By minimizing the amount of time spent around a radiation source, exposures can be minimized as shown by the graph of exposure vs time.  Prior planning is the key to minimizing time spent around radiation.  Preplan the procedure, practice the technique without the radiation source and prepare the tools, equipment and chemicals prior to work.  These will markedly reduce exposure time.
 
 
 
 
 
 

         The next figure shows how radiation drops off with increased distance.  The relationship between radiation and distance is an inverse square relationship.  If the distance from a point source of radiation is doubled, the exposure is only 1/4 of that at the original distance.  Exposure can be minimized by taking this into consideration when choosing storage areas and work spaces.
 
 
 
 
 
 

         The final principle of radiation protection is shielding.  Radiation interacts with the material which it passes through so by properly choosing material placed between the radiation worker and the radiation, the amount of radiation can be reduced.  In choosing shielding, one should consider the type of radiation and the energy of the radiation.  Lead is a very dense material and provides efficient shielding for x-rays and gamma radiation.  High energy beta particles such as those found in ³²P should be shielded with a less dense material such as plexiglass.

Internal Protection
         In addition to performing the necessary bioassay, it is also important for radiation workers to protect against internal exposure to radiation.  Internal exposure occurs when radioactive material enters the body by ingestion, inhalation or absorption through the skin.  Special precautions can be taken to greatly reduce the possibility of internalizing radioactive material.
         To prevent ingestion of radioactive material no eating, drinking, smoking or application of cosmetics is allowed in areas where radioactive materials are used.  No food can be stored in refrigerators where radioactive material is also stored.  Also, to protect against ingestion, pipetting by mouth is prohibited.
         Some radioactive material such as unbound ¹²⁵I, ¹³¹I and some ³⁵S compounds are volatile; therefore, to prevent inhalation, these should be used under the hood.  During iodination charcoal masks and personal air monitors should be worn by persons performing the iodination.  Technique is checked by use of thyroid bioassay before and after the procedure.
         To prevent against absorption of radioactive material, proper use of protective clothing is necessary.  Lab coats, disposable gloves and protective eyewear should be worn at all times while working with radioactive material. Protective clothing should be removed and hands should be washed carefully before leaving the laboratory.

Laboratory Surveys and Instrumentation
         One of the best methods for protection against internal and external exposure is the lab survey.  If performed correctly, the survey can detect areas of contamination which could be sources of exposure.  For example, if the phone is accidentally contaminated by someone who answers it wearing contaminated gloves, the lab survey will find the contamination before it becomes a problem.
        Every time radioactive material is used in the laboratory, it is good practice to survey the area where the material was used as well as survey the person who used it.  This is easily accomplished if the radioactive material is a high energy beta emitter such as ³²P or a gamma and x-ray emitter such as ¹²⁵I.  Upon completion of the experiment, use the survey meter to check the areas that were used and then survey yourself, the cuffs of your lab coat, your hands and the soles of your shoes.  This survey does not have to be documented each time it is done.  Weekly, if radioactive material is used, a more thorough lab survey is required.  Survey forms are available from the health physicist which include a map of your area and spaces for the instruments used and results from your survey.  The survey consists of a dose rate survey and a wipe test survey.  The record must include the date, area surveyed, equipment used, name and signature of the person conducting the survey, a diagram of the areas surveyed and measurements.
         To begin the survey, if the isotope is detectable with a survey meter, check the areas of the lab where work has been performed and where isotopes are stored.  It is also a good practice to check areas that are not supposed to contain radiation, the phone, the door knob, desks, "cold" centrifuges and incubators, etc.  This part of the survey may surprise you.  It is very important to use the survey meter appropriate for your isotope use.  The action level for dose rate surveys is 2 mR/hr in unrestricted areas, on personal clothing and on skin while the action level for restricted areas and protective clothing used only in restricted areas is 10 mR/hr.  This means that if the GM survey yields levels above the action level, you should decontaminate or shield the area and notify radiation safety.
        The survey meter that is best known is  the Geiger Counter or GM Survey Meter.  It is simple in principle, easy to operate, relatively inexpensive, sensitive, reliable and versatile.  It is particularly suited to radiation safety surveys.  The GM can detect gamma or x-ray radiation (¹²⁵I and ⁵¹Cr) and high energy betas (³²P, ⁹⁰Sr).  The GM will not detect the ³H betas and a sensitive probe such as the pancake is necessary to detect ³⁵S and ¹⁴C.
         The GM tube consists of a cylinder with a conductive shell filled with an inert gas at low pressure (1/8 atmosphere).  It has a central wire with a positive charge.  Anytime an ionization occurs, it initiates a chain reaction in which there is a succession of ionizations that cause the control wire to collect a multitude of electrons.  Multiplication of charge produces a signal of around one volt which is used to activate the counting circuit.

 When preparing to use your GM there are four things which need to be checked:
  Calibration
   Battery
  Response to radiation
Scale

         Before performing the survey, check that the meter is in calibration.  Survey meters must be calibrated annually.  A sticker on the side of the meter shows the date of calibration and the date that the meter is due for calibration.  If the meter is out of calibration, contact the Radiation Safety Office to have it calibrated.  If possible, RSO will loan you a meter while yours is being calibrated.  After calibration, the meter is returned to you with a certificate of calibration.  This certificate should be kept in a file in the laboratory.
         Then, a battery check should be performed.  Most of the meters have a scale labeled "BAT."  By scaling your meter to this setting, the needle should move to the corresponding area on the meter face.  Never use an instrument with low battery power since this may result in erroneous readings.  The batteries can be changed in the instrument when necessary without disturbing the calibration.
         Next, a check that the meter is responding to radiation is necessary.  By placing the probe next to a known radiation source such as a stock vial, the meter can be checked for response.  If the meter is not responding to radiation, contact the Radiation Safety Office for repair.  Finally, ensure that you are familiar with reading the meter.  Sometimes this is the most difficult thing about operating the survey meter.  Most instruments have three or more scales which represent multipliers to use with the instrument scale.  The most common are X0.1, X1, X10 and X100.  The instrument can be set initially at either the highest or lowest setting and then moved as necessary to enable reading.  In most cases in the lab, the X0.1 or X1 will be the most appropriate scale.  You read the result from the meter and then multiply by the scale that the meter is set at.  Here are a few examples.
 
