health physics 2: radiation measurements. introduction radiation not detected with our senses need...

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


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


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/

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