Development of high-pressure Xe ionization chambers for gamma-ray spectroscopy
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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

Aleksey Bolotnikov

Brookhaven National Laboratory

Workshop on Xenon-Based Detectors, Berkeley

November 16-18 2009


Electron transport and spectroscopic properties of HPXe

HPXe detector technology (purification, gas filling)

Detectors designs and applications



HPXe is attractive medium for room-temperature gamma-ray detectors

Advantages of Xe

stable, uniform

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/cc

Intrinsic 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

Xe+ 3He mixture irradiated with thermal neutrons

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 measured at different energies

1275 keV

835 keV

Intrinsic resolution, %

392 keV

166 keV

122 keV

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 detectors

High 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)


No effect

0.7 g/cc

0.5 g/cc

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

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:


Xe2* Xe2++e—

At high densities, W decreases because of the formation of the electronic band structure

Changes 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/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 Xe

Electron drift velocity

Electron drift velocity vs. electric field strength in Xe+%H2 mixture at 0.6 g/cm3

Dmitrenko, Romanuk (1980)

  • Comparison of stopping power of

  • several detector materials at 662 keV

  • 10-cm layer of Xe at 0.5 g/cc is equivalent to

  • ~0.5 cm of HgI2

  • ~1 cm of CZT

  • ~1 cm LaBr3

  • ~1.5 cm of NaI(Tl)

  • ~2 cm of Ge

  • The low stopping power can be compensated by the large area and volume, e.g., one 10-liter cylindrical ionization chamber (10 cm diameter and 100 cm long) has ~1000 cm2 area which is equivalent to ~ 500 1 cm3 CZT ( $20 K vs. $500 K) !

  • 10-liter HPXe detector is equivalent to a standard 6” NaI(Tl) detector

Stopping power of HPXe

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:

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



Nuclear Well



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

Xe purification and filling system at BNL

  • Two stages of purification:

    • preliminary purification with high-temperature getters for Xe and H2

    • spark-discharge for fine purification

  • The system ensures high purity of Xe during and after the filling

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 atm

Xe 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, year

Spark 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 detector

Monitoring the electron lifetime by measuring their drift time

Purity monitor

Drift time measurements


The drift time gives a low estimate for the lifetime

Do not try to detect vertical events in HP! 

Measurement of Xe density

  • The dielectric constant e is related to a density

  • The ratio R0=(e-1)/(e+2)is approximately a linear function of density:

    R0 =Ar+Br2,A=10.9 cm3/mole

  • The dielectric constant e can be measured as a capacitance between two adjacent electrodes inside the chamber: e=C/C0

  • During the measurements, a test pulse generator in connected to one electrode while the amplitude of the induced signals is used to estimate the capacitance

  • Accuracy is < 5%

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

Cylindrical geometry:

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 vessels

III. 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 added

HPXe chamber built at BNL

G. Smith, P. Vanier, and G. Mahler, BNL

A portable spectrometer based on BNL’s chamber

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

System built

by G. Smith and P. Vanier

Symmetrical design have several benefits:

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 H2

HPXe chamber designed by MEPhI, Russia

Symmetrical two-drift region chamber

  • This gamma-ray spectrometerwas a part of gamma-ray burst searching instrument onboard MIR space station

  • Continuously operated for > 10 years

Cylindrical geometry improves performance of Xe ionization chambers:

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

Cylindrical ionization chamber with a self-supporting Frisch-grid made of a mesh

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

HV feedthroughs

Supporting insulators



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

1. PMT

2. Cylindrical ionizationchamber

3. Frisch grid

4. Anticoincidence scintillator

5. Electronics

6. Charge sensitive amplifier

7. High voltage supply

8. Ceramic feed-through

9. Hermetical vessel

10. Anode

  • Uses specially designed HV feedthroughs to support the Frisch-grid and the anode

  • Three modifications of ionization chambers were developed based on this design: 0.2, 2.0, and 10 liters

Commercial ionization chambers developed at MEPhI

2-liter chamber

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

0.2-liter chamber

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



Effect of radiation on detectors responses measured by MEPhI group

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 noise

Attempts to build large-volume (10 L) HPXe cylindrical ionization chamber

Developed by


Several designs of the ionization chambers without Frisch-grids have been proposed:


virtual Frisch-grid

Originally, these designs were applied to CZT detectors

Unfortunately, direct copying of these techniques did not work

New designs: greedless ionization chambers

Co-planar grid chambers proposed by the group from University of Michigan

  • Co-planar anode is made of individual SS rods

  • High noise due to capacitance and HV

Co-planar anode

  • Unpredictable electric field distribution around the strips deposited on ceramic

  • Large capacitance

  • No guard ring

Dual-anode cylindrical ionization chamber proposed by CTC

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

Small scale

Large scale

Dual-anode chamber prototype tested at CTC

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

Pulse-height spectra measured with the prototype

Density of Xe is 0.3 g/cm3

Natural mixture of

Thorium isotopes

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.


Outer metal coating

Inner conductive


Ceramic tube


Virtual Frisch-grid ionization chamber developed by CTC

  • This design is proposed for small volume, ~200 cm3, but very robust and mechanically strong device

  • Withstand harsh environments:

    • high-radiation dose

    • high-temperature, up to 200 C

    • strong mechanical vibration

Schematic of the device

Active area is inside a 2-in ceramic tube which has high-resistivity internal and low-resistivity external coatings

Pulse-height spectra measured with prototype

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!

Factors limiting energy resolution of HPXe ionization chambers

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)

Gate current

noise in JFET

Thermal noise in the

JFET channel

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!

Design of the chamber with the segmented anode

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

Design of segmented HPXe ionization chamber

  • 10-liter volume chamber

  • Expected energy resolution 2% at 662 keV

  • Address several problems:

    • minimizes capacitance

    • reduces electronic and acoustic noises

  • We plan to build and test this design next year

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 work


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