Development of high-pressure Xe ionization chambers for gamma-ray spectroscopy Aleksey Bolotnikov Brookhaven National Laboratory Workshop on Xenon-Based Detectors, Berkeley November 16-18 2009. Introduction Electron transport and spectroscopic properties of HPXe
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Development of high-pressure Xe ionization chambers for gamma-ray spectroscopy
Brookhaven National Laboratory
Workshop on Xenon-Based Detectors, Berkeley
November 16-18 2009
Introduction gamma-ray spectroscopy
Electron transport and spectroscopic properties of HPXe
HPXe detector technology (purification, gas filling)
Detectors designs and applications
Advantages of Xe
withstand high radiation and high temperature environments
compressed Xe has high stopping power (Z=54, r=0.5 g/cc)
small Fano-factor, ~0.13 compare 0.06-0.13 for Ge
low cost => large volume
Several competing detectors suitable for room-temperature operation: NaI(Tl), CdZnTe, LaBr3 pushing Xe behind in the areas where the small and compact devices are required
But there are several niches remaining for Xe detectors, e.g., portal and environmental monitors, measurements is harsh environments (high temperature, high radiation)Advantages of high-pressure Xe as detector medium
Despite the small Fano-factor predicted for LXe, its energy resolution is ~6% at 662 keV. This was explained by fluctuations of the recombination rate caused by d-electrons in dense Xe (B. Rodionov)
The statistical limit, ~0.6% at 662 keV, was achieved at low densities
Compressed Xe was proposed as alternative to LXe. It would provide high stopping power while retain good resolution of low pressure gas
However, in reality the intrinsic resolution starts degrades at much low densities that expected
Combination of two effects can explain this dependence: fluctuation of recombination (d-electron model) plus formation of large Xe cluster with liquid-like properties above 0.5 g/ccIntrinsic energy resolution of HPXe
Energy resolution at 662 keV
vs. Xe density
Measurements were taken at high electric field 5-7 KV/cm
Limit on max operation density: < 0.55 g/cm3
In the past, several experiments indicated clusters formation and its step-like density dependence: photoluminescence and photocondactivity measurements.
3He+n -> triton (191 keV ) + proton (573 keV) (neglecting the kinetic energy of the thermal neutron).
Tritons and protons have very small ranges in high-density xenon.
Assuming a uniform density of xenon gas at a density of 0.2 g/cm3, proton and triton ranges are less then 200 m.
Spectra generated by thermal neutrons in Xe+3He mixture
A. Bolozdynya, A. Bolotnikov, J. Richards, A. Proctor, NIM A522, 595-597, 2004.
Intrinsic resolution, %
Xe density, g/cc
Romanuk, Dmitrinko, MEPHI
To overcame electron-ion recombination and achieve intrinsic-level resolution the electric field strength in the drift region should be > 2 KV/cm
2% is comparable with the resolutions of typical CZT and LaBr3 detectors and much better than those obtained for large volume NaI detectorsHigh electric field is required to achieve intrinsic energy resolution in compresses Xe
Energy resolution vs. electric field strength
If clusters formation can be suppressed by heating the gas this may improve energy resolution at densities > 0.6 g/cc?Temperature effect on energy resolution
Relative changes of the total collected charge and energy resolution measured at 5 KV/cm for two densities; 0.5 and 0.7 g/cc (662 keV)
Later, Dmitrenko et al. proved that energy resolution in HPXe (~0.5 g/cc) does not changed up to 200 C. => HPXe can be used at high-temperatures where other techniques do ton work, e.g., responses of semiconductor detectors rapidly degrade with temperature.
W-value at low pressure is ~21 eV this may improve energy resolution at densities > 0.6 g/cc?
The W-value decreases with Xe density approximately as linear function to ~15.6 eV measured at LXe
At low densities such decrease is attributed:
At high densities, W decreases because of the formation of the electronic band structureChanges of the W-value in compressed Xe
Relative changes of the W-value vs. density
Bolotnikov, Ramsey (1995 )
W is ~20 eV at 0.6 g/cc => low amplitude signals in comparison to CZT (~5 eV).
Electronic and other noises are more critical in HPXe.
Low mobility in pure Xe, drift velocity saturates at ~1 mm/ this may improve energy resolution at densities > 0.6 g/cc?ms, 5 cm - 50 ms
The electron transport crossection has a deep minimum around 0.3-1.0 eV (Ramsauer–Townsend effect)
Several admixtures, e.g., H2, N2, CH4 and other organics, increase the mobility
H2 and N2 are the most practical because they can withstand the spark purification
High concentration of H2 requires stronger electric field!
The optimal concentration of H2 is 0.2-0.3% at the electric field of > 2 KV/cm. The velocity increases 2-5 times
Slow detector in comparison to CZT
Pure XeElectron drift velocity
Electron drift velocity vs. electric field strength in Xe+%H2 mixture at 0.6 g/cm3
Dmitrenko, Romanuk (1980)
HPXe detectors provide large effective areas and can replace NaI, CZT and Ge detectors in the areas where large effective area detectors are required.
