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Re: Physics Question

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Fabian Russell <fb@zen.info> writes:

>On Sun, 10 Jul 2016 11:15:03 -0500, Sergio wrote:

>> 
>> found the orgional source, the answer is D, 
>>

>If the answer is 14.4 minutes, then the reaction
>cannot be consecutive first-order reactions,
>A --> B --> C

>The reaction has to be a decay by two separate
>and distinct processes:

>A --> B, or A --> B'

>But this make no sense for radioactive decay because
>a nucleus does not "decay" by gamma emission.  In
>gamma emission the number of protons and neutrons
>does not change.

>Gamma emission is usually the result of a previous decay
>mode which leaves the nucleus in an excited state.  This
>excited state will then emit a gamma photon.  But the
>gamma emission is always a subsequent, or consecutive
>reaction.

>It seems to me that the problem statement is faulty.

See my previous reply on this topic.

Quite often when a nucleus decays, the initial daughter nucleus is an
isomer, an excited state.  Nearly all isomers emit a gamma within
nanoseconds or less, to become the ground state nucleus.  We interpret
this by the parent nucleus decaying by alpha or beta AND a gamma.
Nobody cares that the gamma was produced by a separate process a half a
nanosecond later.

However, there are a few isomers with a longer half-life before they
transmute into something else.  One that confused early nuclear physicists
was Protactinium-234 and Protactinium-234m.  Natural uranium-238 initially
decays via alpha decay to Th-234, which the early researchers called U-X1.  
Th-234/U-X1 decayed by beta into what they called U-X2 which beta decayed
with a 1.175 minute half-life into what they called U-II, which is U-234.

Except there was something wrong.  Some of the U-X2 didn't decay with a
1.175 minute half-life.  It had a longer half-life, over 6 hours. It was
otherwise identical to U-X2, it had the same chemical properties, the
same mass (as far as they could tell), and decayed the same way (beta
minus into U-234).  They called this strange substance U-Z. The decay of
Th-234/U-X1 was always a mix of two things, U-X2 and U-Z, identical except
for their half-lives.

Now we know that Th-234 beta decays into Pa-234m, which is one of those
odd isomers with a relatively long half-life (1.175 minutes).  It usually
beta decays into U-234. But 0.13% of the atoms emit a gamma and become
Pa-234 atoms (the ground state), and Pa-234 (which was the mysterious U-Z)
also beta decays into U-234, but with a half-life of 6.6 hours.

So, not only is the initial problem valid, Pa-234m could be the example
nuclide in the problem, except that the half-lives are wrong.  It does
make sense to state Pa-234m sometimes decays via gamma into Pa-234.
The two are almost the same, but have different half-lives.

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Physics Question Sergio <invalid@invalid.com> - 2016-07-07 08:28 -0500
  Re: Physics Question Poutnik <poutnik4nntp@gmail.com> - 2016-07-08 09:02 +0200
    Re: Physics Question Sergio <invalid@invalid.com> - 2016-07-08 09:19 -0500
    Re: Physics Question moroney@world.std.spaamtrap.com (Michael Moroney) - 2016-07-08 15:06 +0000
      Re: Physics Question Odd Bodkin <bodkinodd@gmail.com> - 2016-07-08 10:11 -0500
        Re: Physics Question benj <benj@nobody.net> - 2016-07-08 17:12 -0400
          Re: Physics Question Odd Bodkin <bodkinodd@gmail.com> - 2016-07-08 16:32 -0500
            Re: Physics Question Sergio <invalid@invalid.com> - 2016-07-10 11:15 -0500
              Re: Physics Question Fabian Russell <fb@zen.info> - 2016-07-10 19:25 +0000
                Re: Physics Question Odd Bodkin <bodkinodd@gmail.com> - 2016-07-10 15:43 -0500
                Re: Physics Question Fabian Russell <fb@zen.info> - 2016-07-10 21:52 +0000
                Re: Physics Question Odd Bodkin <bodkinodd@gmail.com> - 2016-07-11 07:50 -0500
                Re: Physics Question Poutnik <poutnik4nntp@gmail.com> - 2016-07-11 17:04 +0200
                Re: Physics Question moroney@world.std.spaamtrap.com (Michael Moroney) - 2016-07-11 01:20 +0000
      Re: Physics Question Poutnik <poutnik4nntp@gmail.com> - 2016-07-08 19:01 +0200
        Re: Physics Question Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-07-09 21:28 +0200
          Re: Physics Question moroney@world.std.spaamtrap.com (Michael Moroney) - 2016-07-10 02:18 +0000
    Re: Physics Question Fabian Russell <fb@zen.info> - 2016-07-09 17:11 +0000

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