HRRL Electron OSA

Operational Safety Assessment

Low Dose Detector Testing Using the High Repetion Rate Linac (HRRL) at the Particle Beam Laboratory 10/26/2008

1. Accelerator Description and Test Purpose

This document outlines a procedure to determine if a low current of 3 MeV electrons from the High Repetion Rate Linac (HRRL) may be used to test detectors in the HRRL experimental cell. The HRRL is an e-linac located in the basement of the physics building which is capable of 25 ns to 1 usec long beam pulses up to 80 mA peak currents at a repetition rate of 2 kHz. The beam energy can range between 3 and 15 MeV. The HRRL has previously been used to deliver a bremsstrahlung beam to an adjacent experimental cell for several experiments. We would like to compare a measurement of the dose due to operating the HRRL with our Monte Carlo calculations to determine if the HRRL may be used to source a 25 ns wide pulse of 3 MeV electrons.

The goal of this test is measure the dose in public areas around the HRRL experimental cell when the HRRL is configured to deliver a single electron per pulse to the HRRL experimental cell. A MCNPX and GEANT4 Monte Carlo estimates that a 25 nsec long peak beam current of 2 mA would generate less than 2 mrad/hr above a 6" concrete ceiling assumed to exist above the HRRL experimental cell.

exCurrent Due to the short beam pulse, high radiation output is not expected. An estimate of output dose can be made based on scaling the new pulse length and rep. rate with the former 18 MeV system’s characteristics (50ns at 2000 Hz vs. 3 microsecond at 120 Hz). This calculation indicates the new system will produce ~1/3 the dose of the former accelerator, thus, the limiting conditions of operation will be determined by the duty cycle of the electron gun. The test objectives covered by this OSA are to inject beam into the accelerator while keeping the rep. rate below 100 Hz. This test will allow various accelerator-tuning issues to be understood as well as give a scalable indication of total radiation dose as a function of gun pulse duty cycle and providing the information that we need to define the safety envelope and associated operational parameters. For this series of tests the beam will only be directed through the 0° port of the accelerator and the two beam holes into the test area will be blocked with shielding material. The intent of this Operational Safety Assessment is to address the radiation safety related aspects for these experiments.

2. Safety

The initial beam injection test will be monitored by the ISU Technical Safety Office (TSO) to establish safe operations (SO). Upon the establishment of SO the TSO will provide monitoring as determined necessary based on initial operations. To address the operational safety needs shielding, interlocks, warning lights, audible signals, manual scram buttons, radiation survey instruments, and in the shielding properties of the building will be utilized. A diagram is attached.

1. Interlocks. The HRRL power supply is interlocked to the current building interlock system such that the access to the operational cell area will break the interlock chain disabling the accelerator. After the interlocks are set and before the accelerator will operate an audible and visual warning will be activated. Interlock characteristics are as per the standard IAC interlock system including, lights, keys, scrams, inspection buttons, etc., as follows: a. Lights. Rotating beacons, yellow for interlocks set, red for beam on. b. Audible Alarms. After interlocks are set a 30-second audible alarm is sounded. Only after the cessation of the audible alarm will the accelerator operate. c. Keys. A key is required to set interlocks and a separate key must be set to allow the enable button upon the accelerator computer touch screen to actuate. d. Scrams. Scram buttons are collocated with inspection buttons. Depressing a scram will deactivate the interlocks shutting off the accelerator and require the interlock arming procedure to be repeated before resuming operation. e. Inspection Buttons. These buttons must be depressed within two (2) minutes of final interlock setting. f. Area Monitors. When operating limits are established area monitors will be interlocked to the accelerator such that at preset limit the accelerator will be caused to trip off. g. Area Monitor #2. This is an independent monitor that will indicate dose rate at the cell door.

2. Survey instruments. No entrance into the cell radiation area will be allowed after tests without the presence of the operator or operator designee with out the Power Supply Key, and a TSO-approved survey meter.

3. Operators. Only IAC professional staff will operate the HRRL during the tests.

4. Radiation surveys. Surveys of the 1st floor areas and other potential hot spots will be conducted by TSO personnel and/or TSO designees.

5. Radiation Measurements to be made. TSO or designee will make radiation field measurements. a. Left East Wall Beowulf room b. Corridor and Control Area c. Upstairs in W end of Physics Offices d. Outside on W end of addition e. In the vicinity of the klystron