Health Physics 2: Radiation Measurements. Introduction Radiation not detected with our senses Need detectors to confirm presence of radiation Avoid over.

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


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)


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


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


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


“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)


Energy Dependence of Gamma Survey Meter

Page(s): 153 to 154


f-Factor (rads/Roentgen)

Page(s):


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


Car Gate


Conveyor


Truck Monitor


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


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


Direct Reading Dosimeter (DRD)

Page(s): 115-116


Natural leakage of 5-10 mR/day

Keep control DRD in desk!

Do not drop!

Gas Multiplication

Page(s): 117 to 118


secondary ions

Proportional Counters

Page(s): 118 to 119


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


Geiger-Mueller Counter

Page(s): 119 to 124


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”)


Photon Interaction with NaI(Tl) Crystal

Page(s): 126 to 127


NaI(Tl) – PMT Assembly

Page(s): 127


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


Phosphor- PMT Assembly

Page(s): 127


Photomultiplier Tube (PMT)

Page(s): 127 to 129


Electron Multiplication in PMT

Page(s): 127 to 129


Counting Chain (1)

Page(s): 129


Discriminator Action

Page(s): 130


Counting Chain (2)

Page(s): 131


Counting Chain (3)

Page(s): 132


Co-60 Energy Spectrum from NaI(Tl) Detector

Page(s): 136


Energy Spectrum from NaI(Tl) Detector

Page(s): 136


Energy Resolution (FWHM)

Page(s): 136


Energy Transfer in Phosphor

Page(s): 125 to 127


Photoelectric Effect

Page(s): 125 to 127


Well Counter

Page(s):


Liquid Scintillation Counter (1)

Page(s): 132



Page(s): 132



Page(s): 132


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


Scintillators for Alpha and Beta Particles

Page(s): 134 to 135


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)


Film Dosimeter Calibration Curve

Page(s): 138 to 139


Film Dosimeter: Energy Dependence

Page(s): 140 to 141


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.


TLD: X-ray Sensitivity of LiF

Page(s): 143 to 147


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


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


Si or Ge

p-layer

n-layer

Detector Cooling

Page(s): 147 to 149


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).


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


Neutron TLD with Li-6 and Li-7 (1)

Page(s): 149 to 150


Damage Track Neutron Dosimeter

Page(s): 149 to 151


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.


Energy Dependence of Gamma Survey Meter

Page(s): 153 to 154


Rate Meter Response (Time Constant)

Page(s): 152 to 153


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


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)


Master

Page(s): 107 to


Master

Page(s): 107 to


Master

Page(s): 107 to


Master

Page(s): 107 to


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


References

• Nuclear Regulatory Commission Home Page: www.nrc.gov

• teachers [email protected]/NRC/teachers.html

• students [email protected]/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/

Health Physics 2: Radiation Measurements. Introduction Radiation not detected with our senses Need detectors to confirm presence of radiation Avoid over.

Documents

gamma radiation

radiation instrumentsgms

real radiation detectorneed

end of handoutpage numbers

dose ratemr

hourmeasure dose

equivalent dose ratepages

mev betas