I know that the APD's that are designed as drop-in PMT replacements
use linked arrays of smaller APDs so you can have large surface area
but still keep the capacitance down. SensL has some kind of
fast-quench circuit on their array so it quickly dumps its charge
before the next event.
Craig
On Thu, Mar 11, 2010 at 1:28 PM, James Pawley <[log in to unmask]> wrote:
> From the Zeiss Brochure:
>
> The new GaAsP detector technology is not confined to visual
> light excitation. The LSM BiG (binary GaAsP) now also offers
> you multicolor multiphoton imaging with GaAsP performance.
> Two LSM BiG modules can be added to NLO systems
> as transmitted and incident light NDDs, providing 4 ultrasensitive
> detection channels. The LSM 780 NLO and LSM 710 NLO
> let you penetrate deeper and detect more light.
>
> So additional to the internal GaAsP spectral detector you can add a GaAsP
> NDD (BIG detector) for multiphoton imaging. And you can have the GaAsP
> single channel sitting directly over the objective. I think you can also
> couple two GaAsP detectors on an external port of the scan head
>
> best wishes
>
> Andreas
>
> Thanks Andreas,
> Does anyone have any idea of the maker or model number of these BiG
> detectors?
> As I mentioned earlier, the count-rate performance of APD single-photon
> counting systems are limited by the capacitance of the sensor diode. This is
> in turn proportional to the the sensitive area of the detector.
> For this reason, the sensitive areas tend to be small: maybe 100µm diameter.
> As this is much smaller than the ray bundle coming out the back of most
> high-mag, high-NA objectives, it is not obvious how one could "squeeze" the
> light bundle there to match the sensitive area. So I assume that the word
> "big" in your post is relevant and it would be nice to know how big and if
> it works in the counting mode, and whether it is an avalanche diode or
> merely a GaAsP photodiode (no amplification, but no multiplicative noise
> either. Very suitable for transmitted detectors....).
> Cheers,
> Jim P.
> **********************************************
> Prof. James B. Pawley, Ph.
> 608-263-3147
> Room 223, Zoology Research Building, FAX
> 608-265-5315
> 1117 Johnson Ave., Madison, WI, 53706
> [log in to unmask]
> 3D Microscopy of Living Cells Course, June 12-24, 2010, UBC, Vancouver
> Canada
> Info: http://www.3dcourse.ubc.ca/ Applications due by March 15,
> 2010
> "If it ain't diffraction, it must be statistics." Anon.
>
> -----Original Message-----
> From: RICHARD BURRY <[log in to unmask]>
> To: [log in to unmask]
> Sent: Wed, 10 Mar 2010 18:54
> Subject: Re: Zeiss or Olympus
>
> Jim
>
> Thanks for the details of the GaAs and GaAsP detectors. But my question
> remains, does Zeiss use either of these detectors in its 780 multiphoton?
> And does this represent an improvement in S/N over standard PMTs? My
> experience is with the NLO a while ago is that the GaAs detector was more
> sensitive but for samples with a range of emission intensity, it was
> difficult to not saturate some part of the sample.
>
> Dick
>
> ----- Original Message -----
> From: James Pawley <[log in to unmask]>
> Date: Wednesday, March 10, 2010 11:58 am
> Subject: Re: Zeiss or Olympus
> To: [log in to unmask]
>
>> Just to clarify, the 780 has a GaAsP (Gallium Arsenite Phosphate)
>> detector, not GaAs, the difference in quantum efficiency can be seen e.g. in
>> the Webb multiphoton review (Nature Biotechnology 2003, 21, 1369). The
>> drawback is that GaAsP QE drops dramatically for wavelength > 700 nm, but
>> they put a normal PMTs as the two additional channels on the 780, to cover
>> the extended range. By the way GaAsP detectors are PMTs as well, it is just
>> a different material of the photocathode, afterwards the photoelectrons are
>> multiplied in the same way. GaAsP detectors reach 40% quantum efficiency
>> which is about twice as sensitive as a normal PMT. APDs have 60-70% and a
>> back-thinned CCD about 90%., so still a lot of signal is thrown away, not to
>> mention the losses on the way to the detector.
