Fundamentals of Radionuclide Metrology

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1 Fundamentals of Radionuclide Metrology Brian E. Zimmerman, PhD Physical Measurement Laboratory National Institute of Standards and Technology Gaithersburg, MD USA SIM Metrology Workshop Buenos Aires, Argentina 10 November 2011

2 Review Types of radioactivity (I) Alpha decay Emission of an He nucleus (2p+2n) Pu U He2 Beta decay Emission of electron (β - ) or positron/anti-electron (β + ) + neutrino or antineutrino Lu106 72Hf105 + β +ν converts neutron into proton+electron Ga 37 30Zn β +ν converts proton into neutron+antielectron Electron Capture (type of beta decay) Capture of orbital electron into nucleus, competes with β + decay Mn30 55 Fe e +ν converts proton into neutron+antielectron

3 Review Types of radioactivity (II) α and β often result in daughter nucleus being in excited state relaxation to ground state must then take place γ/conversion electron-emission De-excitation through emission of photons (γ-rays) or conversion electrons (competitive processes) Analogous to x-ray and Auger electron emission, except that γ-rays and CE are from nuclear transitions; x-rays and Auger electrons are from atomic transitions. All four types of de-excitation mechanisms are usually observed in nuclear decay. Cascading transitions, parent-daughter decay can make measurement very complex!

4 Measurement of radioactivity -dn/dt = Nλ [λ = ln(2)/t 1/2 ] Typical measurement model R(t) = C/T = R B + A 0 (m/m)ε Γ G(t) f i f j + A x ε x.. A counting process (of emitted radiation) Detection efficiency and correction factors If we could count every event, our job would be easy. Most effort goes into figuring out what we are missing!

5 Radioactivity Measurement vs. Dosimetry Radioactivity Dose Want to know How many How much? How defined Number of spontaneous disintegrations per unit time Amount of energy absorbed per unit mass SI unit Bq (s -1 ) Gy (J kg -1 ) Both rely on measurement of indirect physical quantity (current, voltage, etc.) for quantification.

6 Different radiations, different techniques Choice of technique depends on level scheme of radionuclide being measured!! To give higher degree of confidence, multiple techniques should be used when possible.

7 Primary Standardization Method is self-contained (i.e., measurements of tracer and traced nuclide made simultaneously) and does not rely on external standards for efficiency determination Any corrections made must be small and able to be made with high accuracy Level scheme data may limit degree of primary-ness Not primary methods: HPGe and Si(Li) photon, e -, α, or β spectrometry, ionization chambers This talk will deal with primary methods only!

8 Typical Preparation Scheme for Primary Activity Determinations Stock solution Dilution by factors Point sources for HPGe γ-ray spectrometry (impurities) Counting in 4π γ ionization chambers, dose calibrators Liquid scintillation counting (LSC) with 3 H-standard efficiency tracing or TDCR Point sources for 4π NaI(Tl) γ-ray spectrometry

9 (Anti)Coincidence techniques Two detectors usually required, one for each type of radiation detected (e.g., β-γ, α-γ, β-x, e - -x) α, β, e - channel usually LS or proportional counter photon channel usually Na(I) or HPGe Coincidence counting N 0 N β N γ / (N c ) /(kbq) N N N β γ 0 = fi N βγ N β, N γ, N βγ are β,γ and coincidence counting rates, respectively. f i are various correction factors (1 - N c / N γ ) / N c / N γ Efficiency extrapolation β inefficiency, changed by varying threshold

10 Fixed-geometry or 4π techniques Defined solid angle counting Mostly for α counting, but also for low-e photons with right detector Assumes that only events emitted in f = Ω/4π are detected with nearly 100% efficiency. N 0 = N α /f Small corrections still needed (scattering, primarily) 2π proportional counting α and β, low-e photons Backscattering corrections important 4π counting Proportional counters (α, β, low-e photons) NaI(Tl) well detectors (photons) Efficiencies can approach 100 % Corrections for escape, scattering needed Internal gas counting (4π) Usually low uncertainty (0.1 % %) Sample preparation sometimes difficult

11 Liquid Scintillation (LS) techniques (I) High detection efficiency for α, β : sample is contained in detector CIEMAT/NIST efficiency tracing method Uses 3 H standard and calculational model to determine detection efficiency of a radionuclide of interest Originally developed for pure β emitters Can be applied using commercial LS counters With a LOT of work, can be applied to EC nuclides ( E) W( E) Emax EQ ε = 1- exp - P 0 M 0 2 Emax ( Z,E) de P( Z,E) de 1

Liquid Scintillation (LS) techniques (II) Triple-to-Double Coincidence Ratio (TDCR) Method Uses a specially-designed three photomultiplier tube (PMT) instrument Coincidences refer to the photons

12 Liquid Scintillation (LS) techniques (II) Triple-to-Double Coincidence Ratio (TDCR) Method Uses a specially-designed three photomultiplier tube (PMT) instrument Coincidences refer to the photons emitted from the scintillator NOT the radionuclide! φt φ D Emax E EQ( E)/3λ 3 = S( E)(1 e ) de 0 0 max S( E) EQ( E)/3λ 2 EQ( E)/3λ 3 [ 3(1 e ) 2(1 e ) ] de 1 Assumes equal PMT efficiencies ε D Theoretical ε D vs TDCR, 54 Mn TDCR

13 Metrology with LSC to assure measurement reliability Use different spectrometers differing characteristics: log vs. linear amplification; detection thresholds; dead times; etc. Use a variety of LS cocktail compositions to obviate (or account) chemical composition effects Different scintillation fluids Vary carrier, water concentrations Use a wide quenching / efficiency range so that extrapolated result is efficiency independent Use different techniques for determining detection efficiency Use both CIEMAT/NIST and TDCR whenever possible

14 Uncertainty analysis Usually most time-consuming part of data analysis Requires rigorous understanding of measurement and data treatment process (measurement model) In many cases, error propagation formula cannot be written analytically, therefore other approaches must be taken.

15 CIEMAT/NIST method -- measurement & uncertainty model 3 H standard traced nuclide M odel calculations

16 Evaluating uncertainties Analytical form N 2 f u c ( y) = u i= 1 xi 2 2 ( x i ) Y = f(x 1, X 2,, X N ) measurand Y, input quantities X i Sensitivity analysis f are sensitivity coefficients x i Estimate (or calculate) u 2 (x i ) Sensitivity coefficient can be evaluated by noting effect of x i ± u(x i ) on y Monte Carlo In some techniques, it is impossible or impractical to write the model in a closed, analytical form (complexity of input data, black box instrumentation, etc.) Approach is to assemble large (~100) number of input data sets based on uncertainties of the input values Assumes u(x i ) is normally distributed about x i

17 Summary Radioactivity measurement is a counting process Many methods available for measuring radioactivity Choice of method depends on decay characteristics of radionuclide being measured Use more than one method when possible to increase confidence in result Uncertainty assessment is vital part of measurement process Requires in-depth knowledge of measurement method Many evaluation methods available Choice is based on available information, complexity of measurement model

BUREAU INTERNATIONAL DES POIDS ET MESURES

1 BUREAU INTERNATIONAL DES POIDS ET MESURES International comparison of activity measurements of a solution of 3 H (January 2009) Participating laboratory: T ½ = (4 496.862 d; u = 9.131 d)* Ampoule number