Neutron Beam Characteristics
An ideal neutron beam is:
uniform in neutron intensity in the projected (X-Y) plane across the sample surface,
constant in fluence rate for the entire time during the measurements of all samples. (Because this is rarely the case for a multitude of reasons, a beam monitor is typically used to integrate the fluence for the duration of each recorded spectrum of samples, standards, blanks, etc., then used to normalize them.)
a monoenergetic neutron beam. (fortunately, the most common NDP targets have 1/v capture cross section response to neutrons in the thermal and lower energy spectrum, that is, they do not exhibit resonance capture characteristics. However, even the energy-response absorption of the neutron beam by the sample matrix can change the response. This is especially true if the sample is not uniform in composition.)
a purely neutron beam, containing no gamma or x-ray components.
a parallel (non-divergent) beam.
a high fluence rate beam. (although there have been several outstanding measurements published using low-fluence rate facilities. High fluence rates improve measurements in samples with temporal changes in the target profile.)
In fact, these conditions collectively or individually are not perfectly achieved, thus various techniques are used to assure the reliability of the measurement. For example, the neutron beam and sample position must be closely aligned and reproducible aligned for precise and accurate measurements.
The neutron beam characteristics are readily altered in shape and size by the application of collimators. The neutron beam intensity and energy can also be adjusted by the addition of filters. Beam characteristics for a former NDP facility are shown on this page.
These plots shows the (x-y) neutron intensity profiles (smoothly decreasing from the beam center to the outer edges in about color steps) at the sample plane. The averaged fluence rate over the entire beam is 7.6 E+08 n/cm2/sec (thermal energy equivalent, e.g., 25.4 meV). The true average energy of the neutrons are approximately an orders-of-magnitude or less.
Looking at the neutron intensity profile plotted along the horizontal cross section. Horizontal lines in the plot indicate 1) 4 mm width of the beam, 2) the FWHM of the beam, and 3) the 10 mm width of the beam.
Or, looking at the neutron intensity profile plotted along the vertical cross section. Horizontal lines in the plot indicate the 1) 4 mm width of the beam, 2) the FWHM of the beam, and 3) the 10 mm width of the beam: