[ RadSafe ] Beat phenomena and health physics
Mattias Lantz
Mattias.Lantz at physics.uu.se
Wed Feb 25 03:10:28 CST 2015
A minor correction regarding dead time.
Ed Tupin wrote: "once a photon has interacted with the detector crystal,
the crystal is "dead" and doesn't react with another photon until it has
recovered."
It is not the crystal itself that becomes "dead", the physical processes
(photoelectric effect, Compton scattering, pair production, etc)
continue within the crystal independently of any earlier events.
It is the entire detector system (crystal, semiconductor or other
material where energy from the absorbed ionizing radiation is converted
into an electric signal that needs to be amplified and sent to a data
acquisition system) that becomes "dead" as it is busy with handling the
first event. Depending on the system there can be different parts that
dominate the dead time.
For a Sodium-Iodide (NaI) or Cesium-Iodide (CsI) crystal with
photo-multiplier the full peak height is reached within fractions of a
micro-second, but then there is a tail that is several micro-seconds
long, see for instance the following figure:
http://inspirehep.net/record/556128/files/fig8a.png
For plastic detectors the pulses are shorter, of the order of
nano-seconds. Other detectors may be much slower.
Any absorbed radiation before the tail has gone back to the base line
will have its peak superimposed on the tail of the earlier event. If
that peak is read out independently its energy will be recorded as
higher than what it actually is. And as Ed Tupin said, if they are close
enough in time they will be summed together to one event with an energy
corresponding to the sum of the two events.
Usually the data acquisition system, i.e., the system to get the signal
from the photomultiplier into some sort of pulse height spectra or
counter, will be blocked once the first pulse arrives. The time to
process that pulse may be several hundred micro-seconds or even
milli-seconds, depending on the system. That is usually the dominating
part of the system dead time.
Regarding beat phenomena in gamma detectors I agree with Bill Prestwich.
Each photon is coming from a random event, they are not synchronized
like sound waves or electromagnetic waves from some sort of oscillating
source. Furthermore the absorption within the detector material is also
a random event depending on probabilities (cross sections), where it is
not very likely to have two photons absorbed by the same molecule within
the crystal. Thus, to my understanding, terms like beat, interference,
harmonic intervals, etc, are not of relevance at all.
The energy resolution of a detector system depends on its
characteristics. The following link shows comparisons of the width of
the peaks from the same gamma source with different detectors:
http://www.ortec-online.com/Products-Solutions/Hand-Held-Radioisotope-Identifiers-HPGe-Proposition.aspx
The reason is the uncertainty in the process of creating and collecting
the electrons that give the electric pulse, and the detector material
itself will also play a role (for instance better collection with heavy
elements than with plastic). For a crystal with photomultiplier there
are several steps (conversion of absorbed photon energy to light,
scattering of light within the crystal detector, collection efficiency
of light on the photo-cathode, amplification of number of electrons in
the photo-multiplier, quality of amplifier), each with a certain random
factor. For a HPGe detector several of these steps are eliminated as the
absorbed energy is directly converted into an electric signal, thus the
uncertainty is lower and the resulting peak is better defined. If any
beat phenomena is possible it would be insignificant in comparison with
the other effect.
Best wishes,
Mattias Lantz
--
Mattias Lantz - Researcher
ランツ マティアス
Department of Physics and Astronomy
Division of Applied Nuclear Physics
Uppsala University, Box 516
SE - 751 20, Uppsala, Sweden
email: mattias.lantz at physics.uu.se
On 02/25/2015 06:11 AM, Edward Tupin wrote:
> One of the factors that minimizes/eliminates the "beats" in health physics spectroscopy is the dead time phenomenon in the detectors.
> that is, once a photon has interacted with the detector crystal, the crystal is "dead" and doesn't react with another photon until it has recovered. Thus it would be rare for beats to show up in gamma spectra. Sum peaks do occur - when two photons hit the detector at the same time, the energy is additive and produces a peak that shows at the sum of the two energies in the component peaks. Ed Tupin
> Cell: 202-631-3307
> From: "radsafe-request at health.phys.iit.edu" <radsafe-request at health.phys.iit.edu>
> To: radsafe at health.phys.iit.edu
> Sent: Tuesday, February 24, 2015 1:00 PM
> Subject: RadSafe Digest, Vol 1781, Issue 1
>
>
> Hi Joe,
>
> I think, since photons are emitted randomly, that the interference effects
> that occur with electrically produced sinusoids are not significant.
