I don't get it. 8Be has zero nuclear spin and 4He has zero nuclear spin. How can a nuclear reaction involving them have huge annular momentum?
On Wed, Apr 8, 2015 at 12:28 AM, Eric Walker <[email protected]> wrote: > Hi Bob, > > The possibility you've been drawing attention to, that the result of the > decay of the [8Be]* compound nucleus into two 4He nuclei with little linear > momentum and a great deal of angular momentum makes for an interesting > thought experiment. Out of curiosity, I calculated the energy that would > be needed to break up an alpha particle into either tritium and a proton or > 3He and a neutron, which would be the reverse of these two reactions: > > 3He + n → 4He + Q (19.3 MeV) > t + p → 4He + Q (20.5 MeV) > > As I understand it, this implies that angular momentum sufficient to > produce ~ 19 MeV of centripetal force would be needed to break apart a 4He > into either 3He and a neutron or tritium and a proton. This suggests that > a 4He can carry a large amount of angular momentum before it is likely to > break apart. (I assume the process is probabilistic and that the force > needed lies along a distribution.) > > Further comments inline. > > Eric > > > On Tue, Apr 7, 2015 at 1:35 PM, Bob Cook <[email protected]> wrote: > > However, I know of know reason why the light nuclei cannot have any spin >> quantum number--high or low. Any spin quantum is available. > > > Further to the thought experiment, I think we should make a clear > distinction between two types of "spin" -- there's the actual spinning > motion of a nucleus (e.g., 4He), and there is the spin state of the > nucleus. At higher rates of rotation, a heavy nucleus such as an isotope > of nickel will reconfigure into a higher spin state, presumably through > deformation. In such a state a photon may be emitted, with the nucleus > relaxing into a lower spin state. Here my mental model is of neodymium > magnets spinning around in a clump. When they snap together into a > lower-energy configuration, a photon is emitted through the movement of the > magnets as they snap together. The photon is emitted in a direction and > carries away energy in such a way as to slow the angular movement of the > spinning nucleus a little (by the amount of energy carried away by the > photon). The participants involved in such a transition are the nucleons, > and the energy of the photon that is emitted will correspondingly be in the > keV or MeV range, which is in the nuclear range. > > A light nucleus, such as 4He, does not have a bound excited state. My > understanding is that it cannot deform under high angular momentum into a > higher energy state which will emit a photon when it relaxes. The 4He will > either break apart into lighter constituents under centrifugal forces or it > will not. But I'm guessing that the actual moment-to-moment velocity of > the 4He about its axis of motion is in principle a continuous quantity. If > this is true, perhaps the energy could be released to the environment in > small amounts. > > Where the thought experiment gets interesting is in the supposition that > you and others have already offered in this thread, that charged body such > as a 4He nucleus that is spinning at an incredible rate will set up a > magnetic field. This magnetic field could disturb nearby electrons, > causing them to emit lower energy photons in the process. > > Although I do not see anything special in the 7Li+p to 8Be transition that > has been proposed (and note Jones's point about the gamma that would be > omitted in the process), I think the more general notion of the energy of a > nuclear transition somehow being deposited in angular momentum and then > released in small amounts is a very interesting one. > >
