Recent comments in /f/askscience

taphead739 t1_j4rn0b7 wrote

Reply to comment by Speterius in Why does the existence of magnetic monopoles imply quantized electric charges? by Speterius

It is a natural consequence of the wave-particle duality. If we go back to classical mechanics, angular momentum is present when something moves along a circular path. Very small particles are also waves, and the frequency (number of times the wave function goes up and down per length unit) is a measure for their (angular) momentum. The wave function must now "fit" the circumference - meaning that if you go around the circle the whole 360°, you must end up with the same value of the wave function and are not allowed to have a sudden step. This only works if the circumference is an integer multiple of the wavelength. As a consequence, only certain wavelenghts and frequencies are allowed, and the same is true for angular momentum.

This is a very simplified picture, of course, but I hope it gets the principle across.

7

Speterius OP t1_j4rgooc wrote

Reply to comment by taphead739 in Why does the existence of magnetic monopoles imply quantized electric charges? by Speterius

Thank you for the answer. Super insightful.

> "Since quantum mechanics dictates that angular momentum must be quantized, (....)"

Is the fact that angular momentum must be quantized a postulate of QM or is it derived from something more fundamental? I saw the plank length come up in the Dirac derivation.

I guess I'm looking for some sort of axiom. Something that doesn't follow from anything, but is the lowest level block of QM.

2

PlutoniumChemist t1_j4rcme7 wrote

Reply to comment by kira01070 in If nuclear fission in U-235 causes the atom to be split into 2 smaller atoms (such as Kr-92 and Ba-141) then how is it that U-236 is produced as waste since the U-235 was just split into smaller peices? by Ian98766

Beta minus decay is the only mode of radioactive decay that increases atomic number.

These heavy radionuclides become more and more likely to decay by alpha decay or by spontaneous fission as their atomic numbers increase and less likely to decay by beta minus.

The half lives of these radionuclides also decrease as they get heavier, meaning they don't "survive" long enough to reliably capture neutrons before they decay into something else

By the time you get to Fm, there are no accessible beta minus decaying isotopes of Fm that can be created through neutron capture before the nuclide decays by some other mode.

This logic applies to nuclear reactors. Nuclear weapons have... a much higher neutron flux. This means the Fm isotopes could potentially capture a very large number of neutrons in a very short period of time in order to create an exotic beta minus decaying isotope of Fm before it decays by some other decay mode. Not sure if this was ever observed during the various nuclear weapons testing phases across the world

19

iksbob t1_j4raeuh wrote

Reply to comment by stefab in If nuclear fission in U-235 causes the atom to be split into 2 smaller atoms (such as Kr-92 and Ba-141) then how is it that U-236 is produced as waste since the U-235 was just split into smaller peices? by Ian98766

> high thermal energy increases the average energy of all collisions.

I was thinking that thermal-kinetic energy of a given atom could add, subtract or have no effect on the total impact energy depending on the atom and neutron's direction at the moment of impact. Within a medium, the kinetic direction of a given particle due to thermal effects would be chaotic, so effectively random.

−3

stefab t1_j4r0rcx wrote

Reply to comment by iksbob in If nuclear fission in U-235 causes the atom to be split into 2 smaller atoms (such as Kr-92 and Ba-141) then how is it that U-236 is produced as waste since the U-235 was just split into smaller peices? by Ian98766

Well, no, high thermal energy increases the average energy of all collisions. Such is the definition of thermal energy. Yes, included in the average collision are outlying collisions where there might be excessive energy (you'll find water evaporates even at room temperature, just at a much slower rate), or a far lower collision energy, such as in the case above of U236 being produced instead of nuclear fission.

37

iksbob t1_j4qyvst wrote

Reply to comment by bikedork5000 in If nuclear fission in U-235 causes the atom to be split into 2 smaller atoms (such as Kr-92 and Ba-141) then how is it that U-236 is produced as waste since the U-235 was just split into smaller peices? by Ian98766

> sum of kinetic energy of all the moving massive particles in a given region

So, high thermal energy might increase the energy of one collision, but reduce that of another. Net zero?

25

bikedork5000 t1_j4quv9z wrote

Reply to comment by iksbob in If nuclear fission in U-235 causes the atom to be split into 2 smaller atoms (such as Kr-92 and Ba-141) then how is it that U-236 is produced as waste since the U-235 was just split into smaller peices? by Ian98766

'Temperature' is really just a statistical construct. Thermal energy is a sum of kinetic energy of all the moving massive particles in a given region. You can have a colder area that still contains fast enough neutrons to trigger fusion, but just less of them than in a 'hotter' area in a bomb, for example. But all things being equal, more neutron flux at the appropriate energy/velocity per neutron will equal more fission interactions.

165

iksbob t1_j4qsn4v wrote

Reply to comment by RobusEtCeleritas in If nuclear fission in U-235 causes the atom to be split into 2 smaller atoms (such as Kr-92 and Ba-141) then how is it that U-236 is produced as waste since the U-235 was just split into smaller peices? by Ian98766

Does thermal energy play a role? As in, is a U235 in reactor-like conditions less likely to fuse from a neutron strike of X energy, versus a U235 in the plasma cloud of a thermonuclear device?

76