why is radioactivity measured in several ways such as mCi,dpm,Bq, etc.?

Hello rswchicago,

Radioactivity cannot be measured by the three-dimensional size of the
radioactive material. As a result there are different ways to measure
the amount of radioactivity.
SOURCE: http://hps.org/publicinformation/ate/faqs/radunits.html

A quick summary of what Radioactivity is can be found on the MSN
Encarta website: "Radioactivity, spontaneous disintegration of atomic
nuclei by the emission of subatomic particles called alpha particles
and beta particles, or of electromagnetic rays called X rays and gamma
rays."

Further down in section II of the article, we are told that the alpha
particles "penetrate into aluminum only a few thousandths of a
centimeter", and that beta particles are "nearly 100 times more
penetrating." The third component, gamma rays are even more
penetrating.
SOURCE: http://encarta.msn.com/encnet/refpages/RefArticle.aspx?refid=761569327&pn=1&s=1#s1

Now, back to our discussion of why radioactivity is measured in
several different ways:

From Rice University in Houston Texas, we are told:
"A traditional unit of radioactivity is the Curie (Ci), which is
defined as that quantity of any radioisotope undergoing 2.22 x 10^12
atomic disintegrations per minute (DPM). A milliCurie (mCi) of a
radioisotope undergoes 2.22 x 109 DPM, and a microCurie produces 2.22
x 10^6 DPM."

So while DPM measures the disintegrations per minute of a
radioisotope, Ci or mCi measures the QUANTITY (amount) of any
radioisotope undergoing 2.22 x 10^12 DPM. Different radioactive
materials are going to have different rates of emissions, so in 1975
the Bq replaced the Curie as the preferred international unit of
radioactivity. "One Bq is defined as one atomic disintegration per
second, or 2.703 x 10^-11 dpm."
SOURCE: http://www.ruf.rice.edu/~bioslabs/methods/radioisotopes/rad1.html

Taking a step back now, we have this information so far: 

1 curie (Ci) = 3.7 x 1010 disintegrations/sec. (dps)
1 millicurie (mCi) = 2.22 x 109 disintegration/min. (dpm)
1 microcurie (Ci) = 2.22 x 106 disintegration/min. (dpm)
1 becquerel (Bq)   = 1 disintegration/sec. (dps)
SOURCE: http://www.radsafe.pitt.edu/ManualTraining/SectionII.htm

Going back to the first webpage we looked at, we are given this info:

"The System International of units (SI system) uses the unit of
becquerel (Bq) as its unit of radioactivity. One curie is 37 billion
Bq. Since the Bq represents such a small amount, one is likely to see
a prefix noting a large multiplier used with the Bq as follows:
37 GBq = 37 billion Bq = 1 Curie 
1 MBq = 1 million Bq = ~ 27 microcuries 
1 GBq = 1 billion Bq = ~ 27 millicuries 
1TBq = 1 trillion Bq = ~ 27 Curies"
SOURCE: http://hps.org/publicinformation/ate/faqs/radunits.html

As you can see, the Bq is able to measure much smaller amounts of
radioactivity. We also know that 1 Bq is equal to 1 disintegration per
second, whereas a Curie was 3.7 x 1010 disintegrations/sec.

In my mind at least, it is easier to understand that 1GBq (1 billion
Bq) is equal to 1 billion disintegrations per second. Sure one could
say that 27 millicuries is the equivilant of that amount, but then one
has to always keep in mind that 1 millicurie is equal to 2.22 x 10^12
DPM!

Instead of having a fixed rate of 1 millicurie that is equal to 2.22 x
10^12 DPM (or another number for DPS), it makes it much easier to have
a system of measurement where 1Bq (becquerel) is equal to 1
disintegration per second! Because 1 Bq is also a very minute amount
of radioactivity, we use the prefixes kilo, mega, giga and tera Bq to
measure larger amounts, but it is still always easy to keep in
persepective the fact that 1Bq is equal to 1 disintegration per
second!

The “General Conference on Weights and Measures” decided that the
millicurie was outdated and as a result, they recommended that the Bq
be used to measure the rate of radioactivity. I personally prefer the
Bq way of things, but just as some people prefer the imperial system
over the metric system, the mCi is still used in some circles,
especially in the United States.

As an interesting side note, I found that the Ci and Bq “do not
accurately measure how dangerous a given amount of radiation might be
for humans. For medical purposes, other units of measurement
reflecting this aspect are more appropriate.”
SOURCE: http://www.carolina.com/chemistry/experiments/halflife.asp

For more info on the milliRem (the amount used to measure the amount
of radiation that does damage to or kills a person) see this page:
http://web.mit.edu/newsoffice/tt/1994/jan05/33567.html

I hope this is the answer you were looking for. Please let me know if
you require any clarifications regarding this answer, and I will get
back to you as soon as possible.

tisme-ga


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unbelievable...the answer was perfect..thanks so much!