>
>
>> Andreas
>
>> Hi all,
>
>
>> Indeed, the GaAs and GaAs phosphide QE curves are very impressive.
>> However, it is important to remember what is actually measured to make these
>> curves. PMT curves refer to the fraction of photons striking the
>> photocathode that produce photoelectrons (It is usually measured by allowing
>> a calibrated amount of light to strike the photocathode and using a
>> nano-ammeter to sense the total photoelectron current between the
>> photocathode and all the other electrodes in the PMT). However, depending on
>> the electrode geometry, 10-30% of these photoelectrons don't actually hit
>> the first dynode (D1), and therefore do not contribute to the PMT output.
>
>> Of those photoelectrons that do hit D1, a reasonable fraction fail to
>> excite any secondary electrons, and again, do not contribute to the PMT
>> output. There are many reasons for this but one is just Poisson statistics.
>> If the average gain is say 4, then about 8% of the collisions will result in
>> zero electrons being emitted. However, this effect is again multiplied by
>> geometrical factors where SE produced in different parts of D1 have better
>> or worse chances of actually striking D2 and producing a SE. Signal loss in
>> this way depends a lot on the actual voltage between the photocathode and
>> D1: it will be less when the voltage is higher. Unfortunately, few confocals
>> seem to have been set up in such way that this is always true. On average
>> signal loss by failure to propagate after collision with D1 will be an
>> additional 20-40%.
>
>
>> Finally, the same type of Poisson effects that cause some signal to be
>> lost entirely, cause the amount by which the remainder is amplified to be
>> highly variable (10-90%). This variation degrades the accuracy of the output
>> signal by introducing what is called multiplicative noise. Because this
>> extra noise can only be "overcome" by counting more photons, its presence
>> effectively reduces the effective QE of the device. In the best case, this
>> reduction is about 40% and in the worst case (an exponential gain
>> distribution, approximated by some micro PMTs) 75% (i.e., the QE is reduced
>> to 60% or 25% of what it would have been if all photoelectrons were equally
>> amplified).
>
>
>> As a result, while the peak effective QE of a PMT with a GaAs or GaAsP
>> photocathode is indeed much better than that of the more common S-20
>> photocathode, in terms of its effectiveness in providing an output current
>> that is proportional to the input photon signal, the QE is more in the range
>> of 3 -10% (depending on dynode geometry and first-dynode voltage) than 40%.
>> (The 60% numbers are for APDs rather than for a GaAs or GaAsP photocathode
>> on a PMT.)
>
>
>> The performance can be improved somewhat on the few confocals that allow
>> single-photon counting as this procedure eliminates multiplicative noise.
>> (see below about the limitations imposed by photon counting)
>
>
>> This tedious recital is I hope justified by noting that, at least when
>> EG&G was the major APD supplier, APD performance was not specified in terms
>> of QE but as Photon Detection Efficiency (PDE). Although APDs can be
>> operated in a low gain, proportional mode, their PDE under these conditions
>> is very low (because APD multiplicative noise is very high and at low
>> (non-avalanche breakdown) gain, by far the most likely gain of the initial
>> photoelectron is zero).
>
>
>> Therefore, high PDE (or high QE) AOD units tend to operate at high bias
>> (high, avalanche gain) and this requires circuitry to quench the avalanche
>> breakdown and count the single-photon pulses. Modern units contain both the
>> sensor itself and the pulse counting and avalanche quenching circuits needed
>> for counting the single-photon pulses. In other words (assuming that
>> Hamamatsu follows the EG&G precedent), their QE figures for single-photon
>> counting units already include any losses for non-propagation or
>> multiplicative noise. Therefore, a quoted PID of 60% really does mean that
>> 60% of the photons (of the specified wavelength) that strike the center of
>> the active surface will be accurately counted.
>
>
>> This is about 4-10x better than the performance of a similar GaAs or GaAsP
>> photocathode on a PMT.