>
> Bill
>
> -----Original Message-----
> From: radsafe-bounces at agni.phys.iit.edu
> [mailto:radsafe-bounces at agni.phys.iit.edu] On Behalf Of JPreisig at aol.com
> Sent: Sunday, February 22, 2015 10:57 PM
> To: radsafe at agni.phys.iit.edu
> Subject: [ RadSafe ] Beat phenomena and health physics
>
> Dear Radsafe,
>
> The phenomena of beats (see your undergraduate text in
> physics/mechanics) involves what happens when 2 or more fundamental
> frequencies of a system are close in frequency (or Energy) to one another.
> Two such sinusoidal signals (or more) produce sum and difference
> frequencies, which can be important in problems.
>
> I've been discussing beats concerning the fundamental wobble
> frequencies. I also said something perhaps about beats and the Sodium
> doublet.
>
> Another situation where beats might be important is in health physics
> and perhaps even physics. We Health Physicists measure gamma and other
> spectra using a MultiChannel Analyzer and similar equipment. There may be
> times when our gamma spectra have peaks that are close to one another and
> may produce beat (sum and difference frequency) peaks. Do we observe such
> peaks in our spectra??? Will these beat phenomena affect our physics
> results??? Quite possibly. These extra peaks may affect spectral
> stripping and other results.
>
> Remember, E = h x nu, where nu is the frequency in atomic and/or
> nuclear physics.
>
> Perhaps physicists doing sensitive experiments account for such beat
> phenomena. I don't remember hearing about beat phenomena in my Health
> Physics courses. I first heard about beat phenomena in a physics course or
> two. Then I heard about it again in geophysics research.
>
> Beat phenomena might affect our every day Health Physics work??? Oh
> my.
>
> I seem to remember Cesium-137 having closely spaced decay (energy)
> gamma peaks.???
>
> Regards, Joe Preisig
>
>
>
>
>
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>
>
> Radsafe/Bill,
>
> In Health Physics, the effects may not be significant. In certain
> sensitive physics experiments, the effects may be significant.
>
> Joe Preisig
>
>
>
>
> In a message dated 2/24/2015 9:24:48 A.M. Eastern Standard Time,
> prestwic at mcmaster.ca writes:
>
> Hi Joe,
>
> I think, since photons are emitted randomly, that the interference effects
> that occur with electrically produced sinusoids are not significant.
>
> Bill
>
> -----Original Message-----
> From: radsafe-bounces at agni.phys.iit.edu
> [mailto:radsafe-bounces at agni.phys.iit.edu] On Behalf Of JPreisig at aol.com
> Sent: Sunday, February 22, 2015 10:57 PM
> To: radsafe at agni.phys.iit.edu
> Subject: [ RadSafe ] Beat phenomena and health physics
>
> Dear Radsafe,
>
> The phenomena of beats (see your undergraduate text in
> physics/mechanics) involves what happens when 2 or more fundamental
> frequencies of a system are close in frequency (or Energy) to one
> another.
> Two such sinusoidal signals (or more) produce sum and difference
> frequencies, which can be important in problems.
>
> I've been discussing beats concerning the fundamental wobble
> frequencies. I also said something perhaps about beats and the Sodium
> doublet.
>
> Another situation where beats might be important is in health physics
> and perhaps even physics. We Health Physicists measure gamma and other
> spectra using a MultiChannel Analyzer and similar equipment. There may be
> times when our gamma spectra have peaks that are close to one another and
> may produce beat (sum and difference frequency) peaks. Do we observe such
> peaks in our spectra??? Will these beat phenomena affect our physics
> results??? Quite possibly. These extra peaks may affect spectral
> stripping and other results.
>
> Remember, E = h x nu, where nu is the frequency in atomic and/or
> nuclear physics.
>
> Perhaps physicists doing sensitive experiments account for such beat
> phenomena. I don't remember hearing about beat phenomena in my Health
> Physics courses. I first heard about beat phenomena in a physics course
> or
> two. Then I heard about it again in geophysics research.
>
> Beat phenomena might affect our every day Health Physics work??? Oh
> my.
>
> I seem to remember Cesium-137 having closely spaced decay (energy)
> gamma peaks.???
>
> Regards, Joe Preisig
>
>
> _______________________________________________
> You are currently subscribed to the RadSafe mailing list
>
> Before posting a message to RadSafe be sure to have read and understood the RadSafe rules. These can be found at: http://health.phys.iit.edu/radsaferules.html
>
> For information on how to subscribe or unsubscribe and other settings visit: http://health.phys.iit.edu
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