CALIBRATION AND BACKGROUND STUDIES FOR A LIGHT ...
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i CALIBRATION AND BACKGROUND STUDIES FOR A LIGHT DARK MATTER SEARCH Nan Ma A thesis submitted to the department of Physics at Occidental College in partial fulfillment of the requirements for the degree of Bachelor of Arts with Honors Physics Department Occidental College April 24, 2018
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Transcript of CALIBRATION AND BACKGROUND STUDIES FOR A LIGHT ...
i
CALIBRATION AND BACKGROUND STUDIES
FOR A LIGHT DARK MATTER SEARCH
Nan Ma
A thesis submitted to the department of Physics at Occidental College
in partial fulfillment of the requirements for the
degree of Bachelor of Arts with Honors
Physics Department
Occidental College
April 24, 2018
iii
ABSTRACT
Experiments are combined with GEANT4 (GEometry ANd Tracking) simulations to
predict recoil backgrounds in a DRIFT (Directional Recoil Identification From Tracks) beam
dump experiment. A series of benchmarking experiments measuring neutron attenuation have been
conducted to measure the efficiency of the DRIFT detector compared to GEANT4 simulation. The
calibration run measures an efficiency factor of 0.260 ± 0.003. No shielding and 11cm water
shielding runs demonstrates a consistency in the efficiency. However, the results from 20cm water
run suggest GEANT4 might fail in predicting low-energy neutron recoils, or the efficiency changes
in low-energy regime and therefore an efficiency map is necessary to compare the simulations with
direct measurements, or both. In addition, the results from neutron attenuation through concrete
suggest that the undefined composition LOI (Loss On Ignition) of concrete needs further
investigation. The background rate in DRIFT-IIf due to cosmic-ray protons, neutrons and muons
was predicted to be 5.0 ± 0.5 per day by GEANT4, while 5 days of background run with DRIFT-
IIf yields a rate of 3.1 ± 0.7. The rough agreement between simulation and experiment indicates
the success of GEANT4 in predicting cosmic-ray background for DRIFT detectors. Lastly, a
preliminary prediction for DRIFT-IIg beam-dump experiment at SLAC was established. Although
further improvements to the simulation are necessary to conclude the background rate, the results
set a lower limit of 2.6 ± 0.6 for a stainless-steel vacuum vessel, and suggest a possibility of using
an acrylic vacuum vessel to minimize the cosmic-ray background.
iv
TABLE OF CONTENTS 1 INTRODUCTION................................................................................................................. 1
1.1 The Existence of Dark Matter ......................................................................................... 1 1.2 Beam-Dump Experiment and DRIFT ............................................................................. 2 1.3 Research Methodology ................................................................................................... 5
2 TOOLS ................................................................................................................................... 7 2.1 The DRIFT-IIf Detector .................................................................................................. 7 2.2 Cf-252 Neutron Source ................................................................................................... 9 2.3 Platform......................................................................................................................... 10 2.4 GEANT4 ....................................................................................................................... 11
2.4.1 Simulating Cf-252 neutron source with GEANT4 ............................................................ 11 2.4.2 Simulating cosmic-rays with GEANT4 ............................................................................. 12
3 BENCHMARKING EXPERIMENTS .............................................................................. 14 3.1 Background of Benchmarking Experiment .................................................................. 15 3.2 Calibration and No Shielding Measurements ............................................................... 15
3.2.1 Experimental setup of calibration and no shielding run .................................................... 15 3.2.2 Results for calibration and no shielding runs .................................................................... 16
3.3 DRIFT-IIf with Water Shielding .................................................................................. 19 3.3.1 Experimental setup of neutron attenuation through water................................................. 19 3.3.2 Results for neutron attenuation through water .................................................................. 22
3.4 DRIFT-IIf with Concrete Bricks ................................................................................... 22 3.4.1 Experimental setup of neutron attenuation through concrete ............................................ 23 3.4.2 Composition analysis of concrete bricks ........................................................................... 24 3.4.3 Results for neutron attenuation through concrete .............................................................. 25
4 COSMIC-RAY BACKGROUND IN DRIFT-IIf ............................................................. 27 5 PREDICTING COSMIC-RAY BACKGROUND IN DRIFT-IIg .................................. 30 6 CONCLUSION ................................................................................................................... 33 7 ACKONWLEDGEMENTS ............................................................................................... 34 8 APPENDIX .......................................................................................................................... 35
8.1 Cf-252 SPECTRUM IN GEANT4 ............................................................................... 35 8.2 W-VALUE .................................................................................................................... 37 8.3 COMPOSITION REPORT OF CONCRETE ............................................................... 38 8.4 SIMULATION DATA FOR BENCHMARKING EXPERIEMNT ............................. 40 8.5 SIMULATION DATA FOR COSMIC-RAY BACKGROUND ................................. 41
References .................................................................................................................................... 43
1
1 INTRODUCTION
1.1 The Existence of Dark Matter Dark matter is the one of the most intriguing problems in physics. Its discovery can be
traced back to astrophysical measurements of the orbital velocities of spiral galaxies which were
shown to be significantly higher than predicted [1]. With such large orbital velocities, these stars
should have escaped from the spiral galaxies. Therefore, scientists inferred there must be unseen
matter in these spiral galaxies that forces the stars to stay in their rotation curves. Many other
observational data, such as the measurement of cluster mass based on gravitational lensing,
evidence from cosmic microwave background radiation, and formation of structure in the universe,
also point to the existence of dark matter [1].
The current standard model of cosmology, which explains almost all observations, and
many to the 1% level, suggests that 26.8% of the mass of the universe is dark matter [1]. In contrast,
the density of ordinary matter in universe, known as baryonic matter, is only 4.8 ± 0.3% of the
total mass and energy [2]. Despite its large percentage in the universe, dark matter has not been
observed directly so far because of, apparently, feeble interactions with normal matter [2].
Supersymmetry extends the Standard Model of particle physics, predicting the existence
of a Weakly Interacting Massive Particle (WIMP), one popular candidate for dark matter [3]. It is
named after the fact that WIMP is a massive particle (GeV – TeV), and only interacts with its
surroundings through gravity and a weak force. Direct detection of WIMPs involves searching for
WIMP recoils in low energy (~keV/amu) at very low rates. Many experiments have been looking
for evidence of WIMPs for decades, but none have been detected so far [2]. Absence of theoretical
and experimental support for WIMPs has focused attention to another dark matter candidate, Light
Dark Matter (LDM).
