The BASE Facility is capable of providing fluxes of up to 1E9 protons/cm2-sec (the limit of our standard, continuously reading ion chamber dosimetry), but works best in the 1E7 to 1E8 protons/cm2-sec range. Higher levels of flux are monitored using intermittent faraday cup readings. Standard proton energies include 13.5, 20, 30, 40, 50, and 55 MeV, and are performed in air in Cave 4A. Energies below 10 MeV are run in vacuum in Cave 4B. 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). A cable length of 25 feet will comfortably span the distance from the test bench to the BASE Shack upstairs, from where the experiments are typically monitored.
To tune the beam into the cave, protons generated at our ion source are accelerated through the Cyclotron and tuned down the beamline. The beam is then spread out evenly 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 (typically) +/- 10%.
The ion chamber (pictured) monitors the beam continuously during the experiment. Five concentric rings (with diameters of 1cm, 2cm, 4cm, 6cm, and 8cm) and four quadrants are used to establish and monitor uniformity. Collimators are available with diameters of 1, 2, 3, and 4 inches. 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, generating pulses which are 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. The final processing and indication of ion chamber data provides the user with flux and fluence values for each ring and quadrant, and fluence limits can be set that will stop the beam upon reaching the desired fluence level. In addition to a text log file containing beam data for each run, uniformity information and film exposure profiles are available to the experimenter upon request.
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 (which can range from days to years depending upon beam fluence and part composition), or by sending us a copy of your “rad license” with the required isotope and activity limits.
Drawings for Cave 4A mounting fixtures and components can be found here.