Neutron background detection for a hard X-ray balloon-borne polarimeter

• PoGOLite is a balloon-borne hard X-ray polarimeter. It determines polarisation by measuring the azimuthal angular distribution of Compton scattered photons in a plastic scintillator array. The use of an all-plastic target yields a relatively large, low-mass detection area. The dominant source of background for these measurements has been shown, through Geant4 simulations, to originate from high energy (MeV range) atmospheric neutrons. Neutrons can pass the instrument's Bismuth Germanium Oxide (BGO) anti-coincidence shield undetected and subsequently scatter between plastic scintillator elements to produce a polarisation signature. A passive 15 cm thick polyethylene shield surrounding the polarimeter reduces the neutron induced background by an order of magnitude. The background level remains however significant, prompting the need for active monitoring of the continuously changing neutron flux. For this purpose PoGOLite makes use of a phoswich scintillator cell. The phoswich cell consists of a 5 mm thick Lithium Calcium Aluminium Fluoride (LiCAF) scintillator, used for neutron detection. The LiCAF is surrounded by a BGO anti-coincidence system. This small light weight detector can therefore be used to measure the neutron flux even in high radiation environments. This type of neutron detector was tested on a separate dedicated stratospheric balloon mission in March 2013, called PoGOLino, prior to the PoGOLite flight which took place in July 2013. Results from the test flight and implications for the measurements performed on the PoGOLite flight will be discussed.

Ämnesord

NATURVETENSKAP  -- Fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences (hsv//eng)

Nyckelord

Angular distribution

Balloons

Germanium oxides

Neutron detectors

Neutron flux

Polarimeters

Polarization

Scintillation counters

Aluminium fluoride

Anticoincidence shield

Atmospheric neutrons

Compton scattered photons

High radiation environment

Plastic scintillator

Plastic scintillator arrays

Stratospheric balloon

Meteorological balloons

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