2
The theoretical motivation for LDM, which lies in the MeV-GeV mass range, originates
from the theory for dark sectors. Dark sector theory describes a group of particles that only interact
with normal matter via a new force mediated by a new, massive, gauge boson [4]. Out of those
particles LDM has been shown to be a promising dark matter candidate. Moreover, it is possible
to create such particles with high-intensity accelerators [4], as discussed below.
1.2 Beam-Dump Experiment and DRIFT
A beam-dump experiment is designed to produce and measure Light Dark Matter at
Accelerators (LDMA). As shown in Figure 1, multi-GeV electron beams generated with
accelerators first enter and then interact in a beam dump, which usually made of metal such as
copper or aluminum. The incoming high-energy electrons are capable of producing light dark
matter as shown in the accompanying Feynman diagrams with the help of a massive mediator A’.
A massive amount of shielding is required to stop other, normal particles, from reaching the
detector. Neutrinos will be produced in the beam dump and will make it through the shielding but
are not thought to produce a significant background. Because LDM particles produced in the beam
dump interact only feebly with normal matter, they will easily pass through the shielding and can
interact with the detector. Since overall possibility of LDMA interacting with nucleus is low, a
long-term run is necessary to collect LDMA recoil events.
3
Figure 1. Schematic of beam dump experiment [5]. The beam enters the dump from the left. Particles produced in
the beam dump such as electrons, muons and pions will be stopped by the shielding, while LDM and neutrinos can
get through and reach the detector. Neutrinos do not interact enough to cause a significant background. The left
Feynman diagram shows how the incoming electron beams scatters with a nucleus, producing a matter anti-matter
pair of LDM, in a bremsstrahlung-like process. The right Feynman diagram shows how the LDM scatters with a
nucleus in the detector, creating a recoil track and can therefore be detected.
Because LDMA only interacts with normal matter through a new force mediated by the A’,
a specialized detector is required to detect LDMA events. The Directional Recoil Identification
From Tracks (DRIFT) project has developed highly sensitive and low background detectors to
detect the directionally sensitive ionization created by recoils from WIMPs since 1998. The
Negative Ion Time Projection Chamber (NITPC) is DRIFT’s unique technique that utilizes
electronegative carbon disulfide, CS2 gas, for drifting ionization. CS2 is electronegative and,
therefore, captures electrons created by ionizing recoils [5]. Negative charge drifts as anions and
the diffusion is therefore minimized to the fullest extent possible allowing directional detection.
The DRIFT-IId, a 0.8m3 detector operated 1.3 km underground in England, was found to be
background-free for 54.7 days during a shielded run [6]. Overall, DRIFT has been shown to have
4
a longer drift distance, low background, and more importantly, a sensitivity to low energy recoils
in comparison with other directionally sensitive detectors [6].
Because the detector is not sensitive to the neutrinos, only LDMA events from the beam-
dump will be registered in the detector. One signature of LDMA recoils is the direction of the
recoils. Figure 2(a) shows the tracks of LDMA recoils simulated by SRIM (Stopping and Range
of Ions in Matter), while in Figure 2(b) with the present of cosmic-ray background, the recoils at
low energy are oriented randomly [7]. Thus, DRIFT is able to detect LDMA recoils when a
background is presented. With its unique directional detection of low energy recoils and low
background, DRIFT is an ideal detector for LDMA recoils.
Figure 2. (a): This figure shows SRIM simulation result of the LDMA tracks produced by the recoils without
cosmic-ray background. (b): The figure shows that the tracks of recoils oriented randomly due to the presence of
cosmic-ray backgrounds [8].
An experiment is therefore envisioned, combining the beam-dump experiment with DRIFT
project, to detect LDMA. One possible location being contemplated is the SLAC National
Accelerator Laboratory operated by Stanford University in Menlo Park, California. The
5
experiment would be in an unused tunnel (ESB tunnel) under approximately 6m of dirt and
concrete. At this shallow depth DRIFT will be subject to a background from cosmic-ray. Cosmic-
ray neutron, muon and proton backgrounds are well investigated on both Earth’s surface and deep
underground. However, currently there is insufficient neutron data at shallow depths. Thus, it will
be necessary to measure the in-situ background rate of the DRIFT detector for the beam-dump
experiment.
1.3 Research Methodology
Here experiments are compared to simulations to ensure reliability of the simulations, and
then the simulations are used to predict the background in DRIFT induced by cosmic-rays, as
described in Figure 3. Benchmarking is the first part of the research, where the same experiment
is performed on both the DRIFT detector and the computer simulations in order to deduce an
overall efficiency factor between direct measurements and simulations. Particularly, the neutron
attenuation through shielding is selected as the benchmarking experiment because Cf-252, the
neutron source, has a well-known spectrum.
In the latter half of the research, the cosmic-ray induced background is investigated for two
DRIFT detectors: DRIFT-IIf which is located on the ground floor of Hameetman Science Center
at Occidental College, and DRIFT-IIg for the proposed beam-dump experiment at SLAC. The
cosmic-ray background rate for DRIFT-IIf is simulated and converted to a predicted rate with the
efficiency factor. The predicted rate is then compared with the background measured by DRIFT-
IIf to confirm the reliability of the cosmic-ray simulation. Moreover, computer simulations are
implemented on the DRIFT-IIg beam-dump experiment. Using the efficiency factor, a prediction
of cosmic-ray induced background for DRIFT beam-dump experiment is established.
6
Figure 3. Flowchart of the research methodology.
7
2 TOOLS
A study on measuring the neutron attenuation through shielding was performed as a
benchmarking experiment. The tools involved will be introduced in this chapter, including the
DRIFT-IIf detector, the benchmarking experiment design and the computer simulation software
GEANT4.
2.1 The DRIFT-IIf Detector
The DRIFT-IIf detector, located on the ground floor of Hameetman Science Center at
Occidental College, is one of the DRIFT project’s current detectors. DRIFT-IIf is much smaller
than other detectors which allows for small-scale experimentation and portability.
As the sixth generation of DRIFT detectors, DRIFT-IIf inherits and improves the NITPC
technique. The chamber is filled with a mixture of 40 Torr of CS2 gas and 1 Torr of O2 gas. The
neutral, incoming particles collide with the nuclei of the gas molecules, creating ionization tracks
containing negatively and positively charged CS2. The built-in electric field in the chamber can
drift the anion track to the read-out plane, and an event is registered. The O2 gas facilitates the
creation of minority carriers, which are different types of negative ions that enable distance
measurements of ionization tracks via differential drift speeds [8].
