Techniques of the FLASH Thin Target Experiment Page: 3 of 7
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systematically different from other approaches, and
have allowed a substantial reduction in overall un-
2. Experimental Method
There were several differences between the tech-
niques implemented in this work and other experi-
ments. In the first place, a pulsed, high energy elec-
tron beam was used, where the majority of previ-
ously published electron-induced fluorescence mea-
surements have used a radioactive source. Some ad-
vantages are that a) the monochromatic, penetrat-
ing, beam is easy to model, b) the fiducial light
emission length is well defined, c) light signals can
be strong, and statistics collected quickly, and d)
photomultiplier dark counts are excluded by timing.
A disadvantage is that stray radiation backgrounds
must be reduced by shielding, and their remaining
level must be monitored.
The FLASH (Fluorescence in Air from Show-
ers) experiment was carried out at the Final Focus
Test Beam (FFTB) facility at SLAC, operated at
28.5 GeV with pulses 3 ps long at 10 Hz. An air
gap was provided in the beam vacuum line, with
50 micron thick stainless steel beam windows. The
electron trajectories were effectively parallel, and
the beam spot widths were typically ~ 1 mm.
Measurement of the FFTB beam intensity was
improved substantially for the intensity range of this
experiment. A beam toroid was coupled through a
short cable to a purpose developed amplifier and
bandpass filter , and sent outside the radiation
enclosure for digitization on every beam pulse. By
measuring the response to pulses of charge, on a
wire simulating the electron beam, the calibration
has been established with an uncertainty of 2.7%.
The fluorescence apparatus is illustrated in Fig. 1.
The electron beam entered and left the gas volume
through 25 micron aluminum pressure windows.
The volume was a 25 cm long, 15 cm diameter cylin-
der. A pair of thin, blackened, aluminum tubes,
1.6 cm diameter, coaxial with the beam, and with
a 1.67 cm gap between them, acted to define the
measurement length for fluorescence light, while
suppressing background from the forward-emitted
Cherenkov light. There were two light channels,
heavily baffled against scattered rays, extending
radially from this gap. The pressure volume was ter-
minated with 1.2 cm diameter fused silica windows,
placed at 45 cm from the beam. This distance was
Fig. 1. The experimental setup.
as large as practical in order to eliminate Cherenkov
light production in the fused silica. A right an-
gle reflection, at a UV-enhanced aluminum-coated
mirror, was then used before the light reached the
photomultiplier tubes. This allowed the installation
of heavy lead shielding to protect the tubes from
radiation scattered directly from the beam.
In front of each PMT face was a remotely rotat-
able filter wheel with a sample of the filter material
used in the HiRes telescopes, a thin opaque sheet
used to study backgrounds, and a clear gap. In ad-
dition, there were narrow-band filters used for data
still under study. Ultraviolet LEDs were used to help
monitor stability in the photonics system. Four were
placed in front of the PMT face, outside the fluores-
cence optical envelope. One was mounted on-axis, in
a baffled tube diametrically opposite the light col-
Enclosed in the same shielding as the active pho-
tomultipliers, were similar tubes with their photo-
cathodes optically hooded. These were used as a
continuous monitor of signals from radiation pene-
trating the shielding.
From outside the beam radiation enclosure, the
system could be filled to a selected sub-atmospheric
pressure with dry air, filtered moist air from the
atmosphere, or, for systematic checks, with nitrogen
which fluoresces much more strongly than air, or
ethylene which fluoresces very weakly. The pressure
settings used for data taking were in the range 10 to
3. Optical Calibration and Simulation
For the optical calibration, the thin target cham-
ber (fully assembled) was installed in an environ-
mental chamber in a laboratory at the University of
Utah. Using a temperature controller the tempera-
ture in the environmental chamber was kept at the
average temperature measured in the FFTB tunnel
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Abbasi, R.; Abu-Zayyad, T.; Belov, K.; Belz, J.; U., /Utah; Bergman, D.R. et al. Techniques of the FLASH Thin Target Experiment, article, October 30, 2007; United States. (digital.library.unt.edu/ark:/67531/metadc890745/m1/3/: accessed October 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.