Surveys, Calibrations, and Data Analysis Flashcards
Calibration Principles
What parameters are measured in a full characterization calibration?
- Energy dependence
- Dose rate dependence or dead time
- Linearity
- Interference by other radiation types
- Dependence on temperature, pressure, and humidity
- Effects of radiofrequency, magnetic, and electrostatic fields
- Angular response
Define
MARSSIM
- Multi-Agency Radiation Survey and Site Investigation Manual
- Standard procedures covering survey instrument choice, sampling design, survey grids, survey procedures, and quality assurance procedures.
- Accepted by NRC, EPA, DOE, and DOD
Calibration Principles
What is the NRC recommended instrument calibration procedures?
- Daily or other frequent checks of survey instruments should be supplemented every twelve months with a calibration of each instrument at two points separated by at least 50% of each linear scale that is used routinely or with a calibration at one point near the midpoint of each decade on logarithmic scales that are used routinely.
- Survey instruments should also be calibrated following repair.
- A survey instrument may be considered properly calibrated when the instrument readings are within ± 10% of the calculated or known values for each point checked.
- Readings within ± 20% are considered acceptable if a calibration chart or graph is attached to the instrument.
Alpha Contamination Monitoring
What two instruments are useful for alpha contamination monitoring?
- Portable proportional counter
- Portable scintillation counter
** Instruments must have an extremely thin window in order for the alpha particles to penetrate
Alpha Contamination Monitoring
What are two different models of portable alpha proportional counters (i.e., different kinds of gas)?
- Air at ambient pressure as the counting gas
- Propane gas P-10 gas (10% methane, 90% argon)
* Windows should be really thin with a density thickness of 1 mg cm-2 (0.00025” thick)
Alpha Contamination Monitoring
What is the typical manufacturer data for an alpha probe?
- Sensitive area of 50 - 100 cm2
- Total efficiency of 25% (cpm dpm-1)
Alpha Contamination Monitoring
Air proportional counter advantages and disadvantages compared to gas-flow proportional counter
Advantages
- Lighter in weight
- Easier to startup
Disadvantages
- Useless in medium/high humidity
- Lower eficiency than P-10
- Needs gain amplifiers to increase voltage, which can lead to counting of noise pulses caused by mechanical shock or motion of the connecting cable.
Alpha Contamination Monitoring
Gas flow alpha-beta proportional counter advantages/disadvantages compared to ambient air proportional counter
Advantages
- P-10 is cleaner
- High flow rate eliminates plugged needle valve problems
- Higher efficiency than ambient air proportional counters (efficiency is 50% of 2π alphas and 25% of total disintegrations)
Disadvantages
- You can run out of gas
Alpha Contamination Monitoring
What is a common alpha scintillation detector and how does it operate?
- Silver-activated zinc sulphide, ZnS(Ag)
- The phosphor is usually coated directly on the surface of a piece of clear acrylic plastic
- Due internal reflection the acrylic acts as a light pipe and confines light rays from scintillation in ZnS(Ag)
- A photomultiplier tube attached to the light pipe to quantify the scintillation events
Alpha Contamination Monitoring
Alpha scintillation counter advantages and disadvantages
Advantages
- Less sensitive to microphonics than proportional counters (larger signal produced by scintillator)
Disadvantages
- Less rugged due to fragility of PMT
- Efficiency is 30 - 35%, which is between an ambient air and P-10 proportional counter
Alpha Contamination Monitoring
MARSSIM suggests a scanning speed of _____ when scanning for alpha contamination.
3 - 5 cm per second
Alpha Contamination Monitoring
Define
2π calibration
- The survey meter is artificially set to read one-half the disintegration rate
- The meter reading will represent 100% efficiency for the alphas hitting the detector probe
Alpha Contamination Monitoring
Define
4π calibration
- The meter is artificially adjusted so as to read the disintegration rate of the point source
- The instrument would then respond as if it detected every alpha particle emitted by the source
Alpha Contamination Monitoring
What is the MARRSIM requirement when performing an alpha survey?
- The release limits are specified in dpm / 100 cm2 for surface contamination.
- The alpha meter is often adjusted to a 4π calibration with 100% efficiency.
Alpha Contamination Monitoring
Diagram
2π vs. 4π calibration
Gamma Field Monitoring
What is the standard MARSSIM survey of gamma fields?
- Usually used to estimate contaminated soil concentrations of gamma emitters in open land areas.
- The detector is typically a NaI(Tl) scintillation probe.
Gamma Field Monitoring
What two instruments are useful for gamma ray monitoring presenting an external hazard?
- Geiger-Mueller counter
- Ionization chamber
Gamma Field Monitoring
How do you minimize non-uniformity in the field throughout the a chamber volume?
- Always calibrate the instrument with the long axis of the chamber pointing perpindicular to the source.
- The calibration point should be within 3x the longest dimension of the detector volume.
Gamma Field Monitoring
How do you measure radiation levels near background?
