development of high-pressure xe ionization chambers for gamma-ray spectroscopy aleksey bolotnikov

Brookhaven Science AssociatesU.S. Department of Energy

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

Aleksey BolotnikovBrookhaven National Laboratory

Workshop on Xenon-Based Detectors, Berkeley November 16-18 2009


Outline

• Introduction• Electron transport and spectroscopic properties of HPXe• HPXe detector technology (purification, gas filling)• Detectors designs and applications• Conclusion


Advantages of high-pressure Xe as detector medium

• 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, 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)


Intrinsic energy resolution of HPXe

• 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 -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 (-electron model) plus formation of large Xe cluster with liquid-like properties above 0.5 g/cc

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

Threshold


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.

A. Bolozdynya, A. Bolotnikov, J. Richards, A. Proctor, NIM A522, 595-597, 2004.

Spectra generated by thermal neutrons in Xe+3He mixture


Intrinsic resolution measured at different energies

Intr

insi

c re

solu

tion,

%

Xe density, g/cc

122 keV

166 keV

392 keV

835 keV

1275 keV

Romanuk, Dmitrinko, MEPHI


High electric field is required to achieve intrinsic energy resolution in compresses Xe

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

Energy resolution vs. electric field strength


Temperature effect on energy resolution

If clusters formation can be suppressed by heating the gas this may improve energy

resolution at densities > 0.6 g/cc?

0.5 g/cc 0.7 g/cc

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.

No effect Deteriorate


Changes of the W-value in compressed Xe

• 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:

– Xe+Xe+hvXe2*

– Xe2* Xe2

++e—

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

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.


Electron drift velocity

• Low mobility in pure Xe, drift velocity saturates at ~1 mm/s, 5 cm - 50 s

• 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

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

1.0%

0.5%

0.3%

0.2%

Pure Xe

0.7%

Dmitrenko, Romanuk (1980)


Stopping power of HPXe

• 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

HPXe detectors provide large effective areas and can replace NaI, CZT and Ge detectors in the areas where large effective area detectors are required.


Nuclear WellLogging

Portal security and environmental monitoring

Expected performance and application arias

• 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.


II. HPXe technology

• Challenges:

– 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


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


Xe purification and filling system at BNL

• 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


Spark purifier and purity monitor

Ionization chamber for monitoring Xe purity

Spark Dischargeregion

• 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


Monitoring the electron lifetime by measuring their drift time

• 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

Q

Qt

tt

Q

Q

exp1

t

The drift time gives a low estimate for the lifetime

Purity monitor

t

Do not try to detect vertical events in HP!

Drift time measurements


A. Bolotnikov and B. Ramsey, NIMA, 383 (1996) 619

Measurement of Xe density

• The dielectric constant is related to a density

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

R0 =A+B, A=10.9 cm3/mole

• The dielectric constant can be measured as a capacitance between two adjacent electrodes inside the chamber: =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%


III. Designs of HPXe ionization chambers

• 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


HPXe chamber built at BNL

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

• 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


A portable spectrometer based on BNL’s chamber

System builtby G. Smith and P. Vanier

This chamber set a new record of stable operation time, >12 years

Large Compton continuum is due a significant fraction of dead regions inside the chamber


HPXe chamber designed by MEPhI, Russia

• 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

• This gamma-ray spectrometer was a part of gamma-ray burst searching instrument onboard MIR space station• Continuously operated for > 10 years

Symmetrical two-drift region chamber


Cylindrical ionization chamber with Frisch-grid

Electric field linedistribution near the anode

Schematic of cylindrical ionization chamber• 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 a self-supporting Frisch-grid made of a mesh

Best CZT 6x6x15 mm3

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 longDensity 0.35 g/cc

Bolotnikov, Ramsey, TNS IEEE, 1998

2% Spectroscopic performance similar to CZT detectors and much better than NaI (Tl).

This chamber became a baseline prototype for many other designs

HPXe

HPXe


HV feedthroughs

Supporting insulators

Anode

Grid

Design of a commercial cylindrical ionization chamber with the self-supporting Frisch-grid

Dimensions:Length is up to 100 cmDiameter is up to 12 cmXe pressure ~0.4-0.5 g/cm3

(depending on the chamber’s diameter)

Typical design adopted by vendors (CTC in cooperation with MEPhI)

Ceramic spokes supporting a grid

HV feedthrough adopted from MSFC(bulky, low-cost, for prototyping)


1. PMT

2. Cylindrical ionization chamber

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

Commercial ionization chamber designed by MEPhI

• 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

Energy range (50-5000) keVFWHM at 662 keV 2% keVDensity of Xe 0.4-0.5 g/cm³Diameter 120 mmLength 300 mmTotal mass 6 kgVoltage ± 24 VPower 10 W

2-liter chamber

0.2-liter chamber

The chamber is sealed inside a plastic can to avoid the effect of moisture


Energy spectra measured with ionization chambers developed by MEPhI

Eu-152 Th-232

• Large volume Xe detectors resolves low- and high energy gamma-ray lines

V. Dmitrenko et al, MEPhI


Effect of radiation on detectors responses measured by MEPhI group

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).

HPXe detectors have low activation by neutrons


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

Developed by CTC and MEPhI

• 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


New designs: greedless ionization chambers

• Several designs of the ionization chambers without Frisch-grids have been proposed:– co-planar-grid – virtual Frisch-grid

• Originally, these designs were applied to CZT detectors

• Unfortunately, direct copying of these techniques did not work

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

Co-planar anode• Unpredictable electric field distribution around the strips deposited on ceramic• Large capacitance• No guard ring

• Co-planar anode is made of individual SS rods• High noise due to capacitance and HV


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

Large scale Small scale


Dual-anode chamber prototype tested at CTC

Preamplifiers

High-pressure Vessel

Ceramic Tube

HV Feedthrough

Dual Anode

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 mmWire length 30 cm Spacing 3 mmInner diameter of ceramic tube 90 mm


Pulse-height spectra measured with the prototype

The energy resolution is ~4% FWHM at 662 keV at

electronic noise ~ 14 keV (~270 el)

Natural mixture of Thorium isotopes

Density of Xe is 0.3 g/cm3

This spectrum, collected for thorium isotopes, illustrates the capably of the device to detect

high-energy gamma-rays.


Virtual Frisch-grid ionization chamber developed by CTC

Schematic of the device

Anode

Inner conductivelayer

Outer metal coating

Cathode

Ceramic tube

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

• 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


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

Proportional to the capacitance

Thermal noise in the JFET channel

Gate current noise in JFET

Noise generated by the HV supply

2222 ~ DETFETFETel

CeiQ

222 ~ DETGRIDACOUSTIC CkUQ

Proportional to the detector’s Capacitance. It is beneficial to operate at large (no leakage current)

22 ~ HVHV iQ Use a so-called “tick” operation mode of the HV power supply

Acoustic noise is caused by mechanical vibrations of the electrodes and sound waves in Xe

Large capacitance of the anode is the main problem!

22222ACOUSTICHVelGRID QQQ

W

EFQ

0.6% at 662 keV 1% at 662 keV


Design of the chamber with the segmented anode

Standard design of cylindrical ionization chamber

Charge-sensitive preamplifiers

Magnified region around the anode

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


Conclusions

• 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

development of high-pressure xe ionization chambers for gamma-ray spectroscopy aleksey bolotnikov

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