The detector is stored in a 1m long cylindrical vacuum vessel, as shown in Figure 4. At the
back of the vessel is a 0.3m (1’) long cubic field cage with 10 field rings providing the electric
field of 66kV/m for drifting the ionization track. The field cage encloses a 0.01 m3 fiducial volume,
with a drift distance of 0.3m before the ionization track reaches the read-out plane. The read-out
plane is a 0.2m by 0.2m Multi-Wire Proportional Counter (MWPC), as shown in Figure 5. The
orange lines represent anode wires which are covered by grid wires represented by black lines.
8
Tracks of anions are drifted via the drift field to the MWPC where, with the help from gas
avalanche amplification, the horizontal extent of the ionization is measured. Note that because of
grouping of the wires prior to readout the absolute position of the track in the detector is not
measured [6]. The vertical extent of the track is measured using time of arrival of the ionization.
The absolute vertical position of the track is measured utilizing negative anions.
Figure 4. A picture of inner vacuum vessel. On the back is the field cage with a 0.01 m3 fiducial volume, attached to
the detector. The rest are electronics necessary for the detector.
Figure 5. The Multi-Wire Proportional Counter (MWPC) with anode wires in orange, wrapped by grid wires in
black [10].
9
2.2 Cf-252 Neutron Source
The experiment on neutron attenuation through shielding is designed to benchmark the
computer simulation. Cf-252, an isotope of californium, serves as the neutron source in this
experiment. The spontaneous fission process of Cf-252 can produce neutrons with an energy
distribution shown in Figure 6 [11]. Using Cf-252 as the neutron source is ideal for observing the
reduction effect due to the shielding for recoils in the energy range where the DRIFT-IIf detector
is most sensitive.
Figure 6. The energy distribution of Cf-252 neutrons [10]. The neutron
The Cf-252 source used in the experiment is shown in Figure 7. The source is stored in a
3.2cm long double-walled stainless-steel container, and is placed in a 10cm-tall 2.86cm-thick lead
canister to reduce gamma emissions from the source while allowing neutrons to have to escape.
10
The neutron flux rate of the Cf-252 source used in the experiment is calculated to be 54,300
neutrons per sec.
(a) (b)
Figure 7. (a) Cf-252 source in a lead canister. (b) A picture of the Frontier Technologies Corporation Cf-252 source.
The source is stored in a double-walled stainless-steel container which has been shut by tungsten inert gas (TIG)
welding.
2.3 Platform
A wood platform, as show in Figure 8, was built to support the shielding material above
the detector. Water and concrete were used as shielding for Cf-252 neutrons. The detailed setup of
each shielding material will be discussed in Chapter 3.
Figure 8. Picture of DRIFT-IIf in the 1m cylindrical vacuum vessel
with a 1.7m tall wood platform built for placing shielding material.
11
2.4 GEANT4
GEANT4 (GEometry ANd Tracking), is a software toolkit developed by CERN that
performs Monte Carlo simulations and particle tracking. It is widely used not only in particle
physics and high energy physics, but also for cosmic-ray, radiation and medical therapy
simulations [12]. Programmed with C++, GEANT4 allows users to define the geometry of the
experimental setup, initiate desired particles on target, and register the corresponding physics
information for each particle based on the purpose of the experiment. Qt 5, a cross-platform
application framework, is linked to GEANT4 to visualize the geometry as well as traces of particles,
while R, a software for statistical computing and graphics, is used to analyze the data output from
GEANT4.
Since GEANT4 was originally developed for high energy physics, there are doubts about
whether GEANT4 is suitable for simulating low energy physics such as Cf-252 neutron spectrum.
Thus, the DRIFT-IIf benchmarking experiment with Cf-252 as the neutron source will verify the
accuracy of GEANT4 in a lower energy regime.
2.4.1 Simulating Cf-252 neutron source with GEANT4
In GEANT4, the Cf-252 source in Figure 8 is modeled as an isotropic point neutron source
in a lead canister. The energy distribution of Cf-252 is input as a histogram with the neutron energy
ranging from 0 to 10 MeV, separated by every 0.05 MeV. The complete input spectrum code is
attached in Appendix 8.1.
12
2.4.2 Simulating cosmic-rays with GEANT4
Cosmic-rays are high-energy radiation containing charged particles and nuclei from
outside the solar system [13]. Primary cosmic rays from the source can generate secondary
particles when interacting with the atmosphere. Each cosmic-ray particle has a unique energy and
angular distribution, which also depends on the latitude, altitude, and solar activity. Therefore,
instead of incorporating all the factors and inputting these into GEANT4, it is convenient to import
a library that can describe cosmic-ray spectrum on the Earth’s surface.
The Cosmic-ray Shower Library (CRY Library), a C++ package developed by Lawrence
Livermore National Laboratory, is capable of precisely describing cosmic-ray secondary particles
including neutrons, protons, gammas, electrons, muons and pions [14]. The library has been fully
tested and compared to the known cosmic-ray spectrum and results from other simulation tools
such as Fluka and MCNPX. With CRY Library, it is possible to control the factors of the cosmic-
ray distribution by setting the parameters such as altitude, latitude, date and area of the distribution.
GEANT4 simulation is linked to the CRY Library to generate cosmic-ray neutrons.
To convert the simulation results to real time, it is necessary to know the total flux of each
particle. Figure 9 includes the flux spectrum of cosmic-ray protons, neutrons and muons at sea
level from CRY Library documentation. By finding the area under the curve, the total flux of each
particle is estimated to be Φ𝑝𝑟𝑜𝑡𝑜𝑛 = 6.8𝑚−2𝑠−1 , Φ𝑛𝑒𝑢𝑡𝑟𝑜𝑛 = 162𝑚−2𝑠−1 , and Φ𝑚𝑢𝑜𝑛 =340𝑚−2𝑠−1 [15].
13
Figure 9. The energy spectrums of cosmic-ray proton, neutron and muon at sea level generated by CRY library [14].
14
3 BENCHMARKING EXPERIMENTS
The experiment measuring neutron attenuation through shielding benchmarks the
GEANT4 simulation against a DRIFT-IIf measurement. Specifically, four groups of experiments
were launched to examine the neutron attenuation effect. First, a calibration run measured the
neutron rate when the source was placed on top of the vacuum vessel (33cm away from the
detector). This provides a standard efficiency factor between the simulation and experiment. Then
the Cf-252 source was fixed at the source position (83cm from the detector), and the event rate
was measured where no shielding was inserted between source and detector. The last two
experiments evaluated the neutron rate in detector when water and concrete served separately as
shielding. Ultimately, the goal is to obtain an efficiency factor from calibration, and compare that
with no shielding, water shielding and concrete shielding runs. The setup and results of the
benchmarking experiment will be discussed in detail.