- Microrem meter
- Sensitive range of a micrometer is 0 - 25 urem.
- Detector is usually a scintillation crystal, NaI(Tl), by using a solid-state device instead of a gas-filled counter greatly increases sensitiity.
Beta Radiation Monitoring
What are the two problems for beta fields to a technologist?
- Loose beta contamination can become airborne and/or transferred to objects, followed by ingestion/inhalation.
- Beta fields present an external hazard due to the possibile skin and lens of the eye exposure.
Beta Radiation Monitoring
What two types of detectors are satisfactory for beta contamination surveys?
- Pancake GM
- Beta proportional counter
* Both have an efficiency of approximately 20%
Beta Radiation Monitoring
What is the proper instrument to use for a beta field measurement?
- Ion chamber
- An ion chamber can be used because the electrical signal produced is directly proportional to the energy deposited in the chamber gas.
Mixed Beta-Gamma Monitoring
How do you measure the concentration of a mixed beta-gamma contaminant (dpm / 100 cm2)?
- Pancake GM probe or a beta proportional counter
- The probe would have to be calibrated uniquely for the beta-gamma emitters found in the contamination
- Once efficiency was determined (cpm dpm-1), the scan MDC and scan speed are calculated
Mixed Beta-Gamma Monitoring
How do you measure the beta radiation field of a mixed beta-gamma contaminant (R/hr)?
- The ionization chamber is the only common instrument which reads energy deposited, and is the instrument of choice.
- The gamma reading must be significantly smaller than the beta + gamma reading or else large uncertainties are introduced.
- Take two readings, one with window closed and the other with the window open. When the beta shield is in place (window closed), it shields the chamber volume from any beta component.
- When the beta shield is removed (window open) then ionization is produced in the chamber by both photons and beta particles.
- The difference of the two measurement, produces a beta reading.
- Remember the ion chamber is calibrated for uniform gamma exposure rate, so a beta correction factor must be applied to the beta component to get a beta dose rate.
Neutron Radiation Monitoring
What is the biggest problem in achieving a reasonable neutron calibration?
- Taking proper account of the scattered field.
- Easiest way to check for this problem is to measure the deviation of the dose equivalent rate from the predictions of the inverse square law.
- If scattered neutrons are present, the dose rate will fall off more slowly than calculated.
Neutron Radiation Monitoring
How do you characterize a neutron survey instrument?
The response to thermal, intermediate, and fast neutrons is measured.
Neutron Radiation Monitoring
How do you measure relativisitic neutrons?
- The stable 12C nucleus in a plastic scintillator is activated through a nuclear reaction.
- This leads to a radioactive daugther (11C), which produces light flashes in the plastic scintilator.
- The energy response of the system is flat from 20 MeV to many GeV.
- The instrument is easily sensitive enough to use for radiation protection measurements in uncontrolled areas.
Neutron Radiation Monitoring
How do you measure for fast neutrons (broadly meaning a few eV to 10 MeV)?
- Moderated thermal neutron detector.
- The common moderators inlcude wax and polyethylene in the form of cylinders or spheres.
- The fast/slow neutron survey meter makes use of a boron trifluoroide (BF3) proportional counter inside a cylindrical moderator with an outside thermal neutron shield.
- It is unusually energy dependent and so provides more of an indication of the presences of fast neutrons than actual flux values.
Neutron Radiation Monitoring
How do you measure thermal and slow neutrons?
- Boron trifluoride (BF3) proportional counter.
- This detector exhibits rejection of the associated gamma ray fields up to exposure rates of several 10s of mSv hr-1 (a few R hr-1).
- It can be calibrated to read directly in slow neutron flux.
- The flux can be convereted into a dose equivalent rate in mSv / hr through the use of the conversion factor of 27,2000 n cm-2 second per mSv hr-1.
Neutron Radiation Monitoring
How do you measure thermal and slow neutrons (in a very high radiation field)?
- The proportional detector might experience significant losses due to the deadtime, so foil activation techqniues are used.
- Gold or indium metal foils can be placed in the field and the induced radioactivity is then counted to determine the neutron flux.
Surface Contamination Monitoring
When should a sealed source be taken out of surface?
When a sealed source is found to be leaking over 5 nCi of removable contamination, it must be immediately taken out of service, and then repaired or or disposed of as rad waste.
Surface Contamination Monitoring
Wipe samples must be _____ to avoid cross-contamination.
separately packaged
Surface Contamination Monitoring
How do you take a wipe sample?
- A cloth, paper, plastic foam, or fiberglass disk is wiped overa surface area of 100 cm2 (4” x 4” square) with a gloved hand.
- One exception is DOT provision of 300 cm2 for package smear samples taken prior to transport.
Detector Overview
Alpha Contamination with ZnS(Ag) Alpha Scintillator
What are detectable radiation levels and operational comments?
Radiation Levels
- All
Operational Comments
- Check for window light leaks before use.
- Check background rate often as probe gets contaminated easily.
- Be alert to gamma field interference.
