Excellent and very informative; thank you! > According to previous studies by Rutherford, Chadwick and Ellis a Proton with this energy would only travel 5mm in air, whereas the trail being observed has more than 5cm. Why is the reason for that, and why is it different for positrons? Anderson points out that in some of the photographs a positive particle was identified but the energy of the particle was so high (and/or the magnetic field so low) that the curvature of the track (almost a straight line in this case) wasn't enough to distinguish Protons from Positrons. Anderson points out that there is still some work to be done in the study of energy losses of charged particles going through matter as the energy losses being observed for the Positron are not completely compatible with the theory. Anderson tries to come up with some theories for the origin of the Positron. Note that in 1934 Frédéric and Irène Joliot-Curie ended up discovering the $\beta^+$ decay, a natural process where Positrons are created. Carl Anderson, at the time a 28 years-old researcher at Caltech working under the supervision of Prof. R. A. Milikan, describes in this paper the observation of the positron - the first antiparticle to be detected experimentally. This observation validated Paul Dirac's theoretical prediction of the positron and led to the attribution of the Nobel Prize to Anderson in 1936. Anderson actually did not coin the term positron, but allowed it at the suggestion of the Physical Review journal editor to which he submitted his discovery paper. The Wilson chamber (also known as cloud chamber) is a particle detector used for detecting ionizing radiation. In its most basic form, a Wilson chamber is a sealed environment containing a supersaturated vapor of water or alcohol. When a charged particle interacts with the mixture, the fluid is ionized. The resulting ions act as condensation nuclei, around which a mist will form. Due to many ions being produced along the path of the charged particle a characteristic trail is left.  Figure: Wilson chamber used by Carl Anderson Here is an example of how a [cloud chamber works](https://youtu.be/ZiscokCGOhs?t=446). By emerging the Wilson chamber in a magnetic field it was easy to distinguish the positive particles from the negative ones as, according to Lorentz force, they would curve in opposite directions. immersing*? Good question! For two particles to have the same trajectory under the same magnetic field $B$, according to the Lorentz formula: $$ m_{e^+}v_{e^+}=m_{p}v_{p} $$ which means that the heavier the particle, the slower it will be in order to keep the same curvature. Moderately relativistic charged particles other than electrons/positrons lose energy in matter primarily by ionization. The Bethe-Bloch equation tells us that the mean rate of energy loss (or stopping power) is: $$ -\frac{dE}{dx}= Kz^2\frac{Z}{A}\frac{c^2}{v^2}\big (\frac{1}{2}\ln \frac{2 m_ev^2\gamma^2T_{max}}{I^2}-\frac{v^2}{c^2}-\frac{\delta}{2} \big) $$ For more info on the [Bethe-Bloch formula click here](http://pdg.lbl.gov/2000/passagerpp.pdf). In the case of relatively small velocities the energy loss formula is dominated by the term $\frac{1}{v^2}$ and so: $$ -\frac{dE}{dx} \propto \frac{1}{v^2} $$ From this formula we can see that a proton would lose substantially more energy than the positron since it's travelling at a lower velocity. The point here is that this increased energy losses will have a significant impact in the range of the proton (which I'm not calculating, I'm just giving an idea of how different the ranges can be). Note: If you want to calculate the range $R$ of the particles you need to integrate the energy loss: $$ R = \int_{0}^{R_0}dx = \int_{E_0}^{Mc^2}\frac{1}{\frac{dE}{dx}}dE $$ You will also have to take into account energy losses because of Bremsstrahlung radiation for the positron. Anderson comes up with 4 possible explanations for Fig. 1: 1 - The particle being observed is a Proton. In this case a Proton with that curvature has a Kinetic energy of 300 KeV (to see that we just need to use Lorentz force to calculate the speed of the particle and then compute its Kinetic energy $F=ma=m\frac{v^2}{r}=qvB$). According to previous studies by Rutherford, Chadwick and Ellis a Proton with this energy would only travel 5mm in air, whereas the trail being observed has more than 5cm. This hypothesis is therefore rejected. 2 - The track being observed is actually 2 Electrons. In this case Anderson assumes that the trajectory being observed is actually composed of 2 independent trajectories: one Electron going down from the lead and one Electron going down and entering the lead from above (which curve in the same direction as a positive particle going up). These 2 paths would be perfectly synchronized, giving the impression that they would be the same particle. Anderson discards this hypothesis saying that the probability of such a coincidence is extremely low. 3 - The track is an Electron that enters from above, goes through the lead plate and gains energy (since the trajectory is less curved on the bottom part). This hypothesis is again excluded on a probability basis as an event that that would somehow accelerate an Electron passing through a lead plate is extremely unlikely. 4 - The track is a Photon that enters from above, ejects 2 particles from the lead, one going up (positive) and another going down (negative). In this case we end up with a possibility where the positive particle going up has to be a Positron again!