Instruments and Surveys

How do you measure something that is invisible, odorless, and colorless?

 

 

The Geiger-Mueller Detector (G-M)

 

The most common radiation detection instrument is the Geiger counter.  This instrument is a gas-filled detector that works on the principle that as radiation passes through air or a specific gas, ionization of the molecules occur.  When a voltage is connected across the gas chamber, a current will flow as the positive ions are collected at the cathode and the negative ions are collected at the anode.  This current can be used to power a meter to quantify the radiation.

    

The GM detector is a good sturdy detector used for the detection of beta and gamma radiation.  It will detect most beta, gamma, and x-ray radiation used in the research environment as well as the strong gamma and x-ray radiation used in industrial radiography.

 

 

Pre-Use Checks

 

Check the Battery

Check the Wires for Damage or Shorts (if moving the wire causes meter response with no radiation present, the wire is damaged)

Check the Meter for Obvious Signs of Damage

Ensure the Instrument is in Calibration (date found on instrument sticker)

Check the Instrument Response to a Known Source of Radiation

 

 

What Are We Measuring

 

Radiation - When we measure radiation, we are measuring the dose rate from a radioactive source in order to locate the source and also to establish boundaries.  We use this information to keep our personnel doses as low as reasonably achievable and to protect the public.  Typical units and instrument scales for radiation measurement would be - mR/h, R/h, rad/h, mrad/hr, rem/hr, and mrem/hr.  Remember, for gamma, x-ray, and beta radiation we say:

Roentgens (R) = rads = rems

Radioactive Contamination - When we measure contamination, we are determining how much radioactive material has spilled or is in a location where we do not want it.  Contamination gives off radiation, but is not radiation.  In other words, you can get contamination on your shoe, but not radiation.  Typical units would be counts per second (cps or c/s) or counts per minute (cpm or c/m).

Note: 1 mCi = 3.7 x 107 counts per second and 2.22 x 109 counts per minute (assuming 100% detection efficiency). 

 

 

Survey Techniques

 

Radiation

When looking for unknown radiation levels, walk in the direction of the highest reading until you find the source, or reach the pre-determined radiation reading.

When looking for radiation area readings, hold the detector at waist level and slowly turn 360 degrees (all the way around).  You are looking for the highest reading in an area to record.  Ensure enough points are taken to give a good picture of the radiation gradients.  If work is to be performed in an area, personnel will need to know the high areas to avoid and lower radiation areas where they can rest if they are not exiting.

Listen to the audible response, it is faster than the meter response.

If entering a high radiation field, it is important to understand that G-M meters, especially archaic ones, may fail to zero if in a field higher than full scale.  It is important to be alert to changing radiation readings.  Walk slowly, surveying as you go.  Do not proceed directly to the survey area then turn the instrument on.

 

Contamination

When looking for radioactive contamination, slowly scan the surface within 1/2" with the pancake probe.  A slow scan shouldn't move faster than 2" per second (or 1 foot every six seconds).  If the meter has a response setting, it should be set at the fastest response when looking for contamination.

Listen to the audio and, when an increased count rate is heard, stop and hold the detector over the area of the highest count rate.

The response setting should be turned to the slower setting to obtain a reading.

When surveying yourself, the basic procedure is the same.  Be sure to survey all areas that may have been contaminated - hands, face, front of clothing, seat of pants or dress, and shoes.  When surveying personnel out of an accident area, survey their entire body from the head down.

        

Final personnel release surveys should always be performed in an area with low (normal) background levels.

 

 

Scintillation Crystal Detector

    

Scintillation crystals and photo-multipliers are used to detect and measure gamma and x-ray radiation.  These detectors are especially useful in the research environment for detecting the low-energy photons emitted from I-125.  The detectors can either be hand-held or well-counters.  The well-counters are used to count samples in a low background shielded cavity.  The hand-held models are for laboratory work surveys and personnel surveys.  The pre-use checks and survey techniques are essentially the same as for the GM detector above.   For I-125 detection, the detector will have a thin window that will allow the relatively low-energy photons to reach the scintillating crystal.  These windows are fragile and surveys should be conducted with care to ensure no damage to these windows occurs. 

 

Liquid Scintillation Detectors

Liquid scintillation detectors are used to measure weak beta and x-ray emitters.  They are especially useful in detecting the weak beta emissions coming from common radionuclides used in research such as tritium, carbon-14, and sulfur-35.  A sample is immersed in a solution that will scintillate, or give off flashes of light, when exposed to radiation.  The sample and solution (in a vial) are then moved to a shielded cavity in the counter and a photo-multiplier measures the flashes of light.  These flashes are then converted to radioactivity measurements.

It is important that this counter and the crystal scintillation counter be calibrated to a known radioactive source and their efficiency be checked periodically and anytime the machine is moved or subjected to uncontrolled power failures.  Refer to the specific machine's owner's manual and technical representative for information on how these checks are accomplished.