Note that a parameter NIPs (number of ionization pairs) is utilized in the measurements by
DRIFT-IIf. NIPs can be related to the ionization energy due to collisions with carbon, sulfur and
oxygen molecules by the W-value [16]. The W-values for each element are listed in Appendix 8.2.
The NIPs range, 600 - 6000 determines the collision energy between the incoming particle and
mixture gas that can be detected in DRIFT-IIf. Equivalently, the limit on NIPs is converted to an
energy cut-off for GEANT4 simulation result. The following sections only cite the final result
from GEANT4 measured in number of events predicted per day, while the simulation number data
is attached in Appendix 8.4.
15
3.1 Background of Benchmarking Experiment
Since all benchmarking experiments are subjected to cosmic-ray background, it is
necessary to measure the background rate and subtract that from the neutron event. From Chapter
4, the background rate measured by DRIFT-IIf is 3.1 ± 0.7 events per day.
3.2 Calibration and No Shielding Measurements
3.2.1 Experimental setup of calibration and no shielding run
Two experiments, the calibration run and the no shielding run, were done with Cf-252
source at different distances from the detector with no shielding material between the source and
the detector. Figure 10(a) shows a sketch of the setup for the calibration run. The orange box
represents the lead canister storing the Cf-252 source, and is placed directly on top of the vacuum
vessel, with a 33cm distance between the center of the lead canister and the center of the detector.
Figure 10(b) depicts the no shielding run, where the bottom of the lead canister for Cf-252 was
21.5cm above the wood platform and there is an 83cm distance between the center of the lead
canister and the center of the detector, leaving enough space for adding shielding material in a later
experiment. In fact, for all of the attenuation experiments the position of the source was fixed at
83cm relative to the detector regardless of shielding configuration. This position is referred as the
standard source position.
16
(a) (b)
Figure 10. (a) The drawing (not to scale) of the calibration run where the Cf-252 source is at the top of the detector.
As shown in the purple line, the center of the lead canister is 33cm from the center of the detector.
(b) The drawing platform (not to scale) of the no shielding run where the bottom of the lead canister for Cf-252 is
21.5cm above the wood. The distance between the center of the lead canister and the center of the detector is 83cm,
as marked in purple.
3.2.2 Results for calibration and no shielding runs
Figure 11 shows the experimental results from DRIFT-IIf plotting the horizontal distance
z from the detector versus NIPs for 2.24 days of a calibration run and 1.63 days of a no shielding
run. Each dot in the plot represents a recoil event registered in the detector, and the coordinates of
the event give the z position and the relative recoil energy. Analyzing these plots gives an event
rate of 3840 ± 40 per day for the calibration, and 470 ± 20 per day for the no shielding run.
17
(a)
(b)
Figure 11. (a) Plot of horizontal distance z from the detector versus NIPs for calibration run. Each dot is a single
event. (b) Plot of horizontal distance z from the detector versus NIPs for no shielding run.
0 1000 2000 3000 4000 5000 6000
05
1015
2025
30
Nips vs PI zNeutrons CalibrationNeutronSourceFlat , 211SmoothSV25
2.24 days, 8939 events, 3990 +/− 40 events per day
Anode Nips
z (c
m)
Cathode
MWPC
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Cathode rate = 323 events, 144 +/− 8 events per day
Fiducial rate = 8616 events, 3840 +/− 41 events per day
0 1000 2000 3000 4000 5000 6000
05
1015
2025
30
Nips vs PI zNeutrons All50OhmNoShieldingAttenuationHSC020 , 211SmoothSV25
1.63 days, 814 events, 499 +/− 20 events per day
Anode Nips
z (c
m)
Cathode
MWPC
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Cathode rate = 57 events, 35 +/− 4.6 events per day
Fiducial rate = 757 events, 464 +/− 17 events per day
18
Table 1 summarizes both simulation and experimental results for these two runs. Note that
the column 2 is distance between the center of the Cf-252 source and the center of the detector.
Column 3 tabulates the simulated neutron rate per day in DRIFT-IIf predicted by GEANT4, while
column 4 lists the measured rate by DRIFT-IIf, both within 600 to 6000 NIPs. The last column
calculates the efficiency factor, which by definition is the ratio of experimental result versus
simulated prediction.
Table 1 Summary of the simulated and experimental results for calibration and no shielding run.
Experiment Name
Distance (cm)
Simulated Neutron Rate
(event/day)
Measured Neutron Rate
(event/day)
Efficiency Factor
Calibration Run 33 13800 ± 100 3840 ± 40 0.260 ± 0.003 No Shielding Run 83 1810 ± 40 470 ± 20 0.26 ± 0.01
The calibration run measured an efficiency of 0.260 ± 0.003 for Cf-252 neutrons. The most
recent result for DRIFT maps the efficiency of DRIFT-IId, a 0.5m long detector, as a diagram of
the distance between the cathode and MWPC versus NIPs, shown in Figure 12 [6]. The overall
efficiency is around 30%. Thus, an efficiency of 0.260 ± 0.003 for DRIFT-IIf is within a reasonable
range, and is believed to be the true efficiency with respect to GEANT4 simulation. Both runs
show an efficiency factor that is consistent with each other, indicating both DRIFT-IIf and
GEANT4 are well-performed in these two experiments.
19
Figure 12. The efficiency map for DRIFT-IId. The white pixels represent 100% efficiency
while the red indicates 0% efficiency.
3.3 DRIFT-IIf with Water Shielding
3.3.1 Experimental setup of neutron attenuation through water
The first material used for the neutron attenuation experiment is water, a common and
strong shielding material for neutrons [17]. Figure 13 sketches the setup of the neutron attenuation
through water. Two separate measurements were performed as the lead canister enclosing the Cf-
252 source was immersed in two buckets such that 11cm or 20cm of water were below the canister,
and 18cm or 27cm of water were surrounded by the canister in four directions, as illustrated in
Figure 14 (a) and (b). The runs will be referred as the 11cm and 20cm water runs. A lab jack was
used for the small bucket so that the bottom of the lead canister for Cf-252 was fixed at the standard
source position of 83cm from the center of the detector, as discussed above. Figure 14 (c) and (d)
shows the experimental setup of 11cm and 20cm water run.
20
Figure 13. The drawing (not to scale) of the neutron attenuation through water
where the Cf-252 source is submerged in the middle of the water.
21
(a)
(b)
(c) (d)
Figure 14. (a) The drawing (not to scale) of the position of Cf-252 lead canister with 11cm water below the canister.
