PRIMARY STANDARDS of AIR KERMA for 60 CO and X-RAYS & ABSORBED DOSE in PHOTON and ELECTRON BEAMS. Malcolm McEwen
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1 PRIMARY STANDARDS of AIR KERMA for 60 CO and X-RAYS & ABSORBED DOSE in PHOTON and ELECTRON BEAMS Malcolm McEwen
2 Definitions: Standard instrument/measurement/artifact intended to define, realize, conserve or reproduce a unit or quantity to serve as a reference Primary standard standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of same quantity National standard d standard d recognized by a national decision to serve as the basis for assigning values to other standards of the quantity concerned Secondary standard standard whose value is assigned by comparison with a primary standard of the same quantity
3 So that s clear? I shall not attempt today to further define e [it].but I know it when I see it. Justice Potter Stewart, 1964
4 ENERGY RANGES & QUANTITIES kev low energy x-rays kev medium energy x-rays Cs-137 & Co-60 Co-60 Linac photon (x-ray) beams Linac electron beams Air Kerma Air Kerma Air Kerma Absorbed Dose Absorbed Dose Absorbed Dose
5 1. kv x-rays air kerma The primary standard for kv x-rays is the Free Air Chamber
6 kv x-rays air kerma The primary standard for kv x-rays is the Free Air Chamber K air = X W e 1 1- g Q m air air W e 1 1- g W/e = ev m air?
7 kv x-rays air kerma V air = AL air The air volume is defined mechanically
8 Corrections, corrections.. K air = Q air air V W e air g K att K sc K e K hum P pol P ion K att - attenuation of the primary X-ray beam between the aperture and collecting volume K sc - the extra ionization collected from electrons produced by photons scattered within the chamber K e - ionization lost when electrons strike the collecting electrode.
9 An aside on Humidity We are all familiar with correcting readings for temperature and pressure (forget them here at your peril) K hum Humidity, however, is generally ignored, but for kv primary standards it is an important correction kv Co Relative Humidity (%)
10 Uncertainties Medium Energy Free-Air Chamber Standard Uncertainty (MEES) Type A Type B Ionization Current Volume Positioning Correction Factors (excl. kh) Humidity kh Physical Constants Air-Kerma Rate Combined Uncertainty 0.26
11 Alternative ti designs of free-air chamber 1. Attix design Designed by Attix in 1961 Extensible length of chamber No guarding-electrode system No voltage divider Eliminates stringent power supply requirements Errors resulting from fieldnonuniformity are eliminated
12 Attix chamber operating principle i Front and back move
13 Alternative ti designs of free-air chamber 2. PTB design PTB Physikalisch-Technische Bundesanstalt First developed in the 1920s Coaxial cylindrical design Smaller for the same photon energy than standard design Potentially transportable
14 Size does matter. Secondary electrons should not reach electrodes 50 kv» 70 mm 250 kv» 350 mm Co-60???
15 Size does matter. Secondary electrons should not reach electrodes 50 kv» 70 mm 250 kv» 350 mm Co-60???
16 2. Cs-137 & Co-60 air kerma The primary standard for Cs-137 & Co-60 gamma rays is the Cavity Chamber
17 Cavity theory before 9 am is a cruel and unusual punishment O i th h l l Overview - measure the charge, apply a large number of corrections and factors from tables Detail - consult your textbook
18 What is required? K air = Q W 1 L air en air V e air 1- g g air, air, g K att K sc K an K stem K hum P pol P ion A chamber with a very well defined volume W/e & L g/air Monte Carlo A lot of time
19 3. Co-60 absorbed dose An extension of the cavity chamber to determine absorbed dose is possible Beam Q/m air D air (W/e) air P repl (L/ρ) g,air D g / / / en w en w, g w, g g D w
20 Uncertainties
21 Extrapolation Chambers The biggest problem with cavity chambers is the determination of the effective volume of the chamber Mechanical measurements are the only accurate method but it s difficult to be sure once the chamber is sealed up Extrapolation chambers offer an alternative: D med = Q x W e S med,air 1 A air A volume determination is replaced by an area determination plus a differential length measurement
22 Extrapolation Chambers
23 But D med = Q W x e S med,air 1 A air The A x determination turns out to be not that much easier than a measurement of volume The value of S/ρ isn t very well known except for graphite Long term stability is not likely to be as good as for a fixedvolume chamber (and standards labs love stability). Extrapolation chambers are used, but not tfor external lbeams
24 4. Linac beams absorbed dose There is a very obvious change when we move to linac beams. The standards no longer look all the same Cavity chambers may have different geometries but they re basically identical The primary standard for absorbed dose in linac photon beams is the Calorimeter
25 Aside cavity theory Technically you can use cavity theory to derive absorbed dose for high-energy h x-ray beams. You need a very thick-walled chamber or a very large build-up up cap. Tungsten-walled chambers have been built Build-up caps begin to look like phantoms However, you still don t have the unique situation of Co-60 of accurately knowing the values of W/e and L g/air. An MV cavity standard can therefore only be viewed as a secondary device.
26 Absorbed dose calorimetry Simple to define, a lifetime (well almost) to realize D m = c m T 1. Measure a radiation-induced temperature rise. 2. Apply the specific heat capacity for the material in question. What s the problem? It s not cavity theory!
