phy radioactivity 高手進20點

1. Only electromagentic waves in the x- and gamma rays range cause ionization. Those in the radiofrequency or optical range are NOT ionizing (for such reason, they are called "non-inizing radiation).

It is generally true that higher energy photons (which is the particle nature of electromagnetic waves) cause more ionization. But be aware that this in fact depends very much on the interaction medium through which the photons passed.

2. The "danger" of a radioactive source depends on a number of factors, not only its half-life. The most important factor is its "activity" (i.e. its strength). For radioactive sources of the same activity, the one with long half-life stays longer than the shor-lived one.

3. The "mass per nucleon" is not a crucial factor in determining the stability of a nucleus.

4. As the name implies, "binding energy" is the energy that holds the nucleons together. The higher binding energy, the stronger is the force that glues the nucleons.

5. Pressurized water used in a PWR (Pressurized Water Reactor) acts as a moderator and a coolant. Water contains hydrogen atoms, which slow down fast neutron, without absorbing them, so as to facilitate fission reaction to take place,

6. Because most alphas go through the atom, only few of them scattered at large angle. This is not possible if the +ve and -ve charges inside an atom are evenly distributed.

The whole atom is neutral in charge. Since some alpha particles are bounced back, it shows that the nucleus carries +ve charge (like charges repel). Hence there must be present equal amount of -ve charges (i.e. electrons).

7. No. On ingestion, sources with long half-life are more hazardous to health. Having said that, you need also to consider the "biological half-life" of the source, i.e. the time at which the human body excrete the source and which is dependent on its chemical form.

(to be continued in the 意見 section)

2013-07-21 01:13:14 補充：
(cont'd)
8. Yes. The neutron could come from a source or from natural means.

9. Not necessarily.

10. It is a nuclear reaction, but not a fusion reaction.
By "fusion" reaction, we usually mean two nucleus of comparable masses fused together to from a single nucleus.

Here are some supplementary to the previous answers.

(2) The element with longest half-life is probably bismuth with a predicted half life in the order of 10 billion years (big bang was ~ 14 billion years ago). It is completely harmless as its radioactivity is not measurable.

(3) Helium is heavier than the tritium (isotope 3-H of hydrogen) but is more stable. That's why nuclear fusion is theoretically an energy source.

(5) Water is pressurized in plants to increase its boiling temperature so that it works for T>100C.

(6) The scattering cross section measures an effective nucleus radius which is much smaller than the atomic radius measured by dense atomic packing. This shows most of the atomic volume is filled with vacuum, and essentially all mass are compactly concentrated at the center.

(8) No. Fission could happen even without an external neutron source. We need an external neutron source for power plants only because the nuclear fuel (Uranium, Plutonium, etc) is not dense enough for its own neutron radiation to trigger fission. Recall neutron will decay relatively quickly into proton and electron (and neutrino), and proton has charge and cannot come close to a nucleus. If the nuclear fuel is dense enough, fission can be spontaneous. In fact this is how atom bomb works. There is a primary explosion by ordinary explosive to squeeze the nuclear fuel so that it become critically dense enough for spontaneous fission. A nuclear explosion is actually a secondary explosion.

(10) Technically, it is a fusion reaction. But it cost energy for it to happen.

may i answer in chinese for a little?

1. yes , if you study E4(medical phy),you will know higher frequency EM wave will have lower penetrating power since their energy is higher and being absorbed more by the medium.(個term叫attenuation)

2.It's related to the radioactive source which is not removable.Once the source is inside the body, because the decay half-life is long , it will have bad influence to the body for a longer time

3.this is not correct in my physics knowledge.Stability is related to binding energy.

4.binding energy(Eb) is the energy requred to separate the nucleons of a nucleus, if it is larger, more energy is required to separate the nucleons thus the nucleus is more stable. It's has no correlation between the mass and Eb，你應該知道Eb個curve係1個山丘咁既形狀，當中最高Eb既就係iron，所以nucleon少過iron既會做fusion合大個nucleus，大個既會做fission爆開個nucleus，務求到達iron，但兩個都會release energy.

5.yes, it's used to slow down the fast moving neutron released in the fission reaction by the fuel element to make the chain reaction possible.(neutron太快will stop reaction)

6.in that experiment, most of the alpha particles passed straight forward on their route which shows that 90% above of the spaces of an molecule is empty and the little alpha particles reflected or scattered show that the mass of a molecule is concentrated in the centre .
alpha particle have positive charges ,whose route will be affected by the negatively charged electrons proving that electrons is present in atom.

7.it's has no contradiction since it's related to the radioactive source which is removable. If short half-life is swallowed ,it will decay at a fast speed that it may not be fast enough to be removed by surgery in hospital.hence it's effect is more dangerous.

8.no. it's not

9.no. it's not

10.It's because no energy is released. all nuclear reaction will release energy