Performance characteristics: this may improve energy resolution at densities > 0.6 g/cc?
Energy resolution is 2.0-2.5% at 662 keV and < 1.5% at > 1 MeV
Temperature range 15-200 C
Large volume, large effective area
Relatively low-cost, long-term stability
Large-volume, up to 1000 cm3, high-sensitive spectrometers (single or arrays) for portal applications and environmental monitoring (remotely, without servicing)
Small, < 200 cm3, rugged devices for application in harsh environments (high-radiation, high-temperature, high-vibrations, etc.), e.g., active zone of nuclear reactors, radioactive waste, well-logging.Expected performance and application arias
Portal security and
Challenges: this may improve energy resolution at densities > 0.6 g/cc?
Purification and handling of Xe, electron lifetime, > 1 ms
Design constraints associated with high-pressure, ~50 atm, high-voltage, 20-30 KV, low-outgas materials (high-density ceramic, SS, Al, Ti)
Relatively weak output signals: the W-value in HPXe is ~20 eV ( ~5 eV in CZT)
High-sensitivity to acoustic noise due to high capacitance of bulky electrodes
proportional mode is not possible at high-pressure, > 20 atm
ionization mode: both charge and scintillation light can be used to generate output signals
primary scintillation light signal is weak, but still can be used as a trigger
practically it is difficult to detect light signals in high-pressure chambers
II. HPXe technology
High-pressure cylinder with spark-discharge purifier containing 1000 l of ultra-pure Xe
There are several tricks how to prepare and fill detectors
Practically, we never used getters to purify Xe; we keep them just in case
Spark purifier is the most efficient technique: e.g., it took ~2 hr to purify ~1000 l of Xe, purchased from Spectra Gases, Inc, to a purity level of > 2 ms measured at room temperature at ~60 atmXe purification and filling system at BNL
Spark-discharge produces microscopic Ti dust that trap with high efficiency electronegative molecules such as O2, CO, CO2, H2O, organics
Require ~10 KV bias and ~2 mA
Small amount of Ti dust can be introduced inside a detector during the filling to ensures that Xe gas stays pure inside a detector for very long time, yearSpark purifier and purity monitor
Ionization chamber for monitoring Xe purity
A small ionization chamber placed inside the spark purifier detects the muon tracks (one event per 20-30 sec)
Simple and robust
Allows to measure purity inside the spark purified and inside the detectorMonitoring the electron lifetime by measuring their drift time
Drift time measurements
The drift time gives a low estimate for the lifetime
Do not try to detect vertical events in HP!
R0 =Ar+Br2,A=10.9 cm3/mole
A. Bolotnikov and B. Ramsey, NIMA, 383 (1996) 619
Two geometries have been used for HPXe ionization chambers: parallel plate and cylindrical
Parallel plate geometry
is not suitable for large volume detectors operating at high pressure
detectors have large “dead” regions that result in a significant background caused Compton scattering
optimal for high-pressure vessels with thin walls: ~2 mm of SS or ~5 mm of Al (for 12-cm diameter vessel)
HV can be directly applied to the vessel walls used as a cathode
allows for making large-volume, > 10 l, detectors by using long cylindrical vesselsIII. Designs of HPXe ionization chambers
The drift region was limited by max bias that can be applied, 25 KV, and field ratio
Energy resolution is mainly determined by shielding inefficiency of the Frisch-grid
Xe density 0.5 g/cc
Sensitive volume 0.5 liter
Energy resolution better than 2%
Ti vessel, no hydrogen addedHPXe chamber built at BNL
G. Smith, P. Vanier, and G. Mahler, BNL
Large Compton continuum is due a significant fraction of dead regions inside the chamber
This chamber set a new record of stable operation time, >12 years
by G. Smith and P. Vanier
Symmetrical design have several benefits: applied, 25 KV, and field ratio
slotted anode to reduce the shielding inefficiency
provides more rigid design
Xe density 0.5 g/cc
Sensitive volume 1 liter
Energy resolution better than 2.5%
Uses Al vessel because Ti corrosion in a presence of H2HPXe chamber designed by MEPhI, Russia
Symmetrical two-drift region chamber
shielding inefficiency can be reduced to ~1%
since the field lines concentrate toward the anode, a smaller field ratio is required to get 100% electron transmission => less HV is required (this very important improvements)
The very first attempts to build cylindrical ionization chambers were not successful because they had the Frisch-grids made of stretched wires: very fragile, highly sensitive to acoustic noise. They did not show good performance.
Cylindrical ionization chamber with Frisch-grid
Schematic of cylindrical ionization chamber
Electric field line
distribution near the anode
Significant improvements in mechanical stability and performance of cylindrical ionizations chambers were achieved by using SS or Ni electroformed meshes
Several chambers were developed by group from MFSC/NASA:
5 cm diameter, 20 cm long
Density 0.35 g/cc
Spectroscopic performance similar to CZT detectors and much better than NaI (Tl).
