Primary Monitor
NSRL makes use of a system that provides a live and nondestructive measurement of beam flux continuously throughout every exposure.
The primary monitor of beam flux is a planar ion chamber called “QC1” which is located at the furthest upstream position of the rails, directly after the vacuum window. This ion chamber is connected to a set of recycling integrators—devices that continuously monitor the charge deposited within the chamber through ionization, in discrete counts. The count size is 10 pC in the case of the NSRL recycling integrator system.
For each beam, the fluence in the beam’s uniform area for each QC1 count is known. As such the fluence delivered to the test article can be determined for each spill.
Because the recycling integrator-based system was initially designed for radiobiology, it also has the capability of inhibiting the beam mid-spill when a count threshold is met. While generally not especially useful for physics or electronics testing experiments, this feature can be used to deliver very precise total fluences to test articles.
Scintillator Calibration
When calibrating the beam for use in physics and electronics testing, where the flux/fluence (rather than the dose) is the metric of interest, QC1 is calibrated against a scintillator.
When a beam is prepared, a scintillation detector is placed at the location of the test article within the uniform area of the beam. The signal from the detector is adjusted such that only primary ions are counted and any potential fragments are ignored. The scintillation detector is able to detect and count individual ions as they pass through, so each count on the detector corresponds to one ion traversing the scintillator area. The beam is then enabled, causing a certain number of counts to be accumulated from the scintillator as well as from the recycling integrators reading from QC1. After at least 105 scintillator counts, the total counts accumulated from the scintillator and recycling integrators are related to produce a calibration factor for that beam.
Energy Correction
When the energy of a particular beam is stated, that number refers to the energy at the location of the test article. The beam’s energy loss through stripping foils, a vacuum window, QC1, and the column of air has already been accounted for. As such, unless they have intentionally inserted additional material upstream of the test article, users do not need to correct for any energy loss for the beam ahead of their part.
Limitations at Very Low Flux
Because lower LET ions cause fewer ionizations within the chamber as compared to the same number of higher LET ions, QC1 effectively has a lower resolution for beams using those ions. This allows NSRL to accurately deliver fluxes as low as ~10 ions/cm2/spill with a beam like Tantalum at 310 MeV/u, whereas for a beam like Carbon at 1000 MeV/u a different technique may be required.
Calibrations for Protons
Because the signal from a proton on a scintillator is difficult to distinguish from the signal of background gammas, a different calibration technique must be used. NSRL primarily uses one of two methods depending on whether a high flux or a low flux is desired.
Small Ion Chamber Calibration (High Flux)
For experiments that anticipate needing large proton fluxes, NSRL performs a dose-based calibration after which a conversion is applied to obtain a calibration in units of flux.
A small 1 cm3 volume ion chamber—colloquially referred to as an “Egg” chamber—is connected to a recycling integrator system and placed in the uniform area of the beam. A process identical to the scintillator-based calibration described above is followed, except that recycling integrator counts from the Egg chamber are collected rather than scintillator pulses.
The Egg chamber has a NIST-traceable calibration which allows the total counts to be converted into a measured dose. Using LET in water data from SRIM, a conversion factor of ions/cm2 per unit dose can be obtained. These quantities together are then applied to the QC1 counts versus Egg chamber counts calibration factor determined previously to obtain a factor for QC1 counts versus proton ions/cm2.
Coincidence Scintillator Calibration (Low Flux)
For experiments that anticipate needing small proton fluxes, there is generally not enough ionization within the Egg chamber to allow for good dose resolution from the recycling integrator. This effect is exacerbated with higher energy (and thereby lower LET) protons. As such, in these cases a specialized scintillation detector known as the “coincidence scintillator” is used.
The coincidence scintillator is constructed out of two independent scintillation plastic and phototube assemblies which are adjacent to one another but optically separated. Gammas and other background radiation will generally only interact one of the detectors whereas protons traveling along the beam axis will interact with both effectively instantaneously. As such, a coincidence between the two detectors can be treated as a proton detection.
The coincidence scintillator produces an effectively identical signal to the standard scintillator so the same calibration procedure described above can be followed.
Because protons also produce less ionization within QC1 as well, its own resolution from its recycling integrators can be a limiting factor for very small fluxes. If desired, the coincidence scintillator may be used as the primary flux monitor in place of QC1 provided that there is enough space for it within the beam’s uniform area alongside the test article.
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