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Project ID: HPRI-CT-1999-50005
Financiado con arreglo a: FP5-HUMAN POTENTIAL
País: United Kingdom

Materials suitable for the construction of neutron-efficient image plates with reduced gamma-ray sensitivity

Identification and characterisation of materials suitable for the construction of neutron-efficient image plates with reduced gamma-ray sensitivity and their use in the manufacture of large plates for a working diffractometer.

Examination of the a-sensitivity of Eu(2+) doped BaFBr and BaSrFBr phosphors containing varying quantities of Gd(2)O(3) as neutron converter:

Samples of phosphor with differing proportions of Gd(2)O(3) and BaSrFBr were exposed to known neutron and Fe K X-ray fluxes, and the PSL levels measured using a red laser (lamda=635nm ). While the neutron sensitivity of the plates has a broad maximum between 40/60 and a 60/40 mixture of Gd(2)O(3) and BaSrFBr, the gamma -sensitivity reduced montonically with increased Gd(2)O(3) content to zero for 100% Gd(2)O(3). In this system it is therefore preferable to use a 60/40 or a 70/30 ratio of Gd(2)O(3) to BaSrFBr to significantly reduce gamma-sensitivity while keeping a relatively high neutron DQE.

Measurement of the thermo-luminescent yield of various inorganic borate storage phosphors which are intrinsically neutron sensitive and contain few atoms of high atomic number:

A large series of storage phosphor materials of the type M(2)B(5)O(9)X:Ce(3+), R+ were studied, with M = Sr, Ca; X = Br, Cl; R = Na, K or no extra dopant. These were found to have a much lower gamma-ray sensitivity than the traditional BaSrFBr: Eu mixed with Gd(2)O(3), and the most promising phosphor, Sr(2)B(5)O(9)Br: Ce(3+), was found to give more PSL counts per incident neutron than Gd(2)O(3) doped BaSrFBr. The neutron absorption coefficient for this material is significantly less than for BaSrFBr: Eu/Gd(2)O(3) and requires a thick phosphor layer. This has the effect of reducing the neutron DQE, as the PSL emitted from deep within the phosphor is unable to reach the detector.

It was therefore decided to produce large image plates using a 70/30 ratio of BaSrFBr:Eu/Gd(2)O(3) as the storage phosphor. Using a spraying technique for the phosphor / converter, plates of 400mm x 800mm were made and used for neutron diffraction studies of the protein systems lysozyme and glucose isomerase.


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