4 ms·
How does a photon survive going through the body to make it to the detector? Seems like there is a lot of stuff in the way.
by platz 3y ago
How does a photon survive going through the body to make it to the detector? Seems like there is a lot of stuff in the way.
- eigenket 3y agoA photon from an electron-positron anihilation has frequency around 10^20 Hz, somewhat higher frequency than what we usually call x-rays. Some of them do get absorbed by the body, but enough pass through to be detected usefully.
- magicalhippo 3y agoAbsorption of light by stuff is frequency dependent. At the right frequency, stuff can be almost entirely transparent. Similar to how by detecting infrared light JWST can peer through the dust in galaxies to reveal detail that the couldn't see in optical light using Hubble[1]. Electron-positron annihilation releases photons with an energy of roughly 0.5MeV[2], and as you can see from this graph[3] the absorption is relatively low at those energy levels. Unless I got my math wrong, about 90% of those photons survive traveling through 1 cm of water or fat (which has roughly the same density as water). [1]: https://jwst.nasa.gov/content/about/comparisonWebbVsHubble.html https://jwst.nasa.gov/content/about/comparisonWebbVsHubble.h... [2]: https://en.wikipedia.org/wiki/Electron%E2%80%93positron_annihilation#Low-energy_case https://en.wikipedia.org/wiki/Electron%E2%80%93positron_anni... [3]: https://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/water.html https://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/wat...
- dekhn 3y agohttps://en.wikipedia.org/wiki/Mean_free_path https://en.wikipedia.org/wiki/Mean_free_path