Answer to Question #3759 Submitted to "Ask the Experts"

How can I determine the dose equivalent (mrem/hr, mrem/wk, mrem/yr) using the raw data from a TLD (thermoluminescent dosimeter) that was placed in an office for 22 days?

Two single shots per day are fired from a high voltage (3 MV) pulsed storage machine. The measured raw data on the TLD is 24 mrem. (occupancy factor = 1/4, NCRP 49, Table 4) The reference background is 63 mrem/yr. Fourteen shots were measured during the 22 days. I want to determine the nonoccupational exposure.

I will try to answer your question, stating assumptions that I have to make along the way.

Assumption 1: The "raw data" of 24 mrem on the TLD is the total dose received by the TLD during the 22 days that the TLD was deployed in your office. This means that (1) any TLD reader background signal has been properly subtracted, (2) the signal accumulating in the TLD during any transport or storage time between preparation (annealing) and deployment in your office has been properly subtracted and, (3) the signal accumulating in the TLD during any transport or storage time between removal from your office and processing (reading) has been properly subtracted.

With this assumption, the total dose rate in your office is (24 mrem)/(22 days), or 1.091 mrem per day. The reference background is given as 63 mrem per year which is (63 days)/(365.25 days per year) or 0.172 mrem per day. Thus the average net dose rate to the TLD during the 22 days was (1.091-0.172) = 0.919 mrem per day.

Assumption 2: The net dose rate of 0.919 mrem per day in your office is attributable to the accelerator or other regulated sources under the control of your institution.

Assumption 3: The office occupancy factor of 1/4 attributed in your question to NCRP Report 49 was misread from Table 4. The correct occupancy factor given in NCRP 49 for offices is 1.

Assumption 4: You take 20 business days per year off for vacation and holidays. You work in your office for the remaining 240 days per year.

Assumption 5: All of the accelerator shots occur while you are in your office.

Assumption 6: The average net dose rate of 0.919 mrem per day determined during the 22 days of measurement continues for a year.

Based on the data that you provided and my assumptions, your annual dose received in your office from the accelerator or other regulated sources at your institution is predicted to be (0.919 mrem per day) x (240 work days per year) = 220 mrem.

Tom Gesell, PhD