Re: Is Eric right, or am I? Young's experiment and photomultiplier
Re: Is Eric right, or am I? Young's experiment and
photomultiplier
Source: http://sci.tech−archive.net/Archive/sci.physics/2008−03/msg00657.html
• From: Eric Gisse
• Date: Fri, 7 Mar 2008 13:46:09 −0800 (PST)
On Mar 7, 11:51 am, "hhc...@xxxxxxxxx" wrote:
On Mar 7, 11:53 am, Eric Gisse wrote:
On Mar 7, 7:39 am, "hhc...@xxxxxxxxx" wrote:
On Mar 7, 12:24 am, Eric Gisse
wrote:
In a photomultiplier, the material [called a
scintillator] is made out
of a semiconducting material. When a
photon comes in, it hits the
scintillator, is absorbed, and raises the
energy level(s) of the atoms
that were hit. That isn't a stable situation,
and the system decays
and emits energy in the form of photons with
energies equivalent to
the band gap of the material. Band gap
energies are on the order of an
eV, while emitted photons tend to be more
energetic than that by
orders of magnitude.
Its' called a photomultiplier because one
photon comes in and many,
many more come out. I'd love to believe that
could be inferred, but
Re: Is Eric right, or am I? Young's experiment and photomultiplier 1
Re: Is Eric right, or am I? Young's experiment and photomultiplier
hey, if you could make those logical leaps on
your own we wouldn't be
here.
Eric, what you are desicribing is a totally different device
concept
quite different from than that used in a conventional
photomultiplier,
which is an electron tube with a light sensitive photocathode,
which
when struck by a photon emits electrons. (The photocathode
element of
the tube acts as a conventional photoelectric type device.)
The
difference between a photoelectic tube and a photomultiplier
tube
(PMT) is that the PMT contains a string of "dynodes"
(typically 8 or
more) that repeatedly multiply the electron(s) emitted by the
photocathode in a cascaded avalance type electron
multiplication.
So how is that different than the function of a Geiger−Muller style
detector?
First, sensitivity by a factor approaching 10^8.
Relative to a GM tube?
Second, a scintillation cell combined with a phtomultiplier detect
particles (neutrons, just as one example), while a GM tube detects
only gamma, beta, and weakly alpha radiation.
The principle is /exactly the same/, only the mechanism for
accomplishing the electron cascade is different.
I note that your original post did not even mention cascading
electrons, so perhaps it is you who needs being slapped in the head by
a book. True that GM tubes function by ionizing radiation ionizing the
gas contained within a GM detector, and in turn this causes a
cascading of electrons within the ionized gas.
Re: Is Eric right, or am I? Young's experiment and photomultiplier 2
Re: Is Eric right, or am I? Young's experiment and photomultiplier
A photomultiplier tube, OTOH, contains no gas, detects only photons,
no ionizing radiation (unless you consider the near−UV to be ionizing
radiation, and its dynodes multiple the incident electrons by what I
believe is terned "secondary emission". So, in fact when the primary
electrons emitted from the photocathode strike the first dynode, they
are multiplied by secondary emission and, because of the potential
differences that exit between the dynodes, are attracted to the 2nd
dynode, causing more secondary emission....and so on until the final
dynode and ultimately the anode is reached. This form of cascading has
essentially no conceptual physics connection to what happens in a GM
detector.
I disagree.
The mechanism is different but the idea is the same − the incoming
radiation comes in and knocks off an electron in the detecting medium.
The electron then comes off and is accelerated by the potential in the
apparatus. How the cascade is formed is different, but I don't think
the overall concept is different.
http://en.wikipedia.org/wiki/Secondary_emission
Different mechanism for the same idea − you want to take the small bit
of signal and amplify it.
I had a paragraph written explaining that the function can be
duplicated by a GM tube but it seemed a little tangential to the topic
at hand so I nixed it.
Good choice.
[...]
My guess is that you might be describing the operation of
scintillation media, and not the photomultiplier (PMT) iteslf.
Re: Is Eric right, or am I? Young's experiment and photomultiplier 3
Re: Is Eric right, or am I? Young's experiment and photomultiplier
All I was describing was my current understanding of how a generic
photomultiplier worked. My understanding is that current
photomultipliers use scintillator style detectors and that a tube
style detector was a slightly specialized GM tube.
Sorry, no!!! Your trying to describe how a scintillation cell
operating in concert with a photomultiplier works, while getting the
details a bit confused, which confuses readers who are not physics
students even more than yourself.
Which parts am I getting confused? I mean, other than how the
amplification part works − I know I got that wrong. But I'm pretty
sure I'm not mangling the function of a scintillator cell.
Perhaps you may want to be more specific by identifying the
device
that you are describing?
Just a generic photomultiplier.
But your posts make it appear that you don't know what a "generic
photomultiplier" is, or how it functions. Since I generally view you
as a pretty bright guy, this surprised me. Perhaps you would be well
advised to spend more time in the lab and less time in the books. If
you don't want to do that, at least take a look at the links that I
posted.
I don't know what I have not learned. I made an assumption about how
PMTs worked, and I was wrong.
[...]
OTOH I'll probably learn something here, despite
the worthlessness of the stuff that started it.
Eric, you should now understand what a "photomultiplier" is, and how
it functions.
For myself, your post raised cursiousity in my mind about how a
scintillation crystal or other form of scintillation device converts
Re: Is Eric right, or am I? Young's experiment and photomultiplier 4
Re: Is Eric right, or am I? Young's experiment and photomultiplier
impinging radiation (including neutrons) to light that is readily
detectable down to nearly the level of an individual photon by use of
a photomultiplier tube.
Semiconductor band gap. The incoming radiation excites the electrons
in the semiconductor crystal, which then emits light for the rest of
the detector to amplify into signal.
Everybody wins.
Harry C.
p.s., Perhaps why I am somewhat familiar with this subject is because
I am an 'old fart' and remember seeing how Young's Experiment was
demonstrated to student in undergraduate physical optics courses, plus
hand−on experience in building scintillation based neutron detectors
while still a coop student at Drexel/Princeton. (the Princeton−Penn
accelerator at Forrestal Research Center, after a years student coop
assignment at RCA Laboratories). I was one of the lucky ones on the
coop program assignments! Trust me, I'm not that intelligent, but do
have a very good memory and have had a life−long quest for knowledge.
Being willing to learn is enough, imho.
.
Re: Is Eric right, or am I? Young's experiment and photomultiplier 5