(b) The drawing (not to scale) of the position of Cf-252 lead canister with 20cm water run below the canister.
(c) The side view of the small bucket with 11cm water below the canister.
(d) The top view of the large bucket (trash can) filled with 20cm of water below the canister.
22
3.3.2 Results for neutron attenuation through water
Table 2 lists predicted and measured neutron rates, as well as the efficiency between direct
measurement and simulation. The efficiency factor of the 11cm water attenuation experiment
agrees with that of 0.260 ± 0.003 from calibration, while the efficiency of the 20cm water run
agrees with the standard efficiency within 2V.
Table 2 Summary of the simulated and experimental result for calibration and no shielding run.
Experiment Name
Simulated Neutron Rate
(event/day)
Measured Neutron Rate (event/day)
Efficiency Factor
Calibration Run 13800 ± 100 3840 ± 40 0.260 ± 0.003 11cm water 300 ± 20 71± 5 0.24 ± 0.03 20cm water 75 ± 6 14 ± 2 0.18 ± 0.04
Due to the significant reduction effect of water, the energy of Cf-252 neutrons are heavily
decreased. The discrepancy in the 20cm water run suggests that GEANT4 is not perfectly accurate
in simulating low-energy neutrons, and thus more simulation work need to be done in the future
to test the energy models that are appropriate for low-energy neutrons.
3.4 DRIFT-IIf with Concrete Bricks
Apart from water, concrete bricks are utilized as shielding material for neutrons since they
can easily be organized to the desired shielding structure. The detailed experimental setup as well
as the results are discussed below.
23
3.4.1 Experimental setup of neutron attenuation through concrete
Figure 15 illustrates the setup of the neutron attenuation through concrete experiments.
Bricks were arranged to be in a shape of a concrete cave (six sides) of varying thickness. The
thickness of concrete wall varied from one-layer (5.7cm thick) to three-layers (17cm thick). For
each configuration, though, the distance from the source to the detector remained 83cm from the
detector. A lab jack was used to hold the source and lead canister a fixed distance from the detector
for each configuration. Figure 16 shows a side view and a top view of a 2-layer concrete cave.
Figure 15. The drawing (not to scale) of the neutron attenuation through 1-layer concrete cave.
24
(a) (b)
Figure 16. (a) A side view of the 2-layer (11cm thick) concrete cave. (b) The top view of the 2-layer concrete cave
with the Cf-252 canister centered.
3.4.2 Composition analysis of concrete bricks
The composition of the concrete bricks is crucial in simulating the neutron attenuation
effect through concrete. The elemental composition analysis of the concrete bricks was conducted
by ALS Geochemistry and the complete report is included in Appendix 8.3. The mass fractions of
each compound of the concrete bricks are listed below in Table 3. The LOI in the last row of the
Table 3 stands for loss on ignition.
To input the mass fractions in Table 3 to GEANT4, the mass fraction of Cr2O3 is taken to
be 0.01%, while the LOI is assumed to be water and its mass fraction is adjusted to 29.744% such
that the mass fractions of all compounds add up to be 100% [18].
25
Table 3 Elemental composition of the concrete brick.
Element Mass Percentage (%) SiO2 24.02 Al2O3 5.37 Fe2O3 1.95 CaO 26.27 MgO 10.42 Na2O 0.94 K2O 0.97 Cr2O3 < 0.01 TiO2 0.15 MnO 0.05 P2O5 0.046 SrO 0.02 BaO 0.04 LOI 28.9 Total 99.15
3.4.3 Results for neutron attenuation through concrete
Table 4 summarizes both simulated and measured neutron rates through concrete in
DRIFT-IIf.
Table 4 Summary of the simulated and experimental result for neutron attenuation through concrete.
Experiment Name
Simulated Neutron Rate
(event/day)
Measured Neutron Rate (event/day)
Efficiency Factor
1-layer concrete cave 872 ± 50 330 ± 20 0.38 ± 0.03 2-layer concrete cave 300 ± 20 270 ± 20 0.92 ± 0.09 3-layer concrete cave 130 ± 10 190 ± 10 1.5 ± 0.2
26
The efficiency of the detector in this case is not consistent with the efficiency factor
calculated from the calibration run. Specifically, the simulated neutrons are attenuated more
heavily than the actual neutrons. That is likely a result from assuming water as the LOI from the
analysis, and further research is necessary to solve this problem. These results highlight the
importance of knowing the elemental composition of the shielding material and some of the
difficulties in obtaining it.
Results for all benchmarking experiments are summarized in Table 5. An efficiency of
0.260 ± 0.003 is measured from the calibration run. Neutron attenuation with no shielding and
11cm water shielding experiments indicate consistency in the efficiency measurements, but the
results from 20cm water run suggest GEANT4 might fail in predicting low-energy neutron recoils,
and an efficiency map is necessary to compare the simulations with direct measurements. In
addition, all concrete runs disagree with the standard efficiency 0.260 ± 0.003, suggesting more
research is needed to figure out the LOI in the concrete elemental composition analysis.
Table 5 Summary of the simulated and experimental result for benchmarking experiments.
Experiment Name
Distance (cm)
Simulated Neutron Rate (event/day)
Measured Neutron Rate (event/day)
Efficiency Factor
Calibration Run 33 13800 ± 100 3840 ± 40 0.260 ± 0.003 No Shielding Run 83 1810 ± 40 470 ± 20 0.26 ± 0.01
11cm water 83 300 ± 20 74 ± 6 0.24 ± 0.03 20cm water 83 75 ± 6 17 ± 3 0.18 ± 0.04
1-layer concrete cave 83 872 ± 50 330 ± 20 0.38 ± 0.03 2-layer concrete cave 83 300 ± 20 270 ± 20 0.92 ± 0.09 3-layer concrete cave 83 130 ± 10 190 ± 10 1.5 ± 0.2
27
4 COSMIC-RAY BACKGROUND IN DRIFT-IIf
Cosmic-ray neutrons, protons and muons generate significant backgrounds for DRIFT
detectors on the surface of the earth or at shallow depth through a variety of mechanisms. Cosmic-
ray neutrons and protons can collide elastically with CS2 and O2 molecules inside the vacuum
vessel, leaving ionization tracks and inducing a background. Although the mass of muon is small,
when scattering with metals, it can generate GDR (Giant Dipole Resonance) neutrons which also
increase the background rate [19]. Thus, the simulation needs to quantify the background due to
each particle and determine the major background source for DRIFT-IIf.
