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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 radiation field generated when 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 both estimate 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.  ISU's dose limits for the general public are currently set for 100 mrem/year according to the Radiation Safety Training Guide (Rev 08/07).
Our goal is to inject only one electron per pulse into the HRRL experimental Cell, a current several order of manginute less than 2 mA even at the maximum HRRL rep rate.  

A small apperature collimator will be used to reduce the current into the HRRL experimental cell  from its maximum current of 80 mA to less than 2 mA  peak current inside the HRRL experimental cell.  The beam spot size after the 90 degree bend has been observed to be a 2 cm long oval that is about 1 cm wide and does not exceed this size due to the intrinsic collimation of the dipole bending magnet.  A 2" x 4" x 8" Aluminum block collimator with a 2 mm diameter hole will be placed near the HRRL experimental wall in order to reduce the current.  The current would be reduced by at least a factor of 30 if there were no beam divergence thereby reducing the peak beam current to 2.5 mA resulting in a dose of about 2 mrad/hr above the HRRL ceiling cell if there were no beam loss.  

[[Image:Electrons_hit_Phos_screen.jpg | 300.px]][[Image:Electrons_hit_Phos_screen_2.jpg | 300.px]]


The HRRL ceiling dose is expected to be lower than 2 mrad/hr due to the beam divergence and transport loss into the HRRL experimental cell using the collimation described above.  The collimator will also be used to monitor the peak current on the accelerator side.   The Aluminum collimator with the 1 mm diameter hole will have an aluminum shutter placed in front of it to allow the accelerator operators to tune the electron beam and prevent the transport of electrons into the HRRL experimental cell.   The shutter can be remotely opened after establishing stable accelerator operations and maximum beam spot size.   A FC will be placed on the HRRL experimental cell side of the wall that is able to measure peak currents down to 2mA and can be used as a fast accelerator shutdown interlock to prevent current in the HRRL experimental cell that would exceed a dose 2 mrad/hr in public areas near the cell.  Measurements from this test will be used to define the safety envelope and associated HRRL operational parameters.  

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


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