Physics

The Neutron

The neutron is one of the constituent nucleons of the atomic nucleus (the other is the proton). It has zero electric charge, but a magnetic moment and its mass is about 1840 times that of the electron. Outside the nucleus a free neutron will decay into a proton, electron, and antineutrino with a lifetime of about 15 minutes.

The neutron can be described as a classical particle with mass m but it shows wave character too, which can be described with the deBroglie wave-length . Let m=1.6749 10-27 kg be the neutron mass, v its velocity and h Planck's constant.

The following relation for neutron energy and wave-length hold:

Neutrons can be classified according to their kinetic energy:

Neutron Interaction with Matter

Since the neutron is electrically neutral, it interacts only weakly with matter into which it can penetrate deeply. Contrary to x-rays, which interact dominantly with the electron shell of the atom, the neutron does on the level of the nucleus. Therefore the neutron is quite sensitive to light atoms like hydrogen, oxygen, etc. which have much higher interaction probability with neutrons than with x-rays. In contrast to this, metals comparatively show lower interaction probability with neutrons, thus allowing quite high penetration depth. Elements of similar atomic number Z (= number of protons) likewise differentiate easier with neutrons than with x-rays.

Comparsion between neutrons an X-ray	Comparsion between neutrons an X-ray

Neutron matter interaction probability is high at thermal or cold energies (see Figure 3). Neutrons for transmission radiography are therefore extracted from a moderator (e.g. tank containing heavy water), which slows down neutrons produced in a spallation source or a research reactor in the MeV energy range.
There are a series of isotopes with suitable nuclear reactions at thermal neutron energy, which can be used for neutron detection. Neutron detection at higher neutron energy for imaging purposes is much more difficult.

Narrow Beam Attenuation	Exponential Attenuation Law	Neutron Cross Section
Figure 1	Figure 2	Figure 3

Neutron matter interaction processes can be divided into:

Absorption: neutron beam intensity is reduced but propagation direction remains unchanged
Scattering: neutron beam changes intensity and direction
Refraction: neutron changes direction only

Neutron Transmission

The transmission behaviour of a monoenergetic narrow neutron beam can be described by the basic law of radiation attenuation in matter (See Figures 1 and Figures 2). The ratio between the emerging neutron flux I and incident flux Io is called transmission T. Quantitative data about the material composition (e.g. hydrogen content), can be derived from neutron transmission measurements in the case of known sample shape and dimension d. Hereby the relation defining the macroscopic neutron interaction cross section and the evaluated neutron cross section data for taken from a database are used (See Figures 2 and Figures 3).

The simple exponential attenuation law does not hold for all situations. Thick samples or strongly scattering (e.g. hydrogen) or absorbing materials (e.g. containing strong absorbers like boron, gadolinium. …) show a deviation, due to multiple scattering effects or the need to take the changing neutron energy spectrum into account.

The following greyscale squares illustrate the characteristic differences between x-ray and neutron transmission in various materials:

Thickness 1cm	Thickness 2cm	Thickness 5cm

Neutron Radiography

Neutron radiography aims at the detection of transmission differences in objects. These might be caused by missing material (e.g. pores, cracks,…) or inclusion of material showing alternative transmission behaviour. The transmission differences of such defects depend on their size and isotopic composition.

Their detectability is additionally determined by the characteristics of the neutron source and collimator together with sensitivity of the detector system.