The beam uniformity and flux are determined using an array of five detectors. Each detector is made up with a plastic scintillator coupled to photo-multiplier tubes. Four of the detectors are fixed in position and set up to measure beam particle counting rates continuously at four characteristic points 1.64 inches (4.71 mm) away from the beam axis. The fifth scintillator can be optionally put in to measure the beam particle counting rate right at the beam axis. The sensitive area of each detector is defined by a 0.1 cm^2 aperture, while the intrinsic efficiency is 100% for all practical purposes. The beam uniformity parameter (ranging from 0 to 100%), the axial gain (%), and the beam flux (in particles/cm^2/s) are determined by the control software based on the detector counting rates. The results are displayed and updated once every second.

The following information about beam measurements as well as detector verification procedures have been put into a pdf file for your convenience.

Uniformity

The beam uniformity is calculated by the following equation:

Here, n_i ( i=1..4 ) are the counts from the four detectors surrounding the beam and n_av   is the average count of the four scintillators.

Central Shift

An additional gauge of beam uniformity is the central shift (cs). The central shift is a percentage of how far the beam is off from the central beam axis. It is calculated as follows:

Here, n_i (i = BL, BR, TL, TR) is the count from each detector (TL=top left, BR=bottom right, etc.).

Dosimetry

While the central detector is in place along the central beam axis, the beam flux (particles/s/cm²) is determined solely by the counts per second measured by the central detector.  Once the central detector is removed from the central beam axis the following equation is used to determine the beam flux:

Where n_av is the average counts per second from the four corner detectors (equation given above), A is the sensitive area of each detector (A=0.1 cm²) and ag is the axial gain. The axial gain is the ratio of the counts per second of the central detector to the average counts per second of the four corner detectors and is given by the following:

The equation for n_av is previously noted above.  When the central detector is in place the axial gain is updated every time the central detector has accumulated at least 10,000 counts. While the central detector is taken out of the beam axis, the SEUSS software will use the last value calculated for the axial gain in determining the beam flux (unless otherwise specified) during each run.

The fluence, or total particle count per unit area (counts/cm2), is determined by the following:

Where Nav is the average of the total counts from each of the corner detectors for the duration of a run and ag is the axial gain. The average total counts from the four corner detectors can be expressed as:

Where Ni (i=1 to 4) is the total number of counts from each detector for the duration of the run.

Energy

In addition to the scintillators, a 1-mm thick silicon surface barrier detector located in one of the upstream diagnostic chambers is used to measure the beam particle total energy and to provide information on beam purity.  It can also be used to measure the energy and the energy stragling of the beam after passage through the degrader foils.