Monte Carlo Studies of two Different Conversion Layers for Neutron Measurements with Medipix Silicon Detector.

In 2007 the ventilation system of CNGS failed and investigations showed that the failure was due to Single Event Upset (SEU). Since then there has been increased interest in studies of neutron flux, that can potentially cause SEU. Two Medipix detectors have previously been installed in the CMS cavern on a test basis and have shown to work as intended[1]. More Medipix detectors will be installed to provide high resolution measurements of the particle flux in the vicinity of the CMS, focusing on measurements of the neutron flux. The measurements will provide an important basis to know what precautions to take to avoid another failure due to SEU. The measurements will also constitute a valuably reference to the FLUKA simulations of the general flux in the CMS cavern, that can potentially lead to important corrections of the simulations. Furthermore, measurements from the Medipix detectors will act as a cross check on the hadronic forward detector radiation monitoring system (HF radmon). Bonnos spheres are already installed, but can only provide offline measurements, whereas the Medipix detectors offers online measurements. The Medipix detectors measure the ionization by charged particles, and can differ between different particles from the pattern of activated pixels in an interaction[2]. The detectors can detect neutrons, if these are converted into charged particles in the detector, but the detection efficiency is low. Therefore, to increase the detection efficiency, conversion layers of Lithium Fluoride (LiF) and Polyethylene (PE) are applied to the Medipix detectors to increase the detection efficiency for thermal- and fast neutrons respectively. The picture on the front page shows different conversion layers glued on the Medipix detectors. The aluminium is a beam hardener rather than a conversion layer and is therefore not considered in this study. FLUKA has proved to be a successful tool to simulate of the general particle flux in the CMS detector and -cavern, but simulations of single sub detectors of small geometry, i.e. of the order 1 m to 1 mm, are less experienced by FLUKA experts. In this report, we present the results of FLUKA simulation studies on how conversion layers of PE and LiF effect the detection efficiency of neutrons in the Medipix detectors. The studies aim to answer firstly, if FLUKA is a well suited tool for simulation studies of single, small-scale detectors and secondly, if LiF- and PE conversion layers do increase the detection efficiency of neutrons and for what energies of the incoming neutrons, they provide an increase (or decrease) in the efficiency.