Techniques of the FLASH Thin Target Experiment Page: 4 of 7
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at SLAC. A nitrogen laser  was mounted at a
distance of approximately 2 m from the chamber.
It injected a ~ 160pJ beam pulse of 4 ns at a few
Hz into the chamber along the electron beam axis.
The light beam intensity was decreased by an aper-
ture which was mounted on the beam port facing
the laser. Thus the laser beam was confined to the
center of the chamber and its size was similar to the
size of the e- beam. Because the Rayleigh scattering
signal from air molecules is in the range of ~ 10-6 of
the beam, it is necessary to take great care to sup-
press any source of off-axis laser light. The scattered
light passed through the baffled detector arms, was
reflected by the UV enhanced aluminum coated mir-
ror, passed through a filter or the clear gap in the
filter wheel, and finally reached the PMT. The sig-
nals of the two photomultiplier tubes were digitized
with the ADC system used at the FFTB. Simul-
taneously with the PMT signals, the energy of the
outgoing laser beam was measured by a pyroelectric
energy probe  installed on the opposite side of
the chamber. The 25 mm diam. probe, suitable for
pulsed beams of this type, had a calibration uncer-
tainty of 5%, the largest individual contribution to
the yield uncertainty. Based on Rayleigh scattering
calculations as discussed in  and taking into ac-
count that fluorescence light is emitted isotropically,
it then was possible to calculate the number of ADC
counts per isotropically emitted 337 nm photon per
meter. In order to be able to take measurements at
different pressures inside the chamber, coated glass
windows with close to 100% transmission efficiency
in the UV range were attached to both beam ports.
The thin target chamber was connected to a vac-
uum pump and a pressure gauge. Data were taken
at several different pressures between vacuum and
atmospheric pressure, and a linear fit,
NADC - Need
G - P+ ko
was performed to the data, varying the fit param-
eters G and ko. Here, NADC is the signal counts
recorded for each PMT, Ne6d is the number of
pedestal counts measured in the respective signal
channel, E is the laser pulse energy, P, and T are the
pressure and temperature measured in the chamber,
and ko accounts for the light background from scat-
tering of the laser beam with the chamber material.
S 4.3- 107photons is the expected Rayleigh scat-
tering rate of 337.13 nm light calculated from the
expressions in  for standard pressure (760 torr),
and temperature (288.15 K). After the X2 minimiza-
. 'U - 20
C 30 15
* 25 1.
0 2500 5000 7500 10000 0 2500 5000 7500 10000
Fig. 2. PMT response against Rayleigh scattering intensity,
controlled by changing air pressure. The plots represent equa-
tion 1. The left plot shows the scattering of the data during
each of the pressure runs. The right plot shows the fit to the
mean values of each run.
tion, G represents the calibrated number of ADC
counts per isotropically emitted photon per meter
at 337 nm. Data taken at twelve different pressure
points for the clear aperture is displayed in Figure 2.
It was found that the signal strength, normalized
to the laser intensity, rose linearly with pressure, as
expected from Rayleigh scattering. The intercept at
the vacuum setting corresponded to the background
from errant laser rays. The slope represents G.
Because the detector was moved from the elec-
tron beam line to carry out laser studies, its light
sensitivity was compared in both settings by using
the built-in LEDs. Variations among the LEDs led
to a significant uncertainty contribution of 2.5%,
while possible thermal differences associated with
radiation shielding contributed another 1.1%. Fur-
ther systematic studies involved deriving the filter
transmission efficiency of the HiRes filter at 337 nm
from the Rayleigh scattering calibration of the setup
with the clear aperture and the HiRes filter in place.
The resulting difference in the calibration factors
was compared to results from spectrophotometer
measurements performed before the installation of
the apparatus at SLAC. A systematic uncertainty
of 1.8% has been assigned because of this filter con-
sistency check. Finally, the fit shown in Fig. 2 was
repeated while excluding the lowest pressure point.
The resulting deviation from the original value of G
Since the fluorescence light has a broad range
of wavelengths, the system calibration must be
extended at least over the ranges relevant to the
PMT sensitivity and the HiRes filter material, ap-
proximately 300 - 420nm. This was performed by
using a broad-band mercury lamp as a source for
a monochromator, where the wavelength was se-
lected with a precision of 0.5 nm. The light from the
monochromator was monitored consecutively by
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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/m1/4/: accessed April 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.