Selection, suitability and calibration a personal view

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Selection, suitability and calibration - a
personal view

• Pete Burgess

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Disclaimer!

• Neither my current employer, UKAEA, or my future,
  NPL, have approved this.

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Why do you think you need an instrument?

• Confirming theres nothing there?
• Material going off site
• Confirming the level is the same
• as yesterday?
• Looking where theres no previous numbers?
• Comparison with some limit or other?

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Who are the answers really for?

• Regulator?
• Own workforce?
• Pressure group?
• Customer?
• Research project?

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Where are you doing this?

• Nice, clean, dry lab?
• Oil exploration platform?
• Scrap yard?
• Demolition site?
• After a terrorist event?
• Looking for evidence of discharges?

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How often?

• Continuously ?
• Stack discharges
• Interlock for cell
• Personal dosemeter
• Once a day/week/month/year?
• Level gauge integrity
• On demand?
• Response to an emergency

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Whos doing the monitoring?

• The owner of the problem
• Familiar with the work and the environment but
  not independent
• Surveyor
• Should understand the basics of whats happening
  and should be more independent
• Outside body
• Customer, regulator or appointed contractor
• Less familiar with whats happening and (very)
  independent

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Who is going to look after the kit?

• Own people
• Try and keep the number of types to a minimum
• Contractor
• Should be more used to variety
• Manufacturer
• They are (should be?) totally on top of the job

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What quantity?

• Bulk activity in Bq/g
• Surface activity in Bq/cm2
• Air activity in Bq/m3
• Ambient dose equivalent rate
• Directional dose equivalent rate
• Individual dose, penetrating and superficial

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What are the acceptable uncertainties?

• RP is well behind most activities when it comes
  to working this out.
• Its often difficult to work out what the
  acceptable level is John Dunsters factor of 2?
• Its often even more difficult to work out what
  each technique or piece of kit will produce as an
  uncertainty

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How good is any existing kit?

• Change the kit change the answer. Are you
  prepared for this? What will the interested
  parties say when you tell them both answers are
  OK?
• Classic problem is wearing more than one
  dosemeter
• Approved dosimetry service passive
• Site operators passive
• Own and operators active dosemeters

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What radiations do you really have?

• We (usually) know the original source.
• What actually gets out?
• Sometimes easy, sometimes difficult.
• What can we do to make sure we understand?

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Detect first, measure later

• Sometimes its better to detect, remediate and
  then measure.
• Industrial X-radiography room
• Sodium iodide scintillator to find weaknesses
  relatively poor energy response, high sensitivity
• Remediate lead over panel gaps, adjust door
  fit, limit X-ray head movement
• Confirm residual dose rates are acceptable with
  compensated, end window GM

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Examples

• Cs-137 source in a welded steel can in free air.
• Easy, no betas, no X-rays, not much scatter
• But put it on a bench and watch the scatter jump
  up.
• Low angle Compton scatter from the low Z bench
  material
• A few in dose rate terms but maybe a factor of
  2 in fluence

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Defects

• For any photon beam, scatter through defects is
  by Compton interaction which reduces the mean and
  peak energies.

x100
Intensity
x1
Energy
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Angular range

• Unshielded and unscattered radiation defined
  direction
• Shielding and scatter confuse direction

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Pu handling facility

• A range of Pu isotopes
• Mostly alpha emitters but theres Pu-241.
• Often the major activity although a tiny fraction
  of the mass T0.5 14 y for Pu-241, T0.5 Pu-230
  24000 years
• 1 g Pu-241 has 1700 times the activity of 1 g
  Pu-239
• Pu-241 decays to Am-241 will dominate the gamma
  emissions

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Pu handling facility 2

• Main problem for operational RP is airborne
  activity
• Not easy because of the high dose per unit intake
  for Pu
• each Bq on a filter paper is significant
• - Radon can produce a high background
• For clearance and demolition its surface and
  bulk activity
• Painted in activity no alpha emissions
• Check for gross activity using the L X-rays

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Grime and dirt

• Alphas and soft betas are easily attenuated in
  grime.
• Effective monitoring solid angle falls as grime
  thickness increase

No grease bigger angle to just penetrate the
detector
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Whats out there?

• Range of suppliers has dropped considerably
  recently
• Lots of takeovers
• Thermo Nuclear Enterprises, Mini Instruments, R
  A Stephen, Eberline, Bicron, FAG, DCA, Harshaw,
  Siemens/Plessey, Vinten, Pitman
• Ranges are being rationalised
• But still a bewildering variety in some cases

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Go for more of the same?

• Advantages
• Familiar
• No need to change written instructions etc
• Disadvantages
• Still the same problems
• Looks unenterprising
• Some new functions are genuinely useful
  continuous battery monitoring, wider useful dose
  rate ranges, different detectors, no
  potentiometers to twiddle

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Something different who chooses?

• RPA
• Radiations, quantities, ranges, energies.
• Maintenance
• What will go wrong?
• How easy is it to fix?
• How good is the manufacturer at back-up?
• Bean counter
• Is the supplier solvent?
• User has right of veto!

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Dose rate monitor sensitivity

• Many measurements are at relatively low levels.
• Make sure the instrument will give a reasonably
  stable, reasonably quick, response at the lower
  dose rates.
• Try to avoid working routinely on the bottom
  decade or range.

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Information required

• Type test data real numbers from real
  instruments
• Based on IEC documents dont take a pass/fail
  approach. Some IEC criteria are tight, some are
  slack, some are irrelevant.
• Evaluate for your circumstances
• Dont buy anything without a type test IRRs
  demand it.

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Proper energy and polar response data
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Why the type test?

• Suppliers dont intentionally mislead
• But manufacturers can fail to understand the real
  world
• Different countries have different expectations
  whats industry standard in one country can be
  poor practice in another
• A basis for informed judgement

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Things to look out for

• Failure to danger at high dose rates especially
  on GM instruments with non-European detectors.
• Energy responses with only 3 points, 60, 662 and
  1250 keV.
• Polar responses at high energy only.
• High efficiencies from small area contamination
  monitors

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Trials

• Metrological basic GPG tests before use
• Plus anything thats edgy or unusual in your life
  high temperatures, high RF fields, high gamma
  levels when monitoring for alpha etc
• Field put some miles on it
• Pick some intelligent users
• Ask them to try it everywhere and record the
  readings against the current type
• Show it a hard time, dont be kind

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Summarise and compare with expectations

• If its good, tell your friends
• If its not, give the supplier a chance to answer
  criticisms.
• If they fail to, tell your friends.
• BUT beware litigation
• Make sure your views are sound and you have
  evidence
• Make sure you own the tested unit
• Make sure you have a paper trail.

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Testing

• Understand the strengths and weaknesses
• For new instruments, confirm they behave as the
  type test would indicate
• For periodic testing, has its condition
  deteriorated?
• Function checking, a simple limited test to pick
  up major defects.
• Fit for purpose

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Follow-up

• For new types, look hard at year to year changes.
• Any sign of a widespread problem?
• Is it holding up to expectations?
• Do not be afraid to hassle the supplier or
  manufacturer but do your best to provide useful
  information.
• If you can think of an improvement, tell the
  supplier.

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Opportunities for RPA accreditation points.

• Compare the new with the old.
• Are the readings different?
• If so, why. Look at radiation energies, angular
  distributions, averaging areas, averaging volumes
• Publish!