Large dynamic range small-angle diffractometer

PF1 induced neutron background at D22

P. Timmins and H. Börner to Ch. Vettier

In a specific class of experiments at PF1 actinide samples are exposed to neutrons. This can lead to the production of fast neutrons (as a function of the respective fission cross sections) which can create a significant background on the neighboring instrument D22. Consequently an effort has been made to improve and adapt the shielding at the PF1 site.

During the last cycle a background level was reached (after carrying out several steps of improvement) which is acceptable for most of the D22 experiments. However, when the D22 detector is positioned at large distances (15 - 18 m) from the sample position the remaining background can still cause problems. In particular variations of this background (opening and/or closing of the PF1 shutter) can render the D22 data unusable.

To avoid problems for these sensitive D22 measurements (arising from actinide targets) an agreement has been proposed and achieved by the responsibles of both Instruments: Unless there is an emergency a D22 experimentalist (responsible, local contact or user) will always be contacted by a PF1 experimentalist (responsible, local contact or user) before taking any action concerning the PF1 shutter. They will together agree upon a time when the PF1 shutter may be opened or closed in such a way that D22 measurements are not perturbed. If necessary PF1 might have to postpone the action until this is convenient from the D22 point of view. In case of an emergency, a D22 experimentalist will be informed after the event if nobody can be reached before.

In any case, the D22 data at large detector distances are seriously deteriorated even by the ‘improved’ background. Some very sensitive measurements on D22 may need PF1 not to run at all during a period of a few hours up to a few days. Such cases should be discussed at the scheduling stage, and therefore the D22 responsibles must be informed of actinide samples at the earliest possible date.

Copies of this note will be posted in the D22 and PF1 control cabins.

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