34 | PRM-7000 / 8000 / 9000 Users Guide
The PRM Instrument is calibrated in units of dose equivalent rate (also known as effec-
tive dose rate), referenced to Cesium 137, in milli-Roentgen per hour (mR/hr) or micro-
Sieverts per hour (uSv/hr). Micro-Roentgen per hour (uR/hr) is also provided. For activity
levels, counts per minute (CPM) and counts per second (CPS) are also made available
to the user. To convert the dose rate to the accumulated dose, multiply the number of
hours by the dose rate.
Example 1: On the PRM Instrument the Average Screen indicates 0.025 mR/hr. The
Elapsed time is 3 Days and 6 hours.
What is the accumulated effective dose?
1. 3 Days and 6 hours = (24 x 3) + 6 or 78 hours
2. 78 hours X .025 mR/hr = 1.95 milli-Roentgens
If a safety or other exposure limit is provided in a yearly accumulated dose of millirem, or
microSieverts then divide the yearly dose by the number of hours in a year.
Example 2: The paper entitled “Environmental Radiation” published by the Health
Physics Society (http://hps.org/documents/environmental_radiation_fact_sheet.pdf)
states: ”Each year, a resident of the United States receives an average total dose from
background radiation of about 3.1 mSv.”
Convert this value to an average dose rate of uSv/hr.
1. 3.1 mSv (milliSeivert) per year is the same as 3100 uSv (micro-
Sieverts) per year.
2. The number of hours in a year is 365 days X 24 Hours/day or 8,760
hours.
3. Therefore, the average hourly dose rate would be 3100 uSv / 8760 or
.353 uSv/hr
4. Recall that 1 mR/hr = 10 uSv/hr. Therefore .353 uSv/hr = .035 mR/hr
15.2 What is a Safe Level of Radiation?
There continues to be much debate as to the safe level of radiation exposure. Opinions
range from the position that there is no safe level of radiation exposure to complete ac-
ceptance of the limits established by the global collections of governments and agencies.
The matter remains largely a personal choice and Mazur Instruments offers no specifi c
advice on what constitutes a safe level. The company will leave these discussions to the
global collection of experts in the domain.
A list of web links is included below as a resource for the reader.
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