Fusion vs. Dissociation

Steve Willner wrote in
http://groups.google.com/group/sci.a...10441b2?hl=en& :
SW At high temperatures, molecules tend to dissociate to form
SW individual atoms. At even higher temperatures, the atoms break
SW up into separate nuclei and electrons. And at temperatures
SW higher still (far above anything relevant to stellar
SW atmospheres), the nuclei themselves break up.

In article .com,
"Radium" writes:
Huh? If that was the case, then there would be nuclear fusion would
require much lower temperatures.


Why would you think that? Fusion means combining nucleons, not
separating them.

Fusion is the combining of nucleons. However, you said nuclei break up
at extremely high temperatures. That is fission, not fusion.

Nuclear fusion in stars occurs when the temperature is sufficient to
overcome the Coulomb barrier: i.e., the electrical repulsion of the
protons in colliding nuclei. For p-p fusion, the necessary
temperature is around 10 million kelvins. For the carbon cycle, the
repulsion is 12 times higher, and temperature needs to be higher by
about the same factor (actually a bit less because 12C + p goes by
the strong interaction, not the weak one).

Okay.

Temperature to dissociate molecules is about 1000 K, to ionize atoms
about 10000 K, and to break up nuclei about 10^11 K.

Once again, breaking up a nucleus is fission, not fusion.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
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"Radium" wrote in news:1157684993.657003.116110
@e3g2000cwe.googlegroups.com:

Steve Willner wrote in
http://groups.google.com/group/sci.a...10441b2?hl=en& :
SW At high temperatures, molecules tend to dissociate to form
SW individual atoms. At even higher temperatures, the atoms break
SW up into separate nuclei and electrons. And at temperatures
SW higher still (far above anything relevant to stellar
SW atmospheres), the nuclei themselves break up.

In article .com,
"Radium" writes:
Huh? If that was the case, then there would be nuclear fusion would
require much lower temperatures.


Why would you think that? Fusion means combining nucleons, not
separating them.

Fusion is the combining of nucleons. However, you said nuclei break up
at extremely high temperatures. That is fission, not fusion.

Actually it is normally referred to as photo-disintigration to make clear
that an endothermic process is occurring. This is believed to occur during
the collapse phase of a Type II supernova. This is followed by inverse beta
decay, another endothermic reaction resulting in the bulk of the protons
being converted to neutrons.



Nuclear fusion in stars occurs when the temperature is sufficient to
overcome the Coulomb barrier: i.e., the electrical repulsion of the
protons in colliding nuclei. For p-p fusion, the necessary
temperature is around 10 million kelvins. For the carbon cycle, the
repulsion is 12 times higher, and temperature needs to be higher by
about the same factor (actually a bit less because 12C + p goes by
the strong interaction, not the weak one).

Okay.

Temperature to dissociate molecules is about 1000 K, to ionize atoms
about 10000 K, and to break up nuclei about 10^11 K.

Once again, breaking up a nucleus is fission, not fusion.

The terms fission and fusion are normally reserved for exothermic
reactions. As I said the breaking up of nuclei due to such extreme
temperatures is specifically called photo-disintegration.

Klazmon.



--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

Temperature to dissociate molecules is about 1000 K, to ionize atoms
about 10000 K, and to break up nuclei about 10^11 K.



How do you explain critical mass in a fission reaction? Do the nucleons just
possess the kinetic energy
equivalent to 10*11K? (how do you make a carrat?)

Radium wrote:
Steve Willner wrote in
http://groups.google.com/group/sci.a...10441b2?hl=en& :
SW At high temperatures, molecules tend to dissociate to form
SW individual atoms. At even higher temperatures, the atoms break
SW up into separate nuclei and electrons. And at temperatures
SW higher still (far above anything relevant to stellar
SW atmospheres), the nuclei themselves break up.

In article .com,
"Radium" writes:
Huh? If that was the case, then there would be nuclear fusion would
^^^^^^^^^^^^^^^
require much lower temperatures.

Why would you think that? Fusion means combining nucleons, not
separating them.

Fusion is the combining of nucleons. However, you said nuclei break up
at extremely high temperatures. That is fission, not fusion.

The source of the confusion is that you used the
term "nuclear fusion". Perhaps you should clarify
if that was just a typo or what you meant if not.

HTH
George

In article 9J6Mg.127$Lb5.11@edtnps89,
"Thomas Mickle" writes:
How do you explain critical mass in a fission reaction?

Fission in reactors is an exothermic process. It involves neutrons
-- hence no Coloumb barrier -- and unstable nuclei. Critical mass
means that there's enough fissionable material to absorb the neutrons
and create more of them rather than having most of the neutrons
escape and the reaction fizzle. Fission can take place at extremely
low temperatures, though in usual situations the energy released will
make everything hot. First startup of a nuclear reactor can be at
room temperature (or even colder) as far as the nuclear reaction
itself is concerned. (Pumps, pipes, or other components may require
preheating in some reactor designs, and in general one wants to allow
temperatures to rise gradually, not suddenly, but this is because of
ordinary mechanical properties of materials, nothing to do with the
nuclear reaction itself.) Offhand I can't think of a situation in
astronomy where fission is relevant, but I may be missing something.

Photo-disintegration is an exothermic process, relevant only at
extreme temperatures. If temperature is sufficiently high, any
nucleus -- even a normally stable one -- will be broken apart,
absorbing energy from whatever created the extreme temperature in the
first place. This is analogous to dissociation of molecules or
ionization of atoms, which are endothermic processes that take place
at much lower temperatures.

Mention of one particular physical process doesn't mean that no other
processes exist.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
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