculation of the spectrum was also performed. Aluminium and copper activation foils were selected for the neutron fluence measurement. The activation products in the foils were measured for various levels of moderation using polymethylmethachrylate (PMMA) slabs. Five reactions were considered in the experiment: ²⁷Al(n,?)²⁸Al, ²⁷Al(n,p)²⁷Mg, ²⁷Al (n,a)²⁴Na, ⁶⁵Cu(n,?)⁶⁶Cu and ⁶³Cu(n,2n)⁶²Cu. The activities of these end products after the irradiation period were used to compute their production rates during irradiation. Responses as a function of neutron energy were generated for the production of ²⁸Al, ²⁷Mg, ²⁴Na, ⁶⁶Cu and ⁶²Cu using MCNPX and the LA150N library. The resulting response matrix was binned into 49 energy groups from 10^-10 MeV to 70 MeV and includes responses both for bare foils and moderated foils. The PTB code MXD-FC33 code was used to unfold the activation data. The resulting spectrum is compared to both the MCNPX-computed spectrum as well as a previous measurement by Cupps and Elwyn.