 

Counting Statistics and Efficiency

Two things must be considered when recording radioactivity measurements.

How efficient is your system at counting the emission your are measuring and

Is your measurement statistically significant?

The principles of counting statistics and radiation measurement could fill a book (and have) but we do not have the time or need to go over all the nuances of this field.  What is important is that your measurements have meaning.  In other words, if your machine gives you a reading in counts per minute how does that transfer to decays per minute - or measurement to radioactivity.  The good news here is that most machines will do that for you if you input the radionuclide of concern.   The real important numbers are the minimum detectable counts (MDC) and minimum detectable activity (MDA) for a detector.  These will be used to determine if you can detect and measure activity at the level you need to detect and measure it.  

For any given count time and background reading there are a minimum number of counts that the instrument can detect with statistical reliability. Results lower than this number cannot be distinguished from background. The formula for this number is:

 

MDC = 3 + 4.65 Sqrt (R bkg T bkg ) - for same sample and bkg count time

 

MDC = 3 + 3.29 Sqrt (R bkg T sample(1 + T sample /T bkg) ) - for different count times

 

Where: R bkg = Background counts per minute

T sample = Sample counting time

T bkg = Background counts per minute

The Minimum Detectable Activity (MDA) for a survey needs to be determined to ensure that you can detect the trigger values of contamination and to determine CPM results which are statistically insignificant.  For contamination surveys, you should be able to detect 200 DPM per sample.  Therefore your MDA would be 200 DPM or less.

The formula for MDA is:

MDA = MDC / (Counter Efficiency for Isotope of Concern X Sample Count Time)

 

Measuring Radioactive Contamination with a Deep Well Scintillation Counter or a Liquid Scintillation Counter

We discussed how to survey for contamination with a hand-held detector and stated how samples are counted in a laboratory counter, but we didn't discus how to obtain contamination samples of a workplace to measure in the counters.  We use wipe (or swipe) samples to do this.  Wipes are small, usually round, pieces of filter paper that we use to wipe a surface to determine if there is loose surface contamination present.  If fixed contamination is suspected, a scan using a hand-held detector capable of detecting the radiation will also need to be used.

The fundamental procedure for taking a wipe sample is:

Determine points to be surveyed.

Sample an area known to be clean first.  This will serve as your control, or background, sample.

Choose order of sampling to sample areas of suspected lowest contamination first, moving to areas of highest contamination last.

Sample boundary areas such as hallways for each survey as well as work areas.

Wear gloves when performing surveys in suspected contaminated areas.

Wipe the survey paper over an area of 100 square centimeters for each location.  If this is impractical due to the size of what you are sampling, make a note of this with an estimated area surveyed.  For a 1" wipe paper, a 16" (39.3 cm) "S" should be wiped or an area of 4" by 4" (10 by 10 cm).

The samples should not be allowed to come in contact with each other to prevent cross contamination.

Wear gloves and load the samples into vials for counting.  The order loaded should be the order taken (i.e. lowest contamination to highest).

For liquid scintillation counting, liquid scintillation fluid will need to be added to each vial.  For gamma scintillation counters it does not.

Count the samples per the operating instructions of your counter.  Record results on approved survey form.

Results Requiring Action

It is good work practice to decontaminate any area that has a count rate three times the background count rate.

ALARA (As Low As Reasonably Achievable) limits for restricted area surface contamination for beta emitters is 550 dpm/100cm2 for floors and counter tops and 1100 dpm/100cm2 for radioisotope refrigerators, freezers, and fume hoods.

Maximum contamination levels are listed in Chapter 7 of the Ames Health and Safety Manual, Section 7.4.2.  If these limits are reached or exceeded, immediate decontamination is required.  If work practices routinely cause levels at these limits, the Radiation Safety Officer should be asked to review work practices in order to reduce contamination levels to acceptable standards.

 

Badge Dosimeter

When personnel work in areas where exposure to radiation is expected, they are issued a whole body dosimeter.   These can be TLD's, Film Badges, or Luxel badges as shown above. 

Three things are important regarding these badges:

They are issued to the individual and are not to be shared.

They are to be worn at the collar, chest, or waist - whichever is suspected to receive the highest dose from the work area.

When not in use they should be stored in an area away from radioisotopes.

TLD Ring

Personnel working with radioisotopes that will be manipulated behind a shield will also be issued ring badges to ensure the dose to the hands is measured.  Rings should be worn on the dominant hand (or hand that will be closet to the radioisotope the most), TLD (printed portion) towards the palm (opposite from the way a ring is normally worn), under the glove.

 

Back Home Next

hplogo2.jpg (16555 bytes)
Hosted by www.Geocities.ws

1