Ludlum instruments with microRoentgens per hour (µR/hr) meter faces are typically calibrated with a ¹³⁷Cs source and a pulse generator (pulser). The pulser should possess the capability of producing a negative pulse with a 0.5 µs lead, a 1.5 µs width, and a 5 µs tail. The Ludlum Model 500 supplies the correct pulse size and also has a high voltage (HV) readout which will be necessary for complete calibration.

To begin calibration, adjust the HV to determine the operating voltage. This may be accomplished by turning the HV pot clockwise to increase the count readout until the count rate readout is at a constant rate. If the count readout abruptly goes off the scale, turn the HV pot counter clockwise and start this step over. Then place the detector in a milliRoentgen per hour (mR/hr) level. (There are a few different μR/hr meter faces available.) To determine the correct level, calculate the location of the meter needle at ³⁄₄ of full scale. For example, the Ludlum Model 12S comes standard with a 0 - 3 µR/hr face. Thus, when the instrument is set in the x1000 range, ³⁄₄ scale calculates to the 2000 µR/hr meter face point. This reading is equivalent to 2 mR/hr. (1 mR/hr = 1000 µR/hr)

For instrument using a 2.5 x 2.5 cm (1 x 1 in.) NaI(Tl) crystals, place the detector facing the source beam half the distance of the crystal size. Calibrate the meter scale and then check your calibration at the ¹⁄₄ scale. The reading should be within ±10%. Our Calibration Department is limited to 1 mR/hr or 1000 µR/hr, thus a pulser is mandatory to calibrate lower scales. Once the higher scale is calibrated, connect the instrument to the pulser. Next, increase the counts per minute to equal ³⁄₄ scale on the range calibrated. Then, decrease the counts to equal ¹⁄₄ of the meter face scale. For example, 2 mR/hr (2000 µR/hr) = 475,000 cpm and 1 mR/hr (1000 µR/hr) = 200,000 cpm. Next, decade down both the pulser and the instrument by a factor of 10. Using the same pulser count rate, but by a factor lower, calibrate the next range. Repeat this procedure for all the lower ranges.