BT-6 Thermal Neutron Radiography and Tomography

• Build a high intensity neutron beam line that is dedicated to imaging.

• Neutron fluence rate should be 107 cm-2 sec-1.

• Beam should be uniform so that it can be properly digitized within the dynamic range of the imaging system.

• Dynamic range is typically 14 bit to 16 bit.

• Noise:– Neutron shot noise is not so important.

– Photon shot noise is important

– Electronic digitization noise is important but can be subtracted off

– Most serious noise is that of thermal noise or dark current ’s and fast neutrons can overwhelm pixels and can destroy the CCD

• Ideal neutron source is a cold source; however, no positions are open.

• BT-6 good points:– Brilliant source

– Large beam

– Up to 8 meters possible between source (source means aperture) and sample

– Area will be unused for several years

L

d

Point source defined by pinhole aperture

Neutrons from a divergent source

Image or Neutron shadow region rotated to be viewable

Un-sharp edges due to large aperture

Neutron dark mask (Cd for example)

Geometric sharpness

• Neutron images are essentially shadow images of the neutron beam due to attenuation of the neutrons that pass through an object.

• large uniform areas of illumination are produced by placing a sample a long distance away from a neutron point source.

• Point source is approximated by a circular aperture with a small diameter.

• Geometric sharpness is defined as the ratio of the distance from aperture to sample, L, to the diameter of the aperture, d, as shown in Figure 1.

• A greater ratio, L/d, or geometric sharpness results in higher resolution.

Design Issues

• Need a good filter for ’s and fast neutrons– 10 cm of uncooled bismuth seems to be a good

choice.

• Shielding should remove as much of the penumbra region as possible early on.

Main Entry: um·bra

Pronunciation: '&m-br&

Function: noun

Inflected Form(s): plural umbras or um·brae /-(")brE, -"brI/

Etymology: Latin

Date: 1638

1 : a shaded area

2 a : a conical shadow excluding all light from a given source; specifically : the conical part of the shadow of a celestial body excluding all light from the primary source b : the central dark part of a sunspot

- um·bral /-br&l/ adjective

Main Entry: pen·um·bra

Pronunciation: p&-'n&m-br&

Function: noun

Inflected Form(s): plural pen·um·brae /-(")brE, -"brI/; or -bras

Etymology: New Latin, from Latin paene almost + umbra shadow -- more at UMBRAGE

Date: 1666

1 a : a space of partial illumination (as in an eclipse) between the perfect shadow on all sides and the full light b : a shaded region surrounding the dark central portion of a sunspot

2 : a surrounding or adjoining region in which something exists in a lesser degree : FRINGE

3 : a body of rights held to be guaranteed by implication in a civil constitution

- pen·um·bral /-br&l/ adjective