Picotron 100 streak tubes as a 150-channel photometer Page: 3 of 3
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When a photocathode diameter of 10 mm is used, the FWHM of the best -esolvable Lpot
deteriorates to 30 microns FWHM at the edge of photocathode. By adjustrmnt of focus
voltages, static and dynamic resolution across a photocathode diameter of 10 mm can be kept
uniform at 750 at FWHM. This tube therefore shows the potentiality of recording more than
100 channels simultaneously, with a dynamic range of greater than 5000 at 60 picoseconds
(if used wich a suitable intensifier and film for recording).
Figure 6 shows the static MTF of the Picotron 100 (large tube) which ir kept constant
over an image diameter of 9 mm, showing a limiting resolution of 25 lp/mm at the photoca-
thode, at 4% MTF, taken when the tube is operated at a magnification of 1.3:1. The dynamic
resolution of this tube, measured with a charnel plate intensifier and Kodak 2475 film,
described before is now compared with the dynamic range at 30 picoseconds, using direct
lens coupling to a PAR OMA-1 having an overall system gain (including the collection
efficiency and magnification) of 50. The results are shown in Fig. 7. It can be seen that
a dynamic range of > 1000 at 30 ps is well within the limit of this tube. At 30 picoseconds
with tie results obtained with an OMA-1, the slit width was 100 0 on the photocathode and
it was 15011 at the screen. The dynamic and static measurements of the FWHM of the streak
image at the screen showed a change of resolution, see Fig. 8. The dynamic resolution
of this tube wocid allow it to be used as a 180 channel streak tube.
Dynamic range and SNR
The dynamic range is defined as the range of intensity over which the streak tube
functions linearly. This is a function of the time resolution of the tube.5'" The upper
end is defined as the level of intensity where the instrument shows an apparent broadening
of a pulse of known duration by 20%. The lower end can be defined as a signal level at
which the pulse width of an input signal can be detected at a signal to noise ratio (SNR)
of 1, over the threshold detector noise.
Recording on film from the streak tube directly: Threshold signal per pixel on the
photocathode is defined by an equivalent density of 0.2 on film. This is defined (for
Kodak 2475 film) as the point where SNR is 1. The light level involved is much higher
than that required by photoelectron statistic, limited signal at the first photocathode.
The light level at the film plane is equivalent to 5x10~1 J/cm' at this exposure. At a
streak velocity of 30 ps per mm on a phosphor screen, the threshold current density in the
picotron 100 (large format) tube with an S-1 photocathode was 50 ma/cm' (10 1IA/W photo-
cathode sensitivity), for 30 ps laser pulses of 1:06 micron wavelength. The pulse showed
broadening at an input photocurreat density of 400 ma/cm'.
For a pixel size of 501 radius, this is equivalent to an electron density of 730 per
pixel, at the streak tube photocathode, at threshold recording, without any intensification.
The electron densit- at the level of input intensity where a pulse broadening of 203 is
observed, was ca-culated to be 5840 electrons per pixel.
A simple calculation shows that if the intensification in a streak tube was adequate to
enable single photoelection detection, a dynamic range in excess of 5000 should be within
the linear range of operation of a streak tube of the Picotron 100 or Picotron 10011 type.
The linearity of most type of intensifiers are poorer than a range of 100 to 1. It is
therefore important to point out that a streak tube based picosecond photometer must have
a full calibration of the streak tube, the intensifier and the recording medium.
Conclusions
The capability of the Picotron 100 and the Picotron 1005 to operate as 180 and 100
channel (respectively) photometer for picosecond time resolution is demonstrated.
The streak tubes are charo.cterised for picosecond photometry in terms of number of
photoelectrons per pixel at the streak tube photocathode. These results were obtained
without using any nost-streak intensification as such post streak intensification appear
to confuse the quantitative evaluation at present. It is expected that with comparable
characterization of the intensif ie: , we shall be, able to demonstrate a picosecond phot ometer
with single photeelectron detection capability.
The dynamic range of the Picot-on type streak tubes, at 30 picosecond, and wit hout
intensifiers is shown to be 8, which indicates our claim that this type of tube will resolve
40 picob"econds with a dynamic range in excess of 1000 when used with suitable intensifier.
The dynamic range at 4 picoseconds has previous ly been demonstrated to he 120.
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Majumdar, S.; Weiss, P.B. & Black, J.P. Picotron 100 streak tubes as a 150-channel photometer, article, January 1, 1982; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1055593/m1/3/: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.