>
>
>> This good news is tempered by the fact that, because of the high
>> capacitance of the AOD itself, it is hard to count much faster than, say
>> 30MHz. As 30MHz comes out to an absolute maximum of 60 counts during a 2 µs
>> pixel, this means that at least 50% of your counts will be lost due to pulse
>> pileup when 30 counts arrive per pixel and 10% will be lost at only 6
>> counts/pixel. In other words one has to be very careful to adjust the
>> excitation intensity so as not to "clip" the brightness of those parts of
>> the image that contain a lot of fluorophor.
>
>
>> Lots more on this in The Handbook,
>
>
>> Cheers,
>
>
>> Jim Pawley
>
>
>> **********************************************
>
>> Prof. James B. Pawley, Ph.
>> 608-263-3147
>> Room 223, Zoology Research Building, FAX
>> 608-265-5315
>> 1117 Johnson Ave., Madison, WI, 53706
>> [log in to unmask]
>> 3D Microscopy of Living Cells Course, June 12-24, 2010, UBC, Vancouver
>> Canada
>> Info: http://www.3dcourse.ubc.ca/ Applications due by March 15,
>> 2010
>> "If it ain't diffraction, it must be statistics." Anon.
>
>> Just to mention, should one be stuck with PMTs instead GaAs, one could
>> play with the applied bias voltage to modify dark noise (to
>
> balance the gain versus noise. )
>
> Nice thing with Olympus Kalman filtering is that its use would allow
> increase the bias voltage of PMT
>
>
>
>
>
>
>
>
>
> Thanks
>
>
>
> Axel Central Imaging (IMCB) 6-19B, cell +65 9271.5622
>
>
>
> ________________________________
>
> From: Confocal Microscopy List [mailto:[log in to unmask]] On
> Behalf Of RICHARD BURRY
>> Sent: Wednesday, March 10, 2010 8:17 AM
>> To: [log in to unmask]
>> Subject: Re: Zeiss or Olympus
>
>
>
> Mark
>
>> I was thinking about the use of GaAs ( gallium asrenide) detectors for
>> multiphoton by Zeiss for the NLO. This are not a PMT and have very
>> different properties.
>
>> Dick Burry
>
>> Note: This message may contain confidential information. If this Email/Fax
>> has been sent to you by mistake, please notify the sender and delete it
>> immediately. Thank you.
>
>
>
>
>> -- >
>
> **********************************************
>> Prof. James B. Pawley, Ph.
>> 608-263-3147
>> Room 223, Zoology Research Building, FAX
>> 608-265-5315
>> 1117 Johnson Ave., Madison, WI, 53706
>> [log in to unmask]
>> 3D Microscopy of Living Cells Course, June 12-24, 2010, UBC, Vancouver
>> Canada
>> Info: http://www.3dcourse.ubc.ca/ Applications due by March 15,
>> 2010
>> "If it ain't diffraction, it must be statistics." Anon.
>
> ________________________________
>
>> Spam
>> Not spam
>> Forget previous vote
>
>
> Richard W. Burry, Ph.D.
> Department of Neuroscience, College of Medicine
> Campus Microscopy and Imaging Facility, Director
> The Ohio State University
> Associate Editor, Journal of Histochemistry and Cytochemistry
> 277 Biomedical Research Tower
> 460 West Twelfth Avenue
> Columbus, Ohio 43210
> Voice 614.292.2814 Cell 614.638.3345 Fax 614.247.8849
>
> --
>
> **********************************************
> Prof. James B. Pawley, Ph.
> 608-263-3147
> Room 223, Zoology Research Building, FAX
> 608-265-5315
> 1117 Johnson Ave., Madison, WI, 53706
> [log in to unmask]
> 3D Microscopy of Living Cells Course, June 12-24, 2010, UBC, Vancouver
> Canada
> Info: http://www.3dcourse.ubc.ca/ Applications due by March 15,
> 2010
> "If it ain't diffraction, it must be statistics." Anon.
|