 
 
 

Example 1: 
The needle registers at 3.6 .   When the scales are taken into account the readings are:
0.36 mR/hr on the 0.1 scale (3.6x0.1)
3.6 mR/hr on the 1 scale (3.6x1)
and 36 on the 10 scale (3.6x10).
 
 
 
 
 
 
 

Example 2:
The needle registers at 0.8 (see figure XX).  When the scales are taken into account the readings are:
0.08 mR/hr on the 0.1 scale (0.8x0.1)
0.8 mR/hr on the 1 scale (0.8x1)
8 mR/hr on the 10 scale (0.8x10)
100 mR/ hr on the 100 scale (1x100)
This meter has a special scale for the 100 setting.  The needle registers 1.  When the 100 scale is taken into account the radiation field is measured to be 100 mR/hr.  It  will be under extremely odd circumstances that the 100  scale becomes useful to you.

        It is also a good practice to keep the audio on when your survey meter is in use.  Consider the following scenario.  While working the ³²P, you absently turn on the GM not noticing which scale the meter is set at.  You work, checking instruments and gloves for contamination and the needle does not move.  Then you notice that the scale is set at 100 and that is the reason that the needle has not moved.  When you turn the audio on the meter begins to chirp repeatedly.  The audio responds to radiation in the same manner without regard to the scale.    No matter which scale you are set at, you know a hot spot when you find it.
        Detector probes used with survey meters vary with the type of work being done and the isotopes in use.  The best probe for radiation safety surveys is the pancake probe.  This probe enables one to detect ¹⁴C and ³⁵S which is very difficult if not impossible with other probes.  For estimating exposure rate from a beta or low energy gamma or x-ray source, an end window probe would be suitable.  Estimating exposure rate from a medium to high energy gamma or x-ray source is best done with a side window probe.  If two isotopes of different types or energies are being used in the lab, this probe can be used with the window open and closed to determine the isotope involved in the even of contamination or a spill.
         The final step in the weekly laboratory survey is the wipe test.  This part of the survey will determine whether there is removable contamination in the lab.  Using some absorbent material, filter paper, cotton swabs, etc, smear any area where radioactive material is used.  Then, using the proper detection device, count the wipe to determine if any isotope was removed.  Just as with the dose rate survey, it is a good practice to check areas which should not contain any removable contamination, door knobs, refrigerator doors, etc.  The action levels for surface contamination are 200 dpm/100 cm² in unrestricted areas and on personal clothing and skin and 2000 dpm/100 cm² in restricted areas and on protective clothing used only in the restricted area.  However, it is good practice to decontaminate any area or piece of equipment above 200 dpm/100 cm² anywhere in the lab.  This can be accomplished using gloves and some form of cleaning fluid.
         It will be necessary for you to determine which detection device is necessary for the isotope that you are using.  To detect beta emissions, the liquid scintillation counter is the appropriate detector.  The scintillation cocktail converts the beta energy to light which is then multiplied through the use of a photomultiplier tube.  This is then measured to determine the activity.  In order to determine the activity of the sample counted, it is necessary to determine the efficiency of the detector.  Since the detector cannot count every disintegration, the number of the counts which the detector determines must be corrected using this efficiency.  To determine the efficiency of your counter, you will need to count a sample of known activity and then count for  background.  By knowing  the activity of the standard and by using equation 4, you can determine the efficiency.

In equation 4, cpm is the number of counts per minute determined by the detector, bkg is the number of background counts per minute and dpm is the activity of the standard in disintegrations per minute.  For example, in Radiation Safety we use a H-3 standard to calculate efficiency.  The known number of counts determined by the detector is 272 cpm, the background counts is 10 cpm and the known activity of the standard is 414 dpm.

Using equation 4, we determine the efficiency of the detector to be 63%.
         Finally, to determine the activity of the wipe test, we solve equation 4 for activity (see equation 5).

 Here, cpm is the number of counts determined by the detector when the sample is counted.  For example, a wipe test is counted in the our liquid scintillation counter and the number of counts is 1595 cpm, the background is 9 cpm and from above, the efficiency is 63%.  Using equation 5, the wipe is determined to be 2504 dpm.

Since the result exceeds the contamination limit set for a restricted area, clean up is necessary.  Some liquid scintillation counters will perform the above calculations for you.  Our Packard counter will print out the results in dpm using standards which are run daily.  Consult the operating manual of your counter for more information.
         For counting wipe tests in labs where gamma or x-ray emitters are used, it is necessary to use a gamma counter.  The gamma counter is most frequently a sodium iodide crystal activated with thallium and optically coupled to a photomultiplier tube.  The thallium activator converts the energy absorbed in the crystal to light.  The efficiency calculation for the gamma counter is the same as that of the liquid scintillation counter.  Consult the manuals which accompany your gamma counter for more information.