Borexino: A Scintillator Purification System for a Large Scale Solar
Neutrino Experiment
An on-line purification system for a large-scale low-background liquid
scintillation detector has been tested with the 5-ton liquid scintillation
detector, the Counting Test Facility at Gran Sasso. Counter-current water
extraction removes polar and charged impurities such as ions and metal
salts. Vacuum distillation of the entire scintillator has been applied to
remove both radioactive impurities and chemical impurities that affect the
optical properties of the scintillator. Filtration with a 0.05 mm particulate filter removed suspended dust particles that may contain
U, Th and K. Nitrogen stripping was employed to remove dissolved water
and noble gases
1. INTRODUCTION
Very low backgroud detectors are required to identify rare events, including
solar neutrinos, searches for dark matter, double beta decay events, and
searches for proton decay. Liquid scintillation detectors employed for these
searches require a large mass
The concept of on-line purification has been tested with a 5 ton liquid
scintillation counting test facility
The CTF contains the liquid scintillator within a 2 m diameter sphere
constructed of 0.5 mm thick nylon. The scintillator containment vessel is
suspended within 1000 tons of ultrapure water shielding. Scintillation
light is detected with 100 phototubes with light collectors mounted in the
water viewing the scintillator vessel. The liquid scintillator tested in the
CTF was a solution of 1.5 g/L of PPO
2. RADIOPURITY REQUIREMENTS FOR A SOLAR
NEUTRINO DETECTOR
Borexino is a solar neutrino experiment designed to detect the 7Be
neutrino flux by neutrino-electron scattering in a large scale liquid
scintillation detector. The monoenergetic 7Be neutrino
signature will appear as a step decrease in the intensity of the energy
spectrum at 665 keV. Based on the standard solar model
3. PURIFICATION SYSTEM FOR THE COUNTING TEST
FACILITY
The purification of the scintillator focused on removal of dust, potassium from
the fluor, and radon and its progeny. To minimize problems associated
with Rn emanation, materials of construction were chosen to have a low U
content and low permeability to noble gases. 210Pb from the decay of
222Rn may be deposited on the surface of the scintillator
containment vessel from exposure to radon. This surface contamination is
a continual source of radioactivity, which can be identified by positional
reconstruction of the scintillation events. However, minimizing the
surface contamination can improve the sensitivity of the detector, so during
construction of the scintillator containment vessel exposure to air was kept to
a minimum to keep the 210Pb deposits to a minimum.
The scintillation efficiency and attenuation of the emitted light during
transit through the large volume of the scintillator are also critical for a
large liquid scintillation detector, and any purification methods for improving
radiopurity of the scintillator must not degrade the optical properties of the
scintillator [5,6]. The optical attenuation length at 420 nm, as measured
with a visible spectrophotometer, was used as an indication of the optical
clarity of the scintillator to indicate any degradation of the scintillator.
The general design of the scintillator purification system employed
distillation and water extraction followed by filtration and nitrogen
stripping. Distillation and water extraction were proven effective for removing
7Be impurities from liquid scintillator components in the most
sensitive laboratory experiment [7].
Table 1
Radioactive Impurities in Liquid
Scintillators
U, Th and K - most likely source is
suspended dust
U and Th exist at ca. 10 ppm in the earth’s crust
K is 2.4% of earth’s crust
produces
K -Neutron activation measurements have shown substantial
potassium contamination in the fluor
85Kr - 10.7 yr half life contained in air at ~1 Bq/m3
1 liter air exposure produces
a background of 75 events/day
7Be - Produced cosmogenically from 12C, with
a 55 day half life.
Equilibrium
exposure at surface will produce a background of 50,000 events/day that will
require 1 ½ year to decay to <10 events/day
222Rn
- Contained in underground air of LNGS at ~100 Bq/m3 and underground
water at ~10,000 Bq/m3. Rn emanation from materials of
construction will also result from U
Water extraction was preferred for on-line purification. It could be
applied to the scintillator mixture without altering its composition, it
could be done at room temperature avoiding any thermal degradation of the
scintillator and avoids potential hazards associated with heating pseudocumene
above its flash point
When contacted with water during extraction the scintillator becomes
saturated with water. The solubility of water in pseudocumene at 20°C is approximately 50 ppm, and decreases with decreasing
temperature. Decreasing the temperature of a water-saturated scintillator
results in water condensation, making the scintillator cloudy. To avoid water
condensation the water is stripped from the scintillator after water
extraction. Water has a higher vapor pressure than pseudocumene, and may
be stripped from the scintillator by flowing dry nitrogen through the
scintillator. The water content in the scintillator is reduced by 90% in a
single equilibrium stage at 15°C with a mass flow ratio of nitrogen to
scintillator of 0.04
Distillation of the scintillator mixture is more complicated. In a batch
distillation the PPO/pseudocumene mixture will be depleted of the less volatile
PPO. With a continuous feed it is possible to keep a high concentration
of PPO in the liquid of the boiler vessel, such that the equilibrium
vapor composition above the liquid corresponds to 1.5 g
PPO/L-pseudocumene. By maintaining a concentrated PPO/pseudocumene
solution in the boiler vessel the scintillator was continuously distilled with
negligible loss of PPO. The concentration of PPO required was dependent
on the temperature and pressure in the system. Typically the distillation
was carried out at pressures of ~ 100 torr at 125°C, where the
liquid in the bottom of the boiler vessel had a composition of circa 500
g PPO/L pseudocumene
The general strategy adopted for on-line scintillator purification was to
employ water extraction followed by nitrogen stripping. Vacuum
distillation would be used intermittently to remove chemical impurities that
may degrade the optical properties of the scintillator, as well as removing the
radioactive impurities.
