Subject: Re: [RNPROF] Unbelievable gamma reading
Date: May 3, 2009 12:21:02 PM EDT
Welcome to the larger world of radiation from all sources.
Your observation of radiation from a nuclear medicine patient was probably a
good experience for you to help provide a perspective on various sources of
radiation to which we are all exposed.
While unbelievable to you, specialists in radiation safety know about
medical exposures, so this is not a surprise to us.
According to a report about to be published by the National Council on
Radiation Protection and Measurements (NCRP 160), medical sources of
radiation exposure now account for about 48 % of 620 mrem/year (300
mrem/year) on the average to the US population. Medical applications of
radiation include nuclear medicine (such as use of iodine-131 for thyroid
treatments and thallium-201 for heart/lung stress tests). Both of these
nuclides in our bodies will decrease over time due to radioactive decay and
also by excretion in urine and feces. Thus, the time in our bodies is much
less than the radiological half-life.
Nuclear medicine patients are cautioned to avoid close contact with people
for at least several days. The regulations for medical practice, however,
allow for exposures to family members of up to 500 mrem after a patient is
released from the hospital. This could result in more than 2 mrem in an
hour, but that public exposure limitation does not apply to medical
practice. The three principles of radiation safety are justification,
optimization, and limitation. Presumably the high radiation doses to
patients (and families) are justified by the life-saving value of the
nuclear medicine procedures.
Even if a person could receive as much as 10 mrem in an hour in contact with
the nuclear medicine patient, this would require 50 hours of contact to give
500 mrem. Remember, however, during this time, the actual exposure rate is
going to go down quickly from decay and excretion.
Medical applications of radiation also include chest x-rays (at about 10
mrem per exposure) and CT scans (at 1,000 to 3,000 mrem per scan).
Lastly, as I cautioned about using PM 1730 for measurements on granite
countertops, the same caution applies for measuring I-131. This isotope has
a gamma ray energy of about 380 keV, this is almost 1/2 of the cesium-137
calibration energy of 662 keV. Since the PM1730 uses a CsI detector which
is very energy dependent, if you check the dose vs energy response curve you
will see that the PM1730 over responds to I-131 by about a factor of two
above the reading it would get for Cs-137. Thus, all of your readings are
high by at least a factor of two. Remember also the PM 1730 is intended for
providing rapid and sensitive early warning to first responders. It is not
intended to be used for measuring actual exposures in mR/hour.
For perspective on radon, the report NCRP 160 now says that we get about 37%
of our average total radiation dose (229 mrem a year from radon and thoron
decay products) (at about 1.3 pCi/L in our homes). This means at the EPA
action level we are getting between 600 and 700 mrem a year from radon in
hour homes (compare this with the allowable 500 mrem from nuclear medicine
If a home actually has 8 to 10 pCi/L, the exposure to persons in the home
could be from 1,200 to more than 2,000 mrem in a year. Remember this occurs
for all occupants of the home and for every year, not just a short time as
for nuclear medicine procedures.
I hope you will find this information helpful to provide perspective on
various sources of radiation exposure.
Ray Johnson, MS, PE, FHPS, CHP
Vice President, Training Programs
Dade Moeller & Associates
Radiation Safety Academy Division
438 N. Frederick Avenue, Suite 220
Gaithersburg, Maryland 20877
toll free: 800-871-7930
From: International Web Resource for Radon Professionals
[mailto:RADONPROFESSIONALS@LIST.UIOWA.EDU] On Behalf Of Bill Brodhead
Sent: Saturday, May 02, 2009 11:52 AM
Subject: [RNPROF] Unbelievable gamma reading
I now understand why those connected with nuclear medicine consider radon
I was visiting a home to provide a radon mitigation estimate and happened to
have my PM1703 gamma meter on my belt. As soon as I walked into the house
the 1703 started buzzing. I looked down and was shocked that it was reading
300 uR/hr. I took it off my belt and as I approached the client it shot up
to over a thousand uR/hr. The customer then asked me to stand still as he
walked away and my 1703 quickly decreased to more reasonable numbers.
The client had radiation treatment for thyroid cancer 9 days ago!! At
6 feet away my 1703 read 160 uR/hr. At 3 feet it read 400 uR/hr. At
1 foot it read 1300 uR/hr. The client then called his doctors office and
was told it was OK and he should not be concerned. He was told the half
life was 8 days. He was previously told to not sleep with his wife for two
days and to stay indoors for a week but could go out after that.
As I understand it 2 mR/hr for an hour is a maximum allowable dose.
Well this guy was well beyond that. At nine days his chest was still
reading 3 mR/hr! I calculate it would take 2 months before his chest is
down to the 40 uR/hr above background that some granite countertops emit.
W 610 346-8004 Fax 610 346-8575
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