Detector Overview
Alpha Contamination with Alpha Proportional (Air or Gas) Detector
What are detectable radiation levels and operational comments?
Radiation Levels
- All
Operational Comments
- Check often for probe contamination.
- If using gas, keep an eye on the gas gauge.
- If using air, avoid surveys in humid areas.
Detector Overview
Beta Contamination with Pancake GM
What are detectable radiation levels and operational comments?
Radiation Levels
- All
Operational Comments
- Calibrate with expected beta energy as probe quite energy dependent.
Detector Overview
Beta Contamination with Gas Proportional Counter
What are detectable radiation levels and operational comments?
Radiation Levels
- All
Operational Comments
- Calibrate with expected beta energy as probe quite energy dependent.
- Calibration changes with air pressure.
Detector Overview
Beta Contamination with Plastic Scintillator
What are detectable radiation levels and operational comments?
Radiation Levels
- All
Operational Checks
- Check for window light leaks before use.
- Calibrate with expected beta energy as probe quite energy dependent.
Detector Overview
Gamma Field with Side Wall Energy Compensated Geiger Counter
What are detectable radiation levels and operational comments?
Radiation Levels
- Low to Medium
Operational Comments
- Won’t work for gammas below about 50 keV, i.e., Co-57, I-125.
- Underestimates gammas above 1.5 MeV, i.e. N-16
Detector Overview
Gamma Field with Solid Scintillator (NaI or Tissue Equivalent Plastic)
What are detectable radiation levels and operational comments?
Radiation Levels
- Low to Medium
Operational Comments
- NaI micro R meter very energy sensitive.
Detector Overview
Gamma Field with Pressurized Ion Chamber
What are detectable radiation levels and operational comments?
Radiation Levels
- Low to Medium
Operational Comments
- Verify chamber is still pressurized before use.
Detector Overview
Gamma Field with Ion Chamber
What are detectable radiation levels and operational comments?
Radiation Levels
- High
Operational Comments
- Allow warm-up time.
- Zero meter in low background area.
- Calibration depends on ambient air pressure.
Detector Overview
Gamma Field with High Range Telescoping GM
What are detectable radiation levels and operational comments?
Radiation Levels
- Extreme
Operational Comments
- Instruments such as Teletector and Ludlum Stretch Scope
Detector Overview
Tritium Concentration with Windowless Gas Flow Proportional Counter
What are detectable radiation levels and operational comments?
Radiation Levels
- All
Operational Comments
- Steep learning curve
- Sensitive to barometric pressure, humidity, dust, gas flow rate and static electricity.
- H-3 calibration source fragile.
Detector Overview
Neutron Field with 9-inch rem Ball
What are allowable radiation levels and operational comments?
Radiation Levels
- All
Operational Comments
- Reasonable accuracy for energies between thermal and 10 MeV.
- Overresponds to intermediate neutrons. High gamma field rejection.
Define
What is the MARLAP?
Multi-Agency Radiological Laboratory Analytical Protocols
Provides complementary guidance to the MARSSIM.
- Part I ⇒ Addresses the needs of project planners regarding identifying the required analytical data, evaluating the radiation lab, and validating the data quality.
- Part II ⇒ Devoted to operational aspects of an radioanalytical lab itself. Discusses preseving samples, preparation & separation procedures, lab instrumentation, and quality control issues.
Define
Minimum Detectable Concentration
- The smallest concentration of radioactive material in a sample that will yield a new count (above system background) that will be detected with 95% probability with only 5% probability of falsely concluding that a blank observation represents a “real” signal.
- The MDC amount of radioactivity on a sample will yield a net count rate in the system that just barely exceeds 2 standard deviations for the net count rate.
What is the application of statistical models in radiation protection?
- To check a counting circuit to determine if fluctuations are purely statistical.
- Estimate the precision of a single measurement.
Describe
Chi-squared test
Tests whether the sample variance (σexp2, s2) is significantly less or more than the theoretical estimate of the population variance (σp2).
Population assumptions
- Repetitive counts of Ci
- Same counting interval
- Constant source
Theoretical estimate of σp2 is estimated from sample mean count with the assumption
- Poisson distribution estimated by a normal distribution when counts > 30
What are limits for removeable α and β/γ contamination surveys?
α ⇒ 20 dpm / 100 cm2
β/γ ⇒ 200 dpm / 100 cm2
What are limits for non-removeable α and β/γ contamination surveys?
α ⇒ N/A
β/γ ⇒ < 100 cpm above background AND < 0.5 mR hr-1
What is Chauvenet’s criteria used for?
Evaluates for rejection of data.
What is a “t” test used for?
Indicates the probability that a difference between two counts is not due to chance.
What is a chi-square test used for?
Evaluates counter behavior.
What is a Poisson distribution?
A special case of a binomial distribution applicable when the measured event has a low probability of occurring.
What is a Gaussian distribution?
Represents a distribution of counting data which is symmetrical around the mean count rate.