Figure 17 visualizes the geometric setup in GEANT4. In GEANT4, the concrete ceiling
above DRIFT-IIf is simplified to a 15m by 15m by 0.3m concrete slab. A 0.07m thick concrete
floor, a 0.7m by 1.5m by 0.1m iron table, and the 0.8m3 DRIFT-IIg vacuum vessel were added to
the simulation to recreate the lab setup. A 10m by 10m cosmic-ray source is placed above the
concrete ceiling. Moreover, a 2.5cm thick stainless-steel box is inserted in the simulation as an
estimation of the metal in the building.
28
Figure 17. A side view and a 3D view of cosmic-ray background simulation for DRIFT-IIf. The top and bottom
light grey box in the side view represents the concrete floors above and below the detector. The brown structure is
the wood platform above the detector. The gray solid box in the middle is the 0.3-meter detector. The grey solid slab
next to the wood platform is the iron table in the lab, and the dark grey box on the side represents the vacuum vessel
of DRIFT-IIg. The 2.5cm stainless-steel box wraps both floor and ceiling and is not shown in both pictures.
Table 6 lists the background rate due to protons, muons and neutrons respectively. Adding
up all the numbers gives a simulated background rate of 19 ± 2 per day and 22 ± 2 per day with
and without the 2.5cm-thick stainless steel. Using the standard efficiency factor of 0.260 ± 0.003,
a prediction of a background rate of 5.0 ± 0.5 per day in DRIFT-IIf is established for the no
stainless-steel run and a rate of 5.8 ± 0.6 per day is made to the simulation containing 2.5cm
29
stainless-steel. At the same time, 5 days of background data in DRIFT-IIf yields a rate of 3.1 ± 0.7
per day. Thus, the prediction by GEANT4 with no stainless-steel is closer to the direct
measurement, while 2.5cm might be an overestimation of the stainless-steel in the building.
Table 6 Summary of the background simulation for DRIFT-IIf at Hameetman Science Center.
Shielding Cosmic-
ray Particle
Simulated Background
Rate (per day)
Simulated Total
Background Rate
(per day)
Predicted Total
Background Rate
(per day)
Measured Total
Background Rate
(per day)
0.3m concrete Proton 0.8 ± 0.2
19 ± 2 5.0 ± 0.5
3.1 ± 0.7
Muon 0.36 ± 0.08 Neutron 18 ± 2
0.3m concrete and 2.5cm
stainless-steel
Proton 0.8 ± 0.1 22 ± 2 5.8 ± 0.6 Muon 0.36 ± 0.08
Neutron 21 ± 2
30
5 PREDICTING COSMIC-RAY BACKGROUND IN DRIFT-IIg
It has been proposed for the DRIFT project to bring DRIFT-IIg, a 0.8m3 detector to SLAC
National Accelerator Laboratory to launch the first beam-dump experiment. Thus, it is necessary
to predict and understand the cosmic-ray induced background in DRIFT-IIg and calibrate the
shielding.
The corresponding configuration of the experiment as shown in the drawing of Figure 18,
is set up in GEANT4. To simulate the shallow shielding environment at SLAC, a 30m by 30m by
6m dirt slab is placed 8 meters above the detector. On the top is the cosmic-ray source. Figure 19
shows 300 cosmic-ray muons on target, while 2 GDR neutrons (pink line) are produced.
Figure 18. The drawing of DRIFT-IIg background simulation. From the top to bottom
are the 10m by 10m cosmic-ray source, a 6m-thick dirt slab, and the DRIFT-IIg detector.
31
Figure 19. A side view of the simulation. The top white box is the dirt slab. The bottom small white box is the
vacuum vessel, and inside is the detector. This picture shows 300 cosmic-ray muons on target, generating hits
(yellow dots) in the dirt slab. The lines in pink are the GDR neutrons produced by cosmic-ray muons.
Because dirt is a common material for shielding neutrons, the dirt shielding will filter out
most neutrons and protons from the cosmic rays. Thus, muons will provide the largest background
for the DRIFT-IIg detector at an accelerator. A 10m by 10m cosmic-ray muon distribution is then
used as the source.
Two simulations have been performed with different types of vacuum vessels, a 1cm
stainless-steel vessel and a 10cm acrylic vessel. Corresponding predictions on background rate
were determined for each type of vacuum vessel. An acrylic vacuum vessel is expected to have a
low cosmic-ray muon background rate because muons can scatter with stainless-steel and produce
GDR neutrons, as discussed above. Indeed, the simulation result in Table 7 suggests that compared
32
to the stainless-steel vessel, the acrylic vessel has better performance in shielding the cosmic-ray
muons. The number data is included in Appendix 8.5.
Table 7 Simulations for DRIFT-IIg background
Vacuum Vessel Simulated Rate (per day)
(41 Torr) Predicted Rate (per day)
(41 Torr) 1cm stainless-steel 10 ± 2 2.6 ± 0.6
10cm acrylic 3 ± 3 0.7 ± 0.7
Due to the limited amount of time and computing power, a background simulation with the
appropriate muon source size (100m in length) was not carried out. From previous experiences, it
is clear that until the background rate reaches its maximum, a cosmic-ray source with a larger area
will induce more background. Thus, more simulations need to be performed to accurately
determine the background rate for DRIFT-IIg. However, the predicted background rates from
Table 7 set a lower limit to the background in beam-dump experiment. Moreover, the difference
points out a possibility of using an acrylic vacuum vessel to minimize the cosmic-ray background.
33
6 CONCLUSION
In order to predict the cosmic-ray induced background for DRIFT beam-dump experiment
at SLAC, a series of benchmarking experiments measuring neutron attenuation have been
conducted to measure the efficiency of the DRIFT detector compared to the GEANT4 simulation.
The calibration run measures an efficiency factor of 0.260 ± 0.003. No shielding and 11cm water
shielding runs demonstrate a consistency in the efficiency. However, the results from 20cm water
run suggest GEANT4 might fail in predicting low-energy neutron recoils, or the efficiency changes
in low-energy regime and therefore an efficiency map is necessary to compare the simulations with
direct measurements, or both. The results from neutron attenuation through concrete suggest that
the undefined composition LOI of concrete needs further investigation.
The background rate in DRIFT-IIf due to cosmic-ray protons, neutrons and muons were
predicted to be 5.0 ± 0.5 per day by GEANT4, while 5 days of a background run with the DRIFT-
IIf yields a rate of 3.4 ± 0.8. The agreement between simulation and experiment confirms the
success of GEANT4 in predicting cosmic-ray background for DRIFT detectors.
