Neutron Transmission

Transmission imaging is a technique used for taking images of the inside of an object. This is probably best known from hospitals, where X-rays are used to make an image of a broken bone. The principle of neutron transmission imaging is exactly the same, but instead of X-rays, we use neutrons which have other advantages as we explain below.

The principle of a single transmission image (called a radiogram) is different from a normal picture you take with a camera. With a camera, you record the light that has been reflected back or re-emitted from the surfaces of the object you are taking a picture of. In radiography you record the X-rays, or neutrons in our case, that were not stopped by the object, meaning they pass through the object without diffracting, which is called transmitted. Thus with a normal camera, the “detector” is on the same side of the object as the light source, whereas in transmission imaging the detector and light (or neutron) source are on opposite sides of the object.

But what is it that enables us to see the bones in a leg in hospital radiography? For this, we need to look at how the X-rays, and later neutrons, interact with the different elements in the object of interest. X-rays interact with the electron cloud around the atoms, and since heavier elements have more electrons they have a higher chance of stopping the x-rays than lighter elements. So we see the bones in a hospital transmission image clearly since they contain a higher amount of heavy elements than the surrounding tissue. We refer to the interaction chance of an element as the cross-section, which can be thought of as the perceived size of the atom for the specific probe – X-rays or neutrons. The final transmitted intensity on the detector can be written as:

I = I₀ • exp(-μ•l) beam intensity equals beam intensity before the sample per the exponent of -attenuation coefficient per the length

Where I0 is the intensity of the beam before the sample, l is the length through the sample, and μ mu is the attenuation coefficient. The attenuation coefficient is the sum of the elements’ cross-sections in the sample weighted with the density of the sample. Different parts of a sample can have different attenuation coefficients, which will give different intensities on the detector. A high attenuation coefficient will result in a low transmitted intensity and a low attenuation coefficient will give a high intensity. So for the hospital x-ray image the bones have a high attenuation coefficient resulting in dark parts on the image and the surrounding tissue has a low attenuation coefficient resulting in bright parts on the image. Usually in the X-ray images at hospitals, the doctors prefer to have the contrast is inverted, so that the low-transmission areas (bone) appear write and high-transmission areas (soft tissue) appears dark. You can also think of these inverted contrast images as a map of attenuation: low attenuation gives dark parts on the map, high attenuation gives bright parts on the map.

The principle in neutron radiography is precisely the same, and the only difference is the size of the neutron cross-sections of the elements. Neutrons interact with the nucleus of the atoms and not the electrons like X-rays, and so the size of the neutron cross-sections of the different elements do not follow a simple rule like for the X-rays. With neutrons we are able to distinguish lighter elements from each other, like carbon and hydrogen, and we can even differentiate between different isotopes of the same element like hydrogen and deuterium since they have very different cross-section even though they are chemically identical. This can be utilized in a number of ways.

Figure. Transmission radiograms of an analog camera. (first image) The neutron radiogram clearly shows that the photographic film which is made from organic material with a lot of hydrogen attenuates the beam. (second image) The x-ray radiogram shows clear attenuation in the metal parts of the camera but the photographic film is invisible.