Activation of samples in the secondary neutron flux generated with a 66 MeV proton beam on a radionuclide production target

In this work, a small activation chamber was specially designed to facilitate the irradiation

of selected samples with secondary neutrons behind a radionuclide production target in

the vertical-beam target station (VBTS) at iThemba LABS. The main aim was to assay the radionuclides

formed in the samples, with a special interest in those of medical importance. The

experiment was performed simultaneously with a routine radionuclide production run, during

which the target was bombarded with a 66 MeV proton beam with a nominal beam intensity

of 220 A. The target was a tandem Rb/Ga target for production of the relatively long-lived

radionuclides 82Sr and 68Ge, receiving an accumulated charge of 20 000 Ah. Twenty-two materials

were selected for neutron activation, covering a large mass region from 7 to 209 atomic

mass units (amu). Activated samples ranging from 7Be to 209Bi were assayed for their gammaray

emissions by means of a high-purity germanium (HPGe) detector. Based on these activation

data, physical yields were determined for all uniquely identified reaction products.

The experimental yields measured in this project are compared with yield predictions based

on neutron fluxes obtained from Monte Carlo simulations with the radiation transport code

MNCP 6.1 in conjunction with evaluated cross sections for neutron-induced reactions from the

TENDL-2015 and TENDL-2017 nuclear data libraries. Six different physics selections were

employed in the Monte Carlo simulations, leading to several sets of predicted results.

Activation products from a total of 104 nuclear reactions have been observed cleanly, more

than 40 of which are used in medical applications. In the majority of cases the yields are too low

for this method to be profitably exploited in a routine radionuclide production facility. In several

cases the specific activities of the radionuclides will be too low for use in medical applications.

In a few cases the use of secondary neutrons clearly has potential. Six medically interesting

radionuclides have been singled out for further scrutiny, namely 46Sc, 59Fe, 103Ru,141Ce, 161Tb

and 203Hg. Although the production rates are generally low, the produced quantities are nevertheless

sufficient for research purposes, e.g. radiochemical separation studies, the development

of methods for radiolabeling of biomolecules, calibration sources for radiation detectors, etc.

One of the principle aims of this study is to obtain information that will assist the design

of future target stations for the proposed 70 MeV cyclotron facility at iThemba LABS, on the

aspect of making provision for suitable infrastructure that will exploit the secondary neutrons.

There is a renewed interest worldwide to exploit neutrons emitted from high-power targets,

which iThemba LABS has not hitherto investigated or implemented. As 66 MeV proton beams

are routinely available, close to the design energy of a new generation of commercial 70 MeV

cyclotrons, iThemba LABS has a unique opportunity to make a contribution in the context of

future production facilities, utilizing secondary neutrons, in this important energy region.