Detectors in use for Neutron Radiography

Detectors in use for Neutron Radiography (NR) purposes are those, which are able to measure the neutron field in two dimensions perpendicular to the beam direction.
Therefore, the detector area should be in the order or larger than the beam cross-section. Further boundary conditions are the spatial and time dependent resolution of the detector, which can be very different among the existing detectors systems.
An overview about these parameters is given for the most common systems in the following figure. The inherent detectors properties are mainly given by the detection process, which is a nuclear reaction initiated by the neutrons.

Application rage of detectors for neutron radiography regarding their spatial and time resolution (under the beam conditions provided by the radiography station NEUTRA, SINQ, PSI)

The primary detection reactions for thermal neutrons are mainly neutron capture by an absorbing material emitting secondary radiation, which can is used as the real neutron prove. This is way neutrons have no electric charge and cannot make ionisation directly which is needed for detection.

The most important detection reactions are (for thermal and cold neutrons):

³He + ¹n ® ³H + ¹p + 0.77 MeV

⁶Li + ¹n ® ³H + ⁴He + 4.79 MeV

¹⁰B + ¹n ® ^{(07%) 7}Li + ⁴He + 2.78 MeV
¹⁰B + ¹n ® ^{(93%) 7}Li* + ⁴He + 2.30 MeV ® ⁷Li + ⁴He + g (0.48 MeV)

¹⁵⁵Gd + ¹n ® ¹⁵⁶Gd + g + conversion electrons (7.9 MeV)

¹⁵⁷Gd + ¹n ® ¹⁵⁸Gd + g + conversion electrons (8.5 MeV)

The use of fissile material (U, Pu) for neutron detection in NR is very uncommon.

The further process for imaging in radiography based on the previous reactions is possible in different way:

• by light excitation in a scintillator

• by blackening of a suited film

• by excitation of electronic (metastabile) states in a crystal (imaging plates)

• by creation of micro-traces in special foils (track-etch method)

• by charge separation in a semiconductor material

The following detector systems for radiography have been developed on the basis of the mentioned processes and are in use for different applications:

X-ray film in connection with converter foils from Gd, Dy, oder In. The excitation and blackening of the film is caused by gamma and beta radiation as well as by conversion electrons.
Highly light sensitive CCD camera detectors (cooled in most cases) looking onto the weak light emission from a neutron sensitive scintillator (Li-6 or Gd as neutron absorber).
By the use of image intensifiers, the light intensity can importantly be increased (as intensifier tubes or micro-channel plates). In this way, either less sensitive cameras can be applied or higher frame rates becomes possible.
Imaging Plates contain Gd as neutron absorber and BaFBr:Eu 2+ as the agent which provides the photoluminescence. A imaging plate scanner is extracting the latent image information as digitised data file from the plates by de-excitation caused by a laser signal.
Track-etch-foils are "scratched" by a-particles created in a capture reaction of B-10 with thermal neutrons. These very small tracks can be enlarged so much by chemical treatment (etching in an alkaline bath) that a macroscopic image occurs, which can be digitalized or optical enlarged by optical means.
Flat panels based on amorphous silicon can provide digital information directly and an optical magnification (as with cameras) is not necessary. However, thy have to be placed into the direct beam, which can cause some problems for long term use.

A summary about some important properties of radiography detectors is given in the following table.