This chamber became a baseline prototype for many other designs
Best CZT 6x6x15 mm3
Bolotnikov, Ramsey, TNS IEEE, 1998
Design of a commercial cylindrical ionization chamber with the self-supporting Frisch-grid
Typical design adopted by vendors (CTC in cooperation with MEPhI)
HV feedthrough adopted from MSFC
(bulky, low-cost, for prototyping)
Ceramic spokes supporting a grid
Length is up to 100 cm
Diameter is up to 12 cm
Xe pressure ~0.4-0.5 g/cm3
(depending on the chamber’s diameter)
Commercial ionization chamber designed by MEPhI the self-supporting Frisch-grid
2. Cylindrical ionizationchamber
3. Frisch grid
4. Anticoincidence scintillator
6. Charge sensitive amplifier
7. High voltage supply
8. Ceramic feed-through
9. Hermetical vessel
Energy range (50-5000) keV
FWHM at 662 keV 2% keV
Density of Xe 0.4-0.5 g/cm³
Diameter 120 mm
Length 300 mm
Total mass 6 kg
Voltage ± 24 V
Power 10 W
The chamber is sealed
inside a plastic can to avoid the effect of moisture
Large volume Xe detectors resolves low- and high energy gamma-ray lines
Energy spectra measured with ionization chambers developed by MEPhI
V. Dmitrenko et al, MEPhI
HPXe detectors have low activation by neutrons
Spectra from High Pressure Xenon Detector (120 mm, L=500 mm, M= 1.8kg) before and after activation by Pu-Be neutron source (T=66 hours, fluence= 1.5x1010 neutrons).
Spectra from NaI detector ( 80 mm, L=50 mm, M=0.9 kg) before and after activation by Pu-Be neutron source (T=66 hours, fluence= 1.5x1010 neutrons).
Typical energy resolution measured with large-volume detectors with the Frisch-grids was in the range 3-5%. The main reason is large capacitance of the detector, ~50 pF
The weakest part of these types in ionization chambers is the Frisch-grid which requires high-voltage and increase capacitance and acoustic noiseAttempts to build large-volume (10 L) HPXe cylindrical ionization chamber
CTC and MEPhI
Several designs of the ionization chambers without Frisch-grids have been proposed:
Originally, these designs were applied to CZT detectors
Unfortunately, direct copying of these techniques did not workNew designs: greedless ionization chambers
Co-planar grid chambers proposed by the group from University of Michigan
This design takes advantage of cylindrical geometry and coplanar-grid readout approach
Both wires are at the ground potential (important feature that makes it different from coplanar-grid device)
For the majority of events only one wire (it can be either one) collects electrons but both wires sense uncollected ions => the difference between the signals gives collected charge
Since either wire can be collecting, the differential signal can be negative or positive. This can be sorted electronically.
Problem: multiple interaction point events
Electric field inside inside the chamber
Dual-anode chamber was tested by using simple prototype consisting of two parallel anode wires stretched inside the ceramic tube.
The inner surface of tube was coated with an aluminum layer and used as a cathode.
Geometrical parameters of the chamber:
Wire diameter 0.75 mm
Wire length 30 cm
Spacing 3 mmInner diameter of ceramic tube 90 mm
Density of Xe is 0.3 g/cm3
Natural mixture of
The energy resolution is ~4% FWHM at 662 keV at electronic noise ~ 14 keV (~270 el)
This spectrum, collected for thorium isotopes, illustrates the capably of the device to detect high-energy gamma-rays.
Cathode Frisch-grids have been proposed:
Outer metal coating
AnodeVirtual Frisch-grid ionization chamber developed by CTC
Schematic of the device
Active area is inside a 2-in ceramic tube which has high-resistivity internal and low-resistivity external coatings
This chamber provided very good energy resolution, ~2% FWHM at 662 keV, but its active volume was small.
Despite all the efforts to make virtual Frisch-grid and co-planar grid devices, the classic ionization chambers with actual Frisch-grids demonstrate better performance!
1% at 662 keV
0.6% at 662 keV
Proportional to the detector’s
Capacitance. It is beneficial to operate at large t (no leakage current)
noise in JFET
Thermal noise in the
Use a so-called “tick” operation mode of the HV power supply
Noise generated by the HV supply
Proportional to the capacitance
Acoustic noise is caused by mechanical vibrations of the electrodes and sound waves in Xe
Large capacitance of the anode is the main problem!
Magnified region around the anode
Standard design of cylindrical ionization chamber
We expect < 5 pF capacitance per 5-7 cm wide segment
< 8 keV FWHM electronic noise
HPXe ionization chambers represent a mature technology for making roonm-temperature gamma-ray detectors
HPXe detectors provide spectral performance similar to CZT detectors and much better than NaI(Tl)
Due to low density of gas and bulky design, HPXe detectors have limited area of applications where they can compete with CZT and LaBr3
Large-volume ionization chamber are very promising for portal security applications, while small detectors can be used in harsh environments where other techniques do not workConclusions