PLUTONIUM LUNG MONITOR USING A THIN NAI(TL) CRYSTAL OF LARGE AREA

For the assessment of plutonium(Pu) amount in human lungs, a very efficient detector for low energy photons is necessary because of the low energy of emitted X-rays (17 keV on the average) and the small value of maximum permissible lung burden of 239Pu, 16 nCi. As such a detector, we have constructed a Pu lung monitor employing a combination of a large thin NaI(Tl) crystal and a photomultiplier of large photocathode. Size of the crystal is 8 in. diameter by 5 mm thickness in order to cover a large portion of the lungs, and the photomultiplier used (VMP/11/170) possesses a photocathode of 7 in. dia. that has been chosen for the best uniformity in photocathode sensitivity among five identical tubes. The whole detector was set in a shielding chamber of 20 cm thick iron plus 3 mm thick lead which is being used for the whole body monitor. Extraction of the optimum performance from the detector was attempted first by improving the energy resolution with insertion of light guide (made of Lucite) between the optical window of the crystal and the photocathode. An optimum thickness of the light guide was found to be 2 cm for the present detector. Then, background counts of the detector in the shielding chamber were analyzed and it was found that about 40% of the total background between 16 and 100 keV originated from y-activities in the optical window and the glass envelope of the photomultiplier. Noise of the photomultiplier was dominant below 13 keV when no light guide was used, but interfered with the detection of 17 keV X-rays when light guide was inserted. Consequently, it was decided that no light guide be employed in the final form of the detector with a slight sacrifice in energy resolution. Counting efficiency for 239Pu in the lungs was determined using a phantom that was made of Lucite plates. The detector was placed in a similar geometry to that in measurement of actual human subjects. The counting efficiency was found to be 1.34 × 10-2 counts/sec/nCi of 239Pu. The minimum detectable amount defined as 3 times the standard deviation of background counts is 6.2 nCi of 239Pu for a counting time of 100 min. The background counts include those from degraded gamma-rays of 137Cs and K in a normal subject and correction was made on the counting efficiency due to absorption of X-ray photons by rib bones. The values obtained above are rather tentative ones in view of uncertainties in the method of calibration, but it is well below the maximum permissibie lung burden of 239Pu. Finally, some improvements in the future are discussed. © 1969 Health Physics Society.