Protons The BASE Facility is capable of providing fluxes of up to 1E10 protons/cm2-sec, the limit of our standard (continuously reading) ion chamber dosimetry. Higher levels of flux are monitored using intermittent faraday cup readings. Standard proton energies include 13.5, 20, 30, 40, 50, and 55 MeV/nucleon. Energies below 13.5 MeV/nucleon can be run in vacuum in Cave 4B. All proton testing is performed in air. Shielding materials, laser alignment tools, and mounting fixtures are available. Holes are provided through the cave shielding blocks for connecting additional test equipment, with a distance of approximately 10 feet from the test bench to the top of the shielding block (10 BNC cables are permanently installed and available for use; additional cables can be added).

To tune the beam into the cave, protons created at our ion source are accelerated through the Cyclotron and sent down the beamline. It is then spread out evenly (approx. 6-inch diameter) on the cave phosphor and viewed with a digital camera by the Control Room Operator. Once the visual spread is satisfactory, both an ion chamber and Gafchromic film are used to fine-tune the beam uniformity to +/- 10%.

The ion chamber (pictured) monitors the beam continuously during the experiment. Seven concentric rings (with diameters of 1cm, 2cm, 4cm, 6cm, 8cm, 10cm, and 18cm) and four quadrants are used to establish and monitor uniformity. Beam particles traveling through the nitrogen-filled ion chamber leave a trail of ions that are collected by electrodes, the resulting current of which is measured with the electronics. The current signal from the ion chamber is routed through a current to frequency converter, with the pulses then counted with a (CAMAC) scaler module and fed in to the computer. After the beam has achieved proper uniformity on the ion chamber, an exposure is made with Gafchromic film and scanned using a standard desktop printer. Both uniformity data and film exposure profiles are provided to the experimenter.

The final processing and indication of ion chamber data is performed with LabView software. Flux and fluence values for each ring and quadrant are readily available. In addition, fluence limits can be set that will stop the beam upon reaching the desired fluence level.

Parts activated with protons often take days or weeks to decay back to background levels. These parts can be released by either waiting for activity to decay to background, or preferably by sending us a copy of your “rad license” with the required isotope and activity limits (see your Radiation Safety Officer for information regarding this license).