Qualified users interested in obtaining time on CERF can apply by submitting a request form (.doc) and contacting the CERF Liason at 631-344-3386.
The Gamma Radiation Facility known as CERF houses a cesium-137 gamma ray source which can provide 0.662 MeV gamma rays at a variety of dose rates.
The figure at right 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. 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.
|Distance to Source (cm)||Calibration Dose Rate as of 11/20/1998 (R/min)||Dose Rate on 4/16/2008 (R/min)|
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 the table 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
The figure above 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.
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.