Post on 20-Jan-2016
Health Physics
2: Radiation Measurements
Introduction
• Radiation not detected with our senses
• Need detectors to confirm presence of radiation
• Avoid over – exposures (reddening of skin - 3Gy)
Page(s): 107 Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Detection of Radiation
• Made possible by its interaction with matter(solid, liquid gas)
• Ionization (electrical charges), excitation
• Direct (charged particels) and indirect (photons, neutrons) ionization
Page(s): 108 Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Indirect Ionization by Photon
Incoming
Photon
EjectedElectro
n
Two Basic Types of Radiation Measurements in Health Physics:
• External radiation hazardmeasure exposure rate, dose or dose-rate
• Internal radiation hazardmeasure contamination in working area,
bioassay
Page(s): 107 to 108Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Penetration Power of Radiation
External Radiation Hazard (1)
• Discriminate between particles and gamma radiation using probe - shield
• Measure exposure rate (X/t) or dose rate(mR per hour, mSv per hour)
• Measure dose (integrate dose rate, dosimeter)
Page(s): 108 Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
External Radiation Hazard (2)
continued …
• X-rays, gamma radiation, neutrons
• Energetic beta particles (P-32: 1.7 MeV)
• Neutrons (from accelerators, cyclotrons), fast and thermal neutrons
Page(s): 107 to 108Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Internal Radiation Hazard (1)
• Measure contamination in working area(surface, air, water) “wipe tests” (betas)
• Whole-body counter (gamma emitters)
• Bioassays (thyroid assay, urine analysis)
Page(s): 108Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Internal Radiation Hazard (2)
continued …
• Alpha or beta particles when inhaled or ingested(e.g. tritium vapors in power stations containing H-3 with 18keV betas)
• Boneseekers with long half-lives when inhaled or ingested
(Sr-90: 0.5MeV betas, Pu-239 : 5MeV alphas)
• Any radioactive material that enters the body in large amounts
Page(s): 108 Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Types of Radiation Monitoring
• Area and survey monitoring (portable or fixed detectors)
• Technique or procedure monitoring (DRDs or EPDs)
• Personal Monitoring (TLD “badges”)Page(s): 108
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
“Ideal” Radiation Detector
• Responds to one radiation type only
• Includes radiation quality factor, wR
• Uniform energy response
• Gives equivalent dose (H) or equivalent dose rate
Page(s): 108 Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
“Real” Radiation Detector
• Need to discriminate between particles and gamma radiation using probe - shield
• Non-uniform energy response
• Often gives exposure rate (X / t) only(Milli-Roentgen per hour)
Page(s): 108 Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Energy Dependence of Gamma Survey Meter
Page(s): 153 to 154
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
f-Factor (rads/Roentgen)
Page(s):
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Radiation Instruments
GMs from 1962 to 1999
19621970
1985
1999
Instruments
Example: GM Model
GM Survey Meter
• Dial in mR/hr• Battery check
Electronic Personal Dosimeter(EPD)
Page(s): at end of handoutPage numbers refer to handout:”Chapter 8: Radiation Measurements”.
Electronic Personal Dosimeter(EPD)
Skin doseBody dose
Radiation Instruments
Car Gate
Radiation Instruments
Conveyor
Radiation Instruments
Truck Monitor
Radiation Instruments
Security Gates
Gas Detectors
• Ionization Chambers
• Proportional Counters
• Geiger-Mueller Counters (GMs)
Page(s): 111 to 125Page numbers refer to handout:”Chapter 8: Radiation
Measurements”.
Gas-Filled Detectors
Incident Ionizing
Radiation +++
Anode +
Voltage Source
ElectricalCurrent
Measuring Device
Cathode -
---
Ionization Chamber
Page(s): 113
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Ionization Chamber
• Characteristics
– rel. low sensitivity (ideal as control instrument in high field of nuclear reactors)
– measures exposure rates up to 1000 R / min
• Page(s): 112 to 117 Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Condenser Type Dosimeter
Page(s): 115
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Direct Reading Dosimeter (DRD)
Page(s): 115-116
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Natural leakage of 5-10 mR/day
Keep control DRD in desk!
Do not drop!