Figure 1. Schematic of the purification system
in the counting test facility. The scintillator is circulated from the
scintillator containment vessel in the detector through either counter current
water extraction, or vacuum distillation. After water extraction or
distillation the scintillator is filtered through a 0.05 mm filter and stripped of water and dissolved gases in a nitrogen
stripping column. The scintillator is then returned to the detector
containment vessel.
The inside of the CTF purification plant. The
pipe in the center leads down to the scintillator containment vessel. The
water extraction column is to the right rear of the photo. The nitrogen
stripping column is in the foreground on the right of photo. At the
bottom of the stripping column is the liquid holding vessel where the
scintillator level is controlled to fix the liquid pressure head on the
scintillator containment vessel. The distillation subsystem is to the left. The
steel columns going from the ceiling down are the falling film evaporator and
condenser. Near the floor at the rear center and to the front left are
pumps used for moving the scintillator within the purification plant. The
vessels and piping has all been insulated to minimize heat losses.
Since dust particles may introduce U, Th, and K, particulate matter is a
serious concern as a source of radioactive impurities. To minimize
particulate matter stainless steel vessels and tubing were first mechanically
polished to an Ra 15 finish
The scintillator is contained within a nylon vessel suspended in the water
shield of the CTF detector. The scintillator is purified on-line by
removing scintillator from the containment vessel, passing it through the
purification system and returning it to the containment vessel. The
scintillator volume is 5000 L and the total volume of scintillator within the
purification system is less than 50 liters, so the detector volume does not
change significantly during purification. Since the purified scintillator
is mixed back in with the scintillator in the active detector the removal of
impurities decreases approximately exponentially with a time constant equal to
the residence time of the system
A schematic diagram of the purification system for the CTF is shown in Figure
1. Detailed process and instrumentation diagrams
4. PERFORMANCE OF THE PURIFICATION SYSTEM
The computer control of the processes providing a trouble free automated system
operating 24 hours a day with a minimum of operator attention. The
selection of equipment, materials, and cleaning procedures was judged
satisfactory. We were unable to detect radon emanation from any of the
vessels and piping in the purification system, and could only set an upper limit
of radon emanation of 0.02 Ra/m2/d. The only significant
problem encountered was radon contamination of the scintillator by the nitrogen
stripping operation due to a radon leak in the boil off nitrogen system.
In our efforts to identify the radon the entire purification system was made
vacuum tight, and not merely fluid tight, as is normally the case for such
systems. By thorough leak checking with a helium leak detector, and
modification of some seals, the entire system was sealed to the equivalent of a
high vacuum system.
After the final vessel filling, the scintillator was undisturbed for several
weeks to allow radon to decay, and evaluate the 238U and 232Th
backgrounds, and the 14C background. The 222Rn
eventually reached a constant level of 1.5 ± 0.6 counts/day, as
determined by measurement of the 214Bi-214Po beta-alpha
delayed coincidences 
[10].
4.2. Nitrogen Stripping:
Nitrogen stripping of the scintillator reduced the water content in
pseudocumene to < 20 ppm. Nitrogen stripping in the CTF also removed 85Kr,
and reduced the total background. After the initial filling the presence of 85Kr
was detected by a time correlated events along a minor decay branch
Along this branch 1.3±0.2 events/day
were detected indicating a total 85Kr background of 300 events/day
An increase in the radon level during nitrogen stripping indicated a radon
leak. Since the radon was not accompanied by 85Kr, but was
accompanied by thoron the radon must have result from emanation - and not an
air leak. Tests with the boil-off nitrogen system suggested that the radon
emanation came from a source midway down in the tank, which could have been a
welded joint or perhaps residual dust or dirt. By keeping the liquid
level in the nitrogen boil off nearly full the rise in background due to radon
was minimal.
4.2 Water Extraction
Water extraction of the scintillator was carried out for a total of five cycles
with a scintillator flow of 30 liter/hr
Energy spectra of the backgrounds internal to the scintillator vessel before
and after the water extraction are shown in Figure 2. The spectrum after the
water extraction had a featureless decaying background. The increase in
the background at low energy results principally from the tail of the 14C
peak. The spectrum prior to water extraction shows a distinct peak at 395
keV. The 395 keV peak has been identified as the alpha particles from the
decay of 210Po to 206Pb.
|
|
|
Figure 2. Spectral background of the CTF
|
4.3. Distillation
The distillation system ran three complete cycles operating at 12 liter/hr
Table 2
Backgrounds* in the CTF During Purification Operations
|
Time Period |
Total Background |
Total Internal Background |
85Kr
|
210Po |
|
Initial filling |
2595±215 |
470±90 |
300±45 |
250±40 |
|
After nitrogen stripping |
2325±220 |
285±70 |
25±25 |
265±62 |
|
After water extraction |
2075±195 |
21±47 |
25±25 |
<40 |
|
During distillation |
|
|
50±25 |
0 |
|
After distillation |
2005±195 |
35±67 |
25±25 |
0 |
*events in the energy range of the neutrino window,
250-800 keV
5. CONCLUSIONS
Purification of a large-scale liquid scintillator suitable for the Borexino
solar neutrino detector has been demonstrated. A pseudocumene/PPO
scintillator was purified in place achieving backgrounds corresponding to
<10 events/day in the energy window 250-800 keV. Distillation, water
extraction, filtration and nitrogen stripping were all demonstrated to be
effective at removing the radioactive impurities they were designed to handle.
All the construction materials internal to the purification plant
Acknowledgements
Work supported by the Physics Directorate of the
National Science Foundation under grant PHY9313919 as part of the Borexino
Solar Neutrino Experiment. The results from the Counting Test Facility
have been an effort of the entire Borexino collaboration
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