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Forum: Medical Applications
Re: Question Bragg curve profiles of protons in water (aimsphere)
Date: 06 Jan, 2013
From: Michael H. Kelsey <Michael H. Kelsey>

On Sun, 06 Jan 2013 17:23:01 GMT, aimsphere wrote:
> I simulated a water cuboid (50 m X 25 m X 25 cm) of several layers
> irradiated with a pencil beam of mono energetic protons and tracked only
> the primary particle to estimate its depth dose profile. The aim was to
> calculate Range Vs Primary particle energy (100 MeV to 10 GeV).

What physics list did you use? Was it one of the reference lists (e.g., QGSP_BERT), or your own? If you created your own physics list, can you tell us what processes and models you registered for the protons?

> What I found is that for lower energies e.g. 100 MeV to around 800 MeV,
> the Bragg peak is clearly visible but for higher energies it becomes
> lesser and lesser prominent. And also with increasing energies the
> largest dose deposition occurs at outer layers and reduces continuously
> with depth. I have difficulty in understanding this difference in depth
> dose distribution characteristics.
> I think there is some physical phenomenon that occurs for higher
> energies.

Inleastic proton-nucleus interactions. Basically, the proton hits the nucleus and induces production of secondary hadrons (pions, nucleons knocked out, even production of strange particles at sufficiently high energies).

In an inelastic collision, the original "primary" particle (in your case, the proton) is killed. The secondaries coming out may include protons, but there's no identification of such "new" protons with the original particle. As you can imagine, if those secondaries are themselves energetic, they will in turn have inelastic collisions with other nuclei in the material. This process results in a _hadronic_shower_. The primary particle's energy is subdivided among all of those secondaries, and their ultimate electromagnetic energy-loss interactions with the material.

Thus, your method of deriving the Bragg peak, by accumulating the energy lost by the primary as a function of depth, won't work once the inelastic threshold is passed. Instead, just as you describe, you'll usually see the primary particle apparently lose all of its energy in the first interaction (outermost layer), then disappear.

For high energies, what you need to do is track all the particles, and instrument a _scorer_ which records the total energy deposited in each layer. At low energies, you'll see the characteristic Bragg peak. At higher energies, you'll see a broader depth profile (the shower shape), with the energy deposit at a maximum partway through, then a long tail to higher depths.

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1 Feedback: Re: Bragg curve profiles of protons in water   (aimsphere - 06 Jan, 2013)
(_ Agree: Re: Bragg curve profiles of protons in water   (Michael H. Kelsey - 06 Jan, 2013)
(_ Ok: Re: Bragg curve profiles of protons in water   (aimsphere - 06 Jan, 2013)
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