Gas Multiplication
Page(s): 117 to 118
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
secondary ions
Proportional Counters
Page(s): 118 to 119
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
windowless
Proportional Counter
• Characteristics– Energy information preserved
– Particles yield larger pulses than photons
– Differentiate particle exposure in presence
of photons
– Detects thermal neutrons via n-alpha reaction if tube lined with Boron or if BF3 is
used as filling gas• Page(s): 117 to 119 Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Geiger Plateau
Page(s): 120
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Geiger-Mueller Counter
Page(s): 119 to 124
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
GM Counter
• Characteristics– large dead time (~ 100μs), saturation
– has no energy info.
– high sensitivity (100% for each ionizing event)
– measures low exposure rates (~0.1 mR / hr)• Page(s): 112 to 117 Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Single ImagesGas Detectors: Summary
• Ionization chamber has relatively low sensitivity, good for high radiation fields, has energy info.
• Proportional counter as neutron detector with BF3 as filling gas (slow neutrons undergo n-alpha reaction). Has energy info.
• GM has large dead time (~100 micro-sec), saturation in high radiation field, very sensitive, no energy info.
• Page(s): 111 to 125Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Scintillation Detectors
• Phosphors (NaI(Tl), CsF, BGO, LSO)
• Photomultiplier Tube (PMT)dynodes, counting chain, spectra
• Liquid Scintillation Counting (“wipes”)
Page(s): 125 to 137Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Photon Interaction with NaI(Tl) Crystal
Page(s): 126 to 127
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
NaI(Tl) – PMT Assembly
Page(s): 127
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Scintillator Characteristics
• Phosphors (NaI(Tl), CsF, BGO, LSO)• Photoelectric interaction ~ Z4
• NaI(Tl): reference, decay const. ~ 1μs• CsF : faster than NaI(Tl), TOF PET• BGO : slower but more efficient, PET• LSO : very fast (~1ns), high res. PET
Page(s): 125 to 137Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Phosphor- PMT Assembly
Page(s): 127
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Photomultiplier Tube (PMT)
Page(s): 127 to 129
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Electron Multiplication in PMT
Page(s): 127 to 129
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Counting Chain (1)
Page(s): 129
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Discriminator Action
Page(s): 130
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Counting Chain (2)
Page(s): 131
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Counting Chain (3)
Page(s): 132
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Co-60 Energy Spectrum from NaI(Tl) Detector
Page(s): 136
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Energy Spectrum from NaI(Tl) Detector
Page(s): 136
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Energy Resolution (FWHM)
Page(s): 136
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Energy Transfer in Phosphor
Page(s): 125 to 127
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Photoelectric Effect
Page(s): 125 to 127
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Well Counter
Page(s):
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Liquid Scintillation Counter (1)
Page(s): 132
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Liquid Scintillation Counter (2)
Page(s): 132
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Liquid Scintillation Counter (3)
Page(s): 132
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Liquid Scintillation
• Scintillator in intimate contact with radiation source (mainly alphas and betas)
• Solvent (toluene) and solute(POPOP)• Efficiency for alphas and betas: 50 to 100%• Correct for quenching effects (chemical,
color)• Wave length shifter to match photocathode
response• Page(s): 132 to 133
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Scintillators for Alpha and Beta Particles
Page(s): 134 to 135
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Films and TLDs
• Film dosimeters (badges)body, skin, wrist monitoring
• Thermoluminescent Dosimeters (TLDs)LiF, Al2O3 in many shapes: finger ring TLDvery sensitive, linear responseneutron response possible (Li-6, Li-7)
Page(s): 138 to 147Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Film Dosimeter Calibration Curve
Page(s): 138 to 139
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Film Dosimeter: Energy Dependence
Page(s): 140 to 141
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Single ImagesFilm Dosimeters
• Gamma exposure from ~ 10mR to 1000R.• Need filters to correct energy dependence and
yield beta exposure.• Cd filter allows thermal neutron dose meas.• Wearing period of a few months.• Pros: cheap, permanent record, easy
processing.• Cons: darkening by humidity, heat, H-3 vapor.
