Alpha particles

14-07-2025

Femtometre hand grenade

Most people with some familiarity of physics concepts are (rightfully) scared of gamma radiation. If someone studied a bit further, neutron emitters would be even more frightening, and maybe some respect would be paid to beta particles. However, in many non-technical circles, people fail to even consider alpha particles a threat.

Of course, being as heavy as they are, alpha particles are easily stopped by paper or skin. This is because they are more likely to interact with electrons and nuclei as they tread their path of destruction, depositing a lot of their energy in a very short distance. Because of this, some commonly known radioactive isotopes, like Uranium-235 and Plutonium-239, are mostly safe if not ingested in any capacity. You could hold a sample of Pu239 in your hands, with little more than a paper bag in the way, as Queen Elizabeth II once did.

The working principle is "simple" (although the following explanation is not an unanimous, perfect description): collisions with electrons (electronic stopping power) and nuclei (nuclear stopping power) sap the energy of the incoming particle, and smaller particles with less charge are less likely to interact with atoms in a given volume of material. If you were to stand in a crowd, and threw two balls of varying size in a random direction, the smaller one would travel further before hitting someone square in the face and transmitting its energy.

Although it means that even "light" materials are relatively effective at shielding you from its effects (by taking the brunt of the hit), it means your cells are equally likely to get pummeled by alpha radiation if it enters your body. When ingesting alpha particles, unlike gamma emitters, most of the energy will be deposited in the immediate vicitinity of where the material lodged itself. Additionally, the decay energy of a significant portion of alpha emitters is around 5 MeV, nearly 5 times more what any natural gamma emitter spouts out.

Take for instance Actinium-225 (we'll come back to it later), which deposits all of its 6.88 MeV of alpha decay energy in less than 100 microns: this much energy, concentrated in such a small space is a guaranteed death sentence for any biological material unlucky enough to stand close (really close) to it.

Death offers a helping hand

Precisely because of this, when calculating how much "damage" was dealt to tissue, the absorbed dose is weighted according to the type of radiation. This is called relative biological effectiveness, or RBE. Whilst gamma rays have a RBE of 1, alpha particles and other heavy nuclei have a RBE of 20, meaning they are 20 times as likely (for the same absorbed dose) to cause cell death.

As a matter of fact, gamma rays inside the body can be so safe that Positron Emission Tomography (PET) is a relatively common procedure, where beta minus decay mode isotopes, such as Fluorodeoxyglucose F18 are ingested by the patient. In PET scans, the isotope emits a positron when decaying, which (by virtue of being antimatter), annihilates when in contact with an electron, emitting two gamma rays. Since the gamma rays don't interact much with your body, they filter out virtually unimpeded, which, strangely enough, presents more of a risk to the people around you than to your own body.

Naturally, healthy cells are not the only thing in the body we might want to kill. Unlike regular x-ray/gamma ray based therapies, heavy ion/proton therapies are able to deliver highly localised doses, which can be tuned to peak at a certain distance, and also to go no further than the target. This energy deposition peak is called a Bragg peak, and it's usually carefully tuned to hit a tumour square in the centre. Protons aren't the only calibre of bullet in our arsenal - carbon ions are also used because of their large mass.

Much like bombing in conflicts went from "we'll just flatten this general direction" to "which window do you want me to hit?" over the course of the last century, medical therapies have improved massively in what we call specificity: the ability to erradicate disease without affecting other parts of the body.

As cool as rays sound, there are other methods of delivering radiation-powered death to unwanted lodgers of the human body! Most recently, research has been done on targeted alpha particle therapy (TAPT), which finds cancer in the body, binds to it, and allows Actinium-225 to do its thing. Not only is this incredibly efficient, as Ac225 obliterates anything in its (short) path, but it also spares patients from many of the terrible side effects of conventional cancer therapies.

Short path, long way to go

There is a slight caveat to all of this, however: heavy-ion/proton therapy is not widespread. There are only two centres in the UK, and research on this topic hasn't gone as deep as research for other radiotherapy methods, some of which are over a hundred years old, meaning proton therapy is in its early infancy.

As for TAPT, we're facing a "slight" shortage, as Ac225 is "the world's rarest drug". Actinium-225 has to be sourced from spent fuel rods, nuclear fuel or fabricated in cyclotrons, in microscopic amounts. However, precisely because of how effective it is, we should be able to extract enough of it to treat patients for the next 100 years, although we need a more sustainable solution in the long term.

Despite all of these challenges, many interesting developments have taken place in the last few years. Although proton accelerators are big, bulky and power intensive, some new concepts provide new acceleration schemes with unparalleled portability and efficiency, meaning treatment could be extended to parts of the world which had no access to it up until now. Furthermore, by accelerating the particles further, treatments can diminish the amount of collateral damage even further.

The potential for productive utilisation of alpha radiation sources (and heavy ions/protons) in the future is still very much untapped, and if we're able to make effective use of the many applications of alpha particles, the future looks as bright as the dials on a radium watch.