Initial Design Calculations for a Detection System that will Observe Resonant Excitation of the 680 keV state in 238U Page: 4 of 9
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238U foil
lead
68O keV >
light
Fig. 1.- Illustration of the experimental setup for
detecting NRF in 238U. Here the 680 keV light is
produced by a laser/linac source (Barty & Harte-
mann 2004) and is expected to have an energy res-
olution of about 1 percent. Two types of photons
result from the interaction of incident light with
the uranium foil. NRF photons are produced as
resonantly excited nuclei decay. This process pro-
duces photons with energies of 680 keV and 635
keV. Background photons come from interactions
between incident photons, electrons and atoms in
the sample foil. A lead attenuator is used to filter
out these unwanted photons. A segmented array
of BGO detectors observes photons that survive
the passage through the lead.from other processes and just contribute unwanted
background. A large portion of this background is
comprised of low energy photons. To filter this
out a piece of lead is placed between the foil and
the detector. Since lead preferentially attenuates
low energy photons it can be used to lower back-
grounds while still preserving a good fraction of
the wanted signal.
2.1. Characteristics of the Light Source
2.1.1. Energy Resolution
This beam has some photons within the energy
window capable of resonantly exciting the 680 keV
state in 238U and some photons outside of this
window. The width of the window is determined
by thermal motion of the uranium atoms and is
approximately (Metzger 1956)
Thermal = 0.34 eV. (1)
The width of the incident beam is denoted by
Fbeam. Calculations show that the energy reso-
lution of the beam should be in the sub-percent
range, which implies
Beam < 10 keV. (2)
This is very small compared to the resolution char-
acterizing Bremsstrahlung beams, but is still large
compared to the width in eq. 1 characterizing nu-
clear resonance fluorescence (NRF).
2.1.2. Temporal Characteristics
Temporal characteristics of the light source
place demanding constraints on the detection sys-
tem. This is because the light source does not
emit continuously but instead puts out pulses
with a duration of about one picosecond. Fea-
sible detectors cannot resolve signals this short -
all photons coming within a picosecond window
will be summed together. Since this summing
confuses identification of resonant photons the
system must be designed so that each detector
only observes one or a few photons during each
light source pulse.
2.1.3. Intensity
First estimates suggest that the light source
used in the experiments will produce about 109
photons per pulse. With an energy resolution of2
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Pruet, J. & Hagmann, C. Initial Design Calculations for a Detection System that will Observe Resonant Excitation of the 680 keV state in 238U, report, January 26, 2007; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc881406/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.