• Page(s): 138 to 142Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
TLD: X-ray Sensitivity of LiF
Page(s): 143 to 147
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Single ImagesTLD Dosimeters
• Gamma and beta exposure from ~ 20mR to 106 R
• No energy dependence• Linear dose response over wide range• Tissue equivalence• Thermal neutron dose meas. possible
– (Li-6, Li-7) • Wearing period of up to one year.• Cons: no permanent record, no visual record
• Page(s): 143 to 147Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Special Detectors
• Semiconductor detectors (nuclear diodes)Si(Li), Ge(Li), Ge(hyperpure)
• Thermoluminescent neutron dosimeters Li-6 vs. Li-7
• Damage track neutron dosimeters• Bubble neutron dosimetersPage(s): 147 to end of handoutPage numbers refer to handout:”Chapter 8: Radiation Measurements”.
Semiconductor Detector
Page(s): 147 to 149
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Si or Ge
p-layer
n-layer
Detector Cooling
Page(s): 147 to 149
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Single ImagesSemiconductor Detector
• Acts like a solid state ionization chamber.• Si(Li) (delta E=1.2eV), room temp.• Ge(Li) (delta E= 0.7eV), cool to 77o K .• Hyperpure Ge, cool to 77o K, recyclable.
• Wion-pair ~ 3.5eV : high sensitivity and energy resolution (~1%).
• Tissue beta and gamma dose rate (EPD).
• Page(s): 147 to 148Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Neutron TLD with Li-6 and Li-7 (2)
• Li-6 (7.5%): responds to both gammas and to slow neutrons by n- alpha reaction enrich!
• Li-7 (92.5%): only responds to gammas
• Polyethylene slows down fast neutrons
• Cd captures slow neutrons
• Page(s): 149 to 150Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Neutron TLD with Li-6 and Li-7 (1)
Page(s): 149 to 150
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Damage Track Neutron Dosimeter
Page(s): 149 to 151
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Bubble Neutron Dosimeter• Elastic polymer with suspended
droplets of superheated liquid
• When struck by radiation,droplets form gas bubble
• Bubbles remain fixed in polymerfor permanent visual record
• Calibration in “bubbles per mrem”or “bubbles per Sv”
Page(s): end of handoutPage numbers refer to handout:”Chapter 8: Radiation Measurements”.
Use of Radiation Instruments
• Detection and geometric efficiency
• Time constant and dead time
• Directional response
• Operational checks (battery!), calibrationPage(s): 151 to 156Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Single ImagesProper Use of Rad. Instr.(1)
• Remember: counter efficiency depends on:– geometric efficiency (1/r2)– detection efficiency (particles vs. photons)– detector entry window– detector dead time (~100μs for GM, <1μs
for NaI)
• Respect ‘time constant’ of instrument (wait ~3 time constants for accurate reading). Use ‘fast / slow’ switch.
• Page(s): 151 to 15Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Energy Dependence of Gamma Survey Meter
Page(s): 153 to 154
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Rate Meter Response (Time Constant)
Page(s): 152 to 153
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Single ImagesProper Use of Rad. Instr. (2)
• Respect directional response!– Example: beta window on dosimeter
• Respect warm-up time:– transistors need none– PMTs need a few minutes
• Page(s): 151 to 155Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Single ImagesProper Use of Rad. Instr. (3)
• Routinely perform operational checks:
– battery verification (see mark on dial)
– ratemeter check (pulse generator)
– calibration (built-in check source, official agent)
Page(s): 151 to 155Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Master
Page(s): 107 to
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Master
Page(s): 107 to
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Master
Page(s): 107 to
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Master
Page(s): 107 to
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Shielding
BASIC KNOWLEDGE- DOSE -
The Dose From Being Exposed to Cosmic and Machine Produced Radiation Depends
on:
• Time
• Distance
• Shielding
Energy Response of Ionization Chamber
Page(s): 116 to 117
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
References
• Nuclear Regulatory Commission Home Page: www.nrc.gov
• teachers corner@www.nrc.gov/NRC/teachers.html
• students corner@www.nrc.gov/NRC/STUDENTS/students.html
• Nuclear Energy Institute Home Page: www.nei.org• science club@
www.nei.org/scienceclub/index.html
• Health Physics Society Home Page: www.hps.org• www.hps.org/publicinformation/radfactsheets/