Lastly, a preliminary prediction for the DRIFT-IIg beam-dump experiment at SLAC was
established. Although further improvements are necessary to conclude the background rate, the
results set a lower limit of 2.6 ± 0.6 for a stainless-steel vacuum vessel, and suggest the possibility
of using an acrylic vacuum vessel to minimize the cosmic-ray background.
It has been shown that GEANT4, with proper inputs, is capable of simulating recoil events
in the DRIFT-IIf, even for cosmic ray backgrounds. There is good reason to believe that it will
accurately simulate the DRIFT-IIg beam-dump experiment backgrounds at SLAC. Thus, this work
forms a solid foundation to build a new dark matter experiment at an accelerator.
34
7 ACKONWLEDGEMENTS
I would like to express the deepest appreciation to my mentor and advisor, Dr. Daniel
Snowden-Ifft, who made a huge commitment to this study. Without his encouragement and
mentorship this would hardly have been completed. I would also like to thank Dr. Jean-Luc
Gauvreau for his wisdom and help, and Ethan Heffernan for the great teamwork for the last two
summers. Moreover, I want to acknowledge Dr. Janet Scheel and Dr. Alec Schramm for the advice
on this thesis. Lastly, I would like to thank Occidental College Physics Department, Occidental
Undergraduate Research Center and the Kenneth T. & Eileen L. Norris Foundation for supporting
this research.
35
8 APPENDIX
8.1 Cf-252 SPECTRUM IN GEANT4
The Cf-252 spectrum is input as a macro file using the GPS (General Particle Source)
implemented in GEANT4. The syntax is defined as
/gps/hist/point Ehi weight
where Ehi is the upper edge of the energy bin in the histogram, and weight is the relative height of
the corresponding energy bin. The following code input the Cf-252 energy spectrum from 0 to 10
MeV.
/gps/hist/point 0.05 1.019e+19 /gps/hist/point 0.15 1.78e+19 /gps/hist/point 0.25 2.192e+19 /gps/hist/point 0.35 2.467e+19 /gps/hist/point 0.45 2.656e+19 /gps/hist/point 0.55 2.785e+19 /gps/hist/point 0.65 2.87e+19 /gps/hist/point 0.75 2.92e+19 /gps/hist/point 0.85 2.942e+19 /gps/hist/point 0.95 2.943e+19 /gps/hist/point 1.05 2.924e+19 /gps/hist/point 1.15 2.892e+19 /gps/hist/point 1.25 2.847e+19 /gps/hist/point 1.35 2.792e+19 /gps/hist/point 1.45 2.729e+19 /gps/hist/point 1.55 2.661e+19 /gps/hist/point 1.65 2.587e+19 /gps/hist/point 1.75 2.509e+19 /gps/hist/point 1.85 2.429e+19 /gps/hist/point 1.95 2.347e+19 /gps/hist/point 2.05 2.264e+19 /gps/hist/point 2.15 2.18e+19 /gps/hist/point 2.25 2.096e+19 /gps/hist/point 2.35 2.013e+19 /gps/hist/point 2.45 1.931e+19 /gps/hist/point 2.55 1.85e+19 /gps/hist/point 2.65 1.77e+19 /gps/hist/point 2.75 1.692e+19 /gps/hist/point 2.85 1.615e+19 /gps/hist/point 2.95 1.541e+19 /gps/hist/point 3.05 1.469e+19 /gps/hist/point 3.15 1.399e+19 /gps/hist/point 3.25 1.332e+19 /gps/hist/point 3.35 1.266e+19 /gps/hist/point 3.45 1.203e+19
/gps/hist/point 3.55 1.143e+19 /gps/hist/point 3.65 1.084e+19 /gps/hist/point 3.75 1.029e+19 /gps/hist/point 3.85 9.749e+18 /gps/hist/point 3.95 9.235e+18 /gps/hist/point 4.05 8.743e+18 /gps/hist/point 4.15 8.273e+18 /gps/hist/point 4.25 7.825e+18 /gps/hist/point 4.35 7.397e+18 /gps/hist/point 4.45 6.989e+18 /gps/hist/point 4.55 6.6e+18 /gps/hist/point 4.65 6.231e+18 /gps/hist/point 4.75 5.879e+18 /gps/hist/point 4.85 5.545e+18 /gps/hist/point 4.95 5.228e+18 /gps/hist/point 5.05 4.927e+18 /gps/hist/point 5.15 4.642e+18 /gps/hist/point 5.25 4.372e+18 /gps/hist/point 5.35 4.116e+18 /gps/hist/point 5.45 3.873e+18 /gps/hist/point 5.55 3.644e+18 /gps/hist/point 5.65 3.427e+18 /gps/hist/point 5.75 3.222e+18 /gps/hist/point 5.85 3.028e+18 /gps/hist/point 5.95 2.845e+18 /gps/hist/point 6.05 2.672e+18 /gps/hist/point 6.15 2.509e+18 /gps/hist/point 6.25 2.356e+18 /gps/hist/point 6.35 2.211e+18 /gps/hist/point 6.45 2.074e+18 /gps/hist/point 6.55 1.946e+18 /gps/hist/point 6.65 1.825e+18 /gps/hist/point 6.75 1.711e+18 /gps/hist/point 6.85 1.603e+18 /gps/hist/point 6.95 1.502e+18
36
/gps/hist/point 7.05 1.408e+18 /gps/hist/point 7.15 1.318e+18 /gps/hist/point 7.25 1.235e+18 /gps/hist/point 7.35 1.156e+18 /gps/hist/point 7.45 1.082e+18 /gps/hist/point 7.55 1.012e+18 /gps/hist/point 7.65 9.473e+17 /gps/hist/point 7.75 8.861e+17 /gps/hist/point 7.85 8.288e+17 /gps/hist/point 7.95 7.749e+17 /gps/hist/point 8.05 7.245e+17 /gps/hist/point 8.15 6.772e+17 /gps/hist/point 8.25 6.329e+17 /gps/hist/point 8.35 5.914e+17 /gps/hist/point 8.45 5.524e+17 /gps/hist/point 8.55 5.16e+17
/gps/hist/point 8.65 4.819e+17 /gps/hist/point 8.75 4.5e+17 /gps/hist/point 8.85 4.201e+17 /gps/hist/point 8.95 3.921e+17 /gps/hist/point 9.05 3.66e+17 /gps/hist/point 9.15 3.415e+17 /gps/hist/point 9.25 3.186e+17 /gps/hist/point 9.35 2.972e+17 /gps/hist/point 9.45 2.772e+17 /gps/hist/point 9.55 2.586e+17 /gps/hist/point 9.65 2.411e+17 /gps/hist/point 9.75 2.247e+17 /gps/hist/point 9.85 2.095e+17 /gps/hist/point 9.95 1.953e+17
37
8.2 W-VALUE
The table below lists corresponding W-value for each ionization energy due to collisions
with sulfur, carbon and oxygen. The relation between the ionization energy and the w-value is
approximated to be a continuous function in R with the approx function.
