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I am reading the book The Greatest Story Ever Told ... So Far by Lawrence Krauss, and I came across this in chapter 9:

In 1930 Bothe and his assistant Herbert Becker observed something completely new and unexpected. While bombarding beryllium nuclei with products of nuclear decay called alpha particles (already known to be the nuclei of helium), the two observed the emission of a completely new form of high-energy radiation. This radiation had two unique features. It was more penetrating than the most energetic gamma rays, but like gamma rays, the radiation was composed of electrically neutral particles so that it did not ionise atoms as it passed through matter.

I found the last part confusing:

... It was more penetrating than the most energetic gamma rays, but like gamma rays, the radiation was composed of electrically neutral particles so that it did not ionise atoms as it passed through matter.

My understanding was (and my web searching seems to corroborate this), that gamma rays DO ionise atoms as they pass through matter -- which is why they're so damaging to organisms. But this passage seems to state that gamma rays DO NOT ionise atoms as they pass through matter? Or is this passage specifically referring to certain types of atoms such as lead?

I would greatly appreciate it if people could please take the time to clarify this.

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  • $\begingroup$ Gamma rays do not leave tracks in cloud chambers. Only when they do ionize stuff and transfer energy to electrons, the electron leaves a track. Or pair production, etc. $\endgroup$ – Pieter Dec 25 '17 at 9:23
  • $\begingroup$ @Pieter How does this relate to the passage? $\endgroup$ – The Pointer Dec 25 '17 at 9:24
  • $\begingroup$ It is about what one sees in a cloud chamber or bubble chamber. One can see photoelectric ionization events, pair production, Compton events. But not the tracks of gammas. $\endgroup$ – Pieter Dec 25 '17 at 9:26
  • $\begingroup$ @Pieter So you’re saying that this is the type of experiment that was conducted by Bothe and Becker? And this is why gamma rays do not ionise atoms in this experiment? Because they were radiated within a “cloud chamber”? $\endgroup$ – The Pointer Dec 25 '17 at 9:31
  • $\begingroup$ I try to explain what already said above. You do not see ionisation along their path until they do finally ionised matter. That is they are not prone to braking ergo more penetrant. They go until they interact. A charge particle can ionise stuff all along depending on its starting energy. $\endgroup$ – Alchimista Dec 25 '17 at 20:39
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It is like in this image, with two pair-production events (curvatures caused by a magnetic field):

The tracks are due to the charged particles (electrons and positrons in this case), ionizing atoms on their way because of Coulomb interactions. That is why they loose energy fast in matter, why their range is so limited. Also, there are many more of such ionization events because the energies involved are often low (about 10 eV).

The gammas will be ionizing now and then by Compton scattering and by the photoelectric effect, but those cross sections are lower.

Cloud chambers are just a way of observing the interactions, but the same goes on inside any material.

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  • $\begingroup$ So gamma rays DO ionise atoms as they pass through matter, but they just do it far more rarely than other types of radiation? $\endgroup$ – The Pointer Dec 25 '17 at 9:50
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    $\begingroup$ @ThePointer Yes, gamma rays are ionizing radiation. But they do not leave tracks the way charged particles do. $\endgroup$ – Pieter Dec 25 '17 at 9:57
  • $\begingroup$ How does the gamma ray photon lose part of its energy? $\endgroup$ – DJohnM Dec 25 '17 at 15:23
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    $\begingroup$ @Djohnm There is the Compton effect. $\endgroup$ – Pieter Dec 25 '17 at 17:12
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Your problem here has entirely to do with language and not physics.

The distinction that the authors are making and you are ignoring is that gamma rays ionize when they interact, but not as they travel. This is in contrast to charged particles which generate a small amount of interaction along their entire path.

If we place a very thin sample (much, much less than one radiation length) in front of a gamma source, almost none of the gammas will create ionization in the sample even though a large fraction of them go through it: they pass through without interaction.

On the other hand if we use a thick sample (several radiation lengths) then almost all the gamma rays will create ionization because they interact.

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