## Gamma Radiation Source Facility (CERF)

The Biology Department maintains a Gamma Radiation Facility known as CERF where a gamma ray source (Cesium-137) can be operated to provide gamma rays of energy 0.662 MeV at a variety of dose rates.

A schematic of the irradiation room is shown below.

Figure 1 shows a schematic of the room containing the Cs-137 gamma source.  The source is lowered into the ground to shield users from it when not in use, and raised using a hand pulley system to give the exposures.  The length of time it takes to raise and lower the source provides a limitation to the overall accuracy in delivering small doses.  Photos of the facility are located below, and a high resolution version of the schematic can be obtained here. (jpg 355 kB)

The dose rate is controlled by locating the samples near or far from the source.  Doses are controlled by exposing samples for shorter or longer exposure times.  The most recent set of dose rate calibrations for the CERF were made in 1998.  Those calibrations have been adjusted to account for the source lifetime, and the table below gives the dose rates expected at a variety of distances from the source as of 16 April 2008.

Table 1: Measurements taken in 1998 of the dose rate as a function of distance from the source in centimeters.

 Distance to Source (cm) Calibration Dose Rate as of 11/20/1998 (R/min) Dose Rate on 4/16/2008 (R/min) 80 33.4 26.9 88 28.3 22.8 138 12.6 10.1 180 8.7 7.0 230 5.16 4.2 340 2.48 2.0
The farthest corner of the room is at a distance of 5 meters from the source where the dose rate falls below 1.0 R/minute, extrapolated from the measurements in Table 1 above.

For intermediate distances, the dose can be determined using the function
D = A/(r + ro)2
where D is the dose in Rads per minute, r is the distance from the source in centimeters, and the two free parameters A and ro are 315366 and 17.36 cm, respectively.
Alternatively, in order to calculate the distance for a given dose rate, use the formula
r = (A/D)0.5 - ro

Figure 2 shows the dependence of dose rate in Rads per minute as a function of distance from the source in cm.  The fit is described in the test.  Activity levels have been adjusted to 4/16/2008.

Users interested in applying for time on CERF can apply by submitting a request form that can be obtained here.  The facility is maintained by the CERF Liaison who should be contacted by anyone interested in using it.  He can be contacted by email at rich1@bnl.gov, or by phone (x3386, or pager 0210).

Here are some photographs of the facility.  Click on the thumbnail for a larger version.

For a Cs-137 source emitting 662 keV gamma rays and some soft beta particles, the dose rate is related to the source activity by the relation 103 microSieverts per hour corresponds to 1 GigaBequerel at 1 meter.  At a distance of 1 meter, our dose rate is 18.1 R/m which can be converted to 1.06 x 1014 decays per second, or 8.4 x 108 gammas per cm2 per second.