Table 8 W-value for sulfur, carbon and oxygen.
Sulfur Carbon and Oxygen Ionization Energy (MeV) W-value Ionization Energy (MeV) W-value
0 86.62117 0 61.02855 0.01 86.62117 0.01 61.02855 0.02 77.27357 0.02 50.66522 0.03 72.08742 0.03 45.51282 0.04 67.98117 0.04 42.2875 0.05 64.70497 0.05 40.22856 0.06 62.08685 0.06 38.4983 0.07 60.00573 0.07 37.29523 0.08 58.37514 0.08 36.28725 0.09 57.13312 0.09 35.33232 0.10 55.94284 0.10 34.64847 0.11 54.80115 0.11 33.99059 0.12 53.70513 0.12 33.46114 0.13 52.65209 0.13 33.04931 0.14 51.63955 0.14 32.64749 0.15 50.90533 0.15 32.35249 0.16 50.19171 0.16 32.06276 0.17 49.49781 0.17 31.77818 0.18 48.82284 0.18 31.49861 0.19 48.383 0.19 31.31494 0.2 47.95101 0.2 31.22391
1.00E+06 47.95101 1.00E+06 31.22391
38
8.3 COMPOSITION REPORT OF CONCRETE
39
40
8.4 SIMULATION DATA FOR BENCHMARKING EXPERIMENT
The raw data for the benchmarking experiment from GEANT4 are recorded in terms of
neutron recoils captured out of total number of neutrons on target. The 41 Torr CS2 and O2 mixture
gas in DRIFT-IIf detector is compressed by a factor of 25 to speed up the simulation. The intensity
of the Cf-252 source is estimated to be 60000 neutrons per second. Thus, to convert the number of
neutrons 𝑛𝑐𝑎𝑝 captured in the simulation to simulated neutron rate per day 𝑟𝑠 , the following
calculation is applied
𝑡 = 25 × ( 𝑛𝑡𝑜𝑡𝑎𝑙60000 𝑛𝑒𝑢𝑡𝑟𝑜𝑛/𝑠 24 × 3600𝑠1𝑑 )−1 (1)
and
𝑟𝑠 = 𝑛𝑐𝑎𝑝𝑡 (2) where 𝑡 is the time simulated for 41 Torr of mixture gas, and 𝑛𝑡𝑜𝑡𝑎𝑙 is the total number of neutrons
on target. The simulation result for the benchmarking experiment and the corresponding neutron
recoil rate simulated for DRIFT-IIf are tabulated below.
Table 9 Simulation data for benchmarking experiments.
Experiment Name
Simulated Neutron on Target
(M)
Simulated Number of Neutrons
Captured
Simulated Neutron Recoil Rate at 41 Torr
(per day) Calibration Run 340 21119 0.260 ± 0.003
No Shielding Run 239 1817 0.26 ± 0.01 17.5cm water 134 170 0.23 ± 0.03 26.6cm water 452 142 0.24 ± 0.03
1-layer concrete cave 83 304 0.18 ± 0.04 2-layer concrete cave 146 181 0.92 ± 0.09 3-layer concrete cave 327 173 1.5 ± 0.2
41
8.5 SIMULATION DATA FOR COSMIC-RAY BACKGROUND
Similar to the benchmarking experiment, GEANT4 outputs number of recoils captured for
the cosmic-ray background simulation. The cosmic-ray neutron, muon and proton fluxes at the
surface of earth are known to be 162/(𝑚2 ∙ 𝑠), 340/(𝑚2 ∙ 𝑠) and 6.8/(𝑚2 ∙ 𝑠) respectively [13].
Thus, the responding times simulated in terms of the total number of neutrons and muons on target
are given by
𝑡𝑛𝑒𝑢𝑡𝑟𝑜𝑛 = 25 × (𝐴 𝑛𝑛𝑒𝑢𝑡𝑟𝑜𝑛,𝑡𝑜𝑡𝑎𝑙162/(𝑚2 ∙ 𝑠) 24 × 3600𝑠1𝑑 )−1 (3)
𝑡𝑚𝑢𝑜𝑛 = 25 × (𝐴 𝑛𝑚𝑢𝑜𝑛,𝑡𝑜𝑡𝑎𝑙340/(𝑚2 ∙ 𝑠) 24 × 3600𝑠1𝑑 )−1 (4)
𝑡𝑝𝑟𝑜𝑡𝑜𝑛 = 25 × (𝐴 𝑛𝑚𝑢𝑜𝑛,𝑡𝑜𝑡𝑎𝑙6.8/(𝑚2 ∙ 𝑠) 24 × 3600𝑠1𝑑 )−1 (5)
where A is the area of the cosmic-ray source. Then the simulated event rate 𝑟𝑠 in GEANT4 is
𝑟𝑠 = 𝑛𝑐𝑎𝑝𝑡 (6)
where 𝑛𝑐𝑎𝑝 is the number of recoils captured provided by GEANT4, and t is the time simulated
for a neutron, muon or proton from Eq. 3, 4 and 5.
Using Eq. 3, 4, 5 and 6, the simulated background event rate for DRIFT-IIf and DRIFT-
IIg are calculated and listed below.
42
Table 10 Simulation data for DRIFT-IIf cosmic-ray background.
Shielding
Particle Simulated Neutron on
Target (M)
Simulated Number of Neutrons
Captured
Simulated Neutron Recoil Rate at 41
Torr (per day)
0.3m concrete Proton 78 20 0.8 ± 0.2 Muon 3 877 0.36 ± 0.08
Neutron 116 458 18 ± 2 0.3m concrete and 2.5cm stainless-
steel
Proton 39 16 0.8 ± 0.1 Muon 3 877 0.36 ± 0.08
Neutron 148 506 21 ± 2
Table 11 Simulations for DRIFT-IIg cosmic-ray muon background.
Vacuum Vessel Particle
Simulated Neutron on Target
(M)
Simulated Number of Neutrons Captured
Simulated Rate (per day) (41 Torr)
1cm stainless-steel
Muon 223 19 10 ± 2
10cm acrylic Muon 46 1 3 ± 3
43
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