27 Why is measuring absolute dose is difficult? i) Doses of interest are small ii) The quantity required is the dose in an undisturbed phantom. iii) The quantity required is the dose at a point in this phantom. iv) Scattered radiation contributes a significant proportion of the absorbed dose v) Optimization of the measurement is difficult vi) Dose is material dependent
28 Absorbed dose calorimeter the basic components R th+wire R 2 R 3 V out bridge circuit R 4 thermistor V m bridge output signal calorimeter phantom isolation Vout(V) Time (s)
29 D = c ΔT ΔT will depend on the material but for radiotherapy dosimetry it s always small: Dose = 2 Gy ΔT (water) = 0.5 mk ΔT (graphite) = 2.9 mk Our target uncertainty for ΔT is 0.1%, which means sub-μk precision. This is why you don t often find calorimeters in the clinic.
30 D = c ΔT We re measuring a temperature rise due to the energy absorbed from the radiation beam. We therefore need a very stable background against which h we can measure this temperature t rise.
31 D = c ΔT Two options Passive temperature control (thermal isolation) Active temperature control (feedback system)
32 D = c T What is used for the value of the specific heat capacity depends on the calorimeter design. 3 main approaches: 1. Apply a value from tables certain materials (e.g. water) have a well known value of c 2. Measure c for a sample of the material used in the calorimeter 3. Evaluate an effective value of c for the complete calorimeter in situ
33 Corrections, corrections.. Things are never that simple: D m = c m T Πk i Correction factors are very dependent on the specific calorimeter design (and there e are lots) but may include: Perturbation corrections Perturbation corrections Conversion from one material to another Beam uniformity correction (dose averaging) Radiochemistry
34 The Calorimeter Gallery
35 A. Domen-type graphite calorimeter The calorimetric equivalent of the Farmer chamber
36 Uncertainties This is dose to graphite
37 Dose conversion Utilises the photon-fluence scaling theorem Requires knowledge of virtual source position and corrections for: Scatter Attenuation Pair production Of, course, you can use Monte Carlo, if you must.
38 Water/graphite scaling Overall uncertainty in D w ~ 0.5%
39 B. Other graphite calorimeters Calorimetrists can be quite creative The remarkably simple UK, circa 1989
40 The impressively The impressively complex France, 2007
41 The carefully optimized BIPM, 2009
42 C. Water calorimeter development
43 Water calorimetry the big problems 1. Convection 2. Radiochemistry i 3. Containment Water calorimetry the solutions 1. Operate at 4 C 2. High purity water, known composition of dissolved gases 3. Careful design coupled with detailed thermal modelling
44 D. The NRC water calorimeter Combines designs of Domen and Seuntjens
45 D. The NRC water calorimeter Combines designs of Domen and Seuntjens
46 D. The NRC water calorimeter Combines designs of Domen and Seuntjens
47 Uncertainties Source and type of uncertainty (in %) 60 Co 10 MV Type A Reproducibility dt/mu Monitor reproducibility Type B c w,p (specific heat capacity) < < Thermistor sensitivity k c (heat loss) k p (vessel perturbation) k HD (heat defect) k dd (profile non-uniformity) Positioning calorimeter, probes and vessel P dd P ion P pol P TP Humidity Positioning chamber Overall N D,w Not a single MC-derived factor
48 Size matters (II) The NRC water calorimeter is big and heavy: 85 cm cube, 50 kg Moving it within the lab is bad enough Can we make something smaller?
49 Size matters (II)
50 What about electron beams? Standards for electron beam dosimetry have lagged behind photon standards: 1974 NIST graphite calorimeter 1990s NPL therapy-level l graphite calorimeter 1988 first calibration service for linac photon beams 1998 first service for electron beams Very few institutions presently involved
51 Problem #1 Electron range
52 Problem #2 Net charge
53 Electron beam calorimetry As for photon beams both graphite and water calorimeters are in use at primary laboratories Water calorimetry is particularly tricky due to perturbation ti issues of containment t vessel coupled with electron range But it has been done, and at a university!
54 McGill University Water Calorimeter Stewart, Seuntjens, et al Pancake-type vessel and vertical beam geometry allow measurements down to 6 MeV
55 5. Non-calorimetric standards for MV beams What is often forgotten in the discussion of standards is that the radiation facilities available can be crucial to the development and use of the detectors.
56 METAS (the Swiss Primary Laboratory) Scanditronix MM22 Microtron
57 METAS (the Swiss Primary Laboratory)
58 A primary standard based on the Fricke dosimeter
59 A primary standard based on the Fricke dosimeter
60
61
62
63 G-value determination G(Fe 3+ ), normalis sed data 2007 data E s (MeV)
64
65 Comparison of standards
66 kv x-rays air kerma Air kerma, relativee to BIPM tralia ands oland UK Italy ngary nada ussia many China ustria USA ance 0.990
67 2. k Q measurements in photon beams Seuntjens et al Elekta TG-51 values Quadratic fit k Q %dd10 x
68 3. k Q measurements in electron beams 1.10 TG-51 k' R50 Relative e energy dep pendence TRS-398 k Q,Qint NPL graphite calorimeter METAS Fricke McGill water calorimeter NRC water calorimeter R 50 (cm)
69 Acknowledgements e Gerhard Stucki METAS Kristin Stewart, Jan Seuntjens McGill University David Burns BIPM Carl Ross, John McCaffrey, Hong Shen NRC Simon Duane, Alan DuSautoy, Hugo Palmans NPL Josian Daures LNHB
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