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Q & A: Radioactive Decay

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Most recent answer: 12/15/2014
Q:
In radioactivity, when alpha-rays break up an atom, as a helium nucleus is lost, the change in mass number could be -4 and the change in atomic number -2 in the affected atom. what if the atom which it is breaking up, has an atomic number of below 2 (ie. helium or hydrogen) or has a mass number of below 4 (also hydrogen and helium). Also, in both alpha-radiation and beta-radiation, particles are lost and energy is prouced. Where do the lost particles go and why id energy produced. Thanks.
- Julia Kawai (age 15)
HBS, London, England
A:
Hi Julia,

You are right. A nucleus which decays by emitting an alpha particle loses two neutrons and two protons, in order to change its atomic number by -2 and mass number by -4. So a nucleus with an atomic number of 2 or less or a mass number of 4 or less cannot decay by emission of an alpha particle -- there is less than an alpha particle there in the first place. Typically it is the very heavy nuclei or ones which have a superabundance of neutrons which decay by emitting alpha particles.

In nuclear decay, energy isn't really "created", it merely shifts from one form to another. Some nuclear decays actually create some new particles where there weren't any before. If a nucleus decays by beta radiation, one of its neutrons turns into a proton and it emits an electron and an electron antineutrino. The whole process is mediated by the Weak Nuclear Force. Neutrons do this all by themselves if they are not bound up inside a nucleus, and the half-life of this process is about 11 minutes. The electron and neutrino are created during the process, and the proton is just a little bit lighter than a neutron. E=mc**2, and so the little bit of mass difference between the neutron and proton is enought to not only make the electron and its antineutrino, but to give them some energy to fly away.

The electron interacts with other charged particles because it is charged itself. They will slow down and stop when passing through materials, depositing their energy. If they have lots of energy, they may knock another electron off of the atom or molecule they strike, thus "ionizing" it. Ionizing radiation is a potential hazard because it changes the chemical nature of the molecules which are ionized. These may re-form into different molecules. If these molecules are part of the body, burns and/or cancerous changes in the DNA material may result.

Alpha particles have two units of charge and tend to stop more easily than beta particles (which are electrons). Small amounts of shielding are usually enough to keep alpha and beta particles from straying. They become nasty however when the radioactive materials which produce them are ingested. Then damage to tissues may occur even for these very short-ranged particles.

Gamma rays (photons), like their lower-energy cousins X-rays, may penetrate through solid objects to cause problems elsewhere. Lead and tungsten are good shields against gamma rays. Free neutrons are also sometimes produced in nuclear reactions. These may diffuse slowly through materials and get captured by nuclei, which may then decay by beta radiation, changing their atomic number and hence their chemistry. It is good to shield well against free neutrons.

Energy is released in nuclear decay because the total energy of the nucleus after decay is less than that before. If the nucleus falls apart into two pieces (as in alpha decay or in the more dramatic nuclear fission), the pieces are more tightly bound than the original nucleus, and energy is given up in that way. If a nucleus has many more neutrons than protons, the neutrons occupy filled orbitals in the nucleus that fill independently of those for protons. If a neutron decays into a proton, it may then drop into a lower shell and release energy that way. In all cases, the total energy of the system (including all the masses of the particles) is the same before and after, but some of the binding energy (or difference between a proton and neutron mass) may get converted into the kinetic energy of an alpha particle or to produce an electron and antineutrino and to give them kinetic energy. These fast-moving particles continue to fly until they hit something, and that object may be damaged in some way when the energy is deposited.

Tom

(published on 10/22/2007)

Follow-Up #1: what becomes of radioactive decay products?

Q:
I am a middle school teacher who has had a little training in identifying radioactive material using a geiger counter. At a workshop I attended, we were given common radioactive materials to show kids who aren't aware of their danger (examples; smoke detector, small rock of uranite, paint from Mexico, radium in a watch dial, etc.). My concern is that keeping this material at my home, even though outside in my shed, is still dangerous to me and my family even though I was told the heavy plastic containers keep the particles inside. I want to know where these particles go once I open the container. Are they jumping out of the box waiting to bounce into me or what the heck happens to them?
- Denise (age 54)
Albuquerque, NM USA
A:
Nice question.

Assuming that you don't get a geiger reading outside the storage box, you're ok. Those fission products become incorporated into the materials they hit. They aren't hopping around. Some materials can form radioactive isotopes when exposed to some types of radiation. If the box is radioactive, again you should pick that up on your geiger counter.

Mike W.

(published on 03/18/2011)

Follow-Up #2: Attenuation of gamma rays by lead

Q:
Gamma rays have strong penetrating power since they are produced by the radioactivity materials. NOW! HOW CAN THEY BE HELD IN A LEAD CONTAINER OF WHICH GAMMA RAYS DO PENETRATE THROUGH A LEAD CONTAINER? BY WHAT MARGIN DO YOU THINK A NORMAL HUMAN BEING CAN SURVIVE THE SHOCK OF THESE STRONG RAYS?
- Godfrey kapempe (age 17)
lusaka Zambia
A:

Hello Godfrey,

Au contraire.    Lead is one of the best shields of gamma rays.  Uranium is only slightly better.  Attenuation of gamma rays is usually expressed in terms of an attenuation length,  λ,  as I(x) = I0 exp(-x/ λ), where I0 is the initial intensity.   The coefficient λ is a constant that reflects the total probability of a photon being scattered or absorbed.  It is a complicated function of the Z of its nucleus and A, its atomic number.   If you are interested in details you can look at  , section 27.4.1.

 

LeeH

 


(published on 12/15/2014)

Follow-up on this answer.