Bragg–Gray, TG-51, kQ and kTP — the chain that turns a coulomb into absorbed dose.
For Calibrating physicists·8 min
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You will be able to
01State Bragg–Gray in one breath: fluence, stopping power, cavity.
02Apply kTP before you trust a reading.
03Know what kQ is answering: this beam is not cobalt.
Chapter 01
A hole that tells the truth
Bragg–Gray: if a small gas cavity does not perturb the electron fluence, dose to the medium is dose to the gas times the stopping-power ratio. That sentence is why an ion chamber in water can speak gray. Spencer–Attix refines which electrons count. The cavity must be small. The walls must be matched. Humidity, polarity, recombination sit on top.
A chamber reading is not a dose until the chain is applied.
Simulation
Bragg-Gray relation — Change the numbers; the scene follows.
Variables
D_med = 0.0113 Gy
Dw=Dg⋅sˉw,g
Keep thisA chamber reading is not a dose until the chain is applied.
Air density changes with temperature and pressure. kTP refers the mass of air in the cavity back to the calibration climate. Skip it on a high-pressure day and you will be systematically wrong, in the same direction, all afternoon.
Simulation
Temperature-pressure correction — Change the numbers; the scene follows.
Variables
k_TP = 1
kTP=(273.15+T0)(273.15+T)PP0
Keep thisRead T and P at the chamber, not at the door.
TG-51 and TRS-398 turn N-D-w, kQ, kTP, Pion and polarity into absorbed dose to water at zref under a 10 by 10. kQ answers: this beam quality is not cobalt-60. Measure %dd(10)x or TPR20,10 properly. Then the gray you report is traceable.
Simulation
Absorbed dose — TG-51 — Change the numbers; the scene follows.
Variables
D_w = 0.6054 Gy
Keep thisTraceability is a chain of documents, not a feeling.