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High-purity Germanium (HPGe) Detectors

hpge detector

Germanium (HPGe) detectors are semiconductor diodes having a p-i-n structure in which the intrinsic (I) region is sensitive to ionizing radiation, particularly x rays and gamma rays. Under reverse bias, an electric field extends across the intrinsic or depleted region. When photons interact with the material within the depleted volume of a detector, charge carriers (holes and electrons) are produced and are swept by the electric field to the P and N electrodes. This charge, which is in proportion to the energy deposited in the detector by the incoming photon, is converted into a voltage pulse by an integral charge sensitive preamplifier.

Because germanium has relatively low band gap, these detectors must be cooled in order to reduce the thermal generation of charge carriers (thus reverse leakage current) to an acceptable level. Otherwise, leakage current induced noise destroys the energy resolution of the germanium detector. Liquid nitrogen, which has a temperature of 77 °K is the common cooling medium for such detectors. The germanium detector is mounted in a vacuum chamber which is attached to or inserted into an LN2 Dewar. The sensitive detector surfaces are thus protected from moisture and condensible contaminants.

Standard Electrode Coaxial Ge Detectors (SEGe)

Description

The conventional coaxial germanium detector is often referred to as Pure Ge, HPGe, Intrinsic Ge, or Hyperpure Ge. Regardless of the superlative used, the detector is basically a cylinder of germanium with an n-type contact on the outer surface, and a p-type contact on the surface of an axial well. The germanium has a net impurity level of around 1010 atoms/cc so that with moderate reverse bias, the entire volume between the electrodes is depleted, and an electric field extends across this active region. Photon interaction within this region produces charge carriers which are swept by the electric field to their collecting electrodes, where a charge sensitive preamplifier converts this charge into a voltage pulse proportional to the energy deposited in the detector.

The n and p contacts, or electrodes, are typically diffused lithium and implanted boron respectively. The outer n-type diffused lithium contact is about 0.5 mm thick. The inner contact is about 0.3 µm thick. A surface barrier may be substituted for the implanted boron with equal results.

The CANBERRA Coaxial Ge detector can be shipped and stored without cooling. However, long term stability is best preserved by keeping the detector cold. Like all germanium detectors, it must be cooled when it is used to avoid excessive thermally-generated leakage current. The non-perishable nature of this detector widens the application of Ge spectrometers to include field use of portable spectrometers.

The useful energy range of the Coaxial Ge detector is 40 keV to more than 10 MeV. The resolution and peak shapes are excellent and are available over a wide range of efficiencies. A list of available models is given in the accompanying table.

Features

  • Energy range from 40 keV to >10 MeV
  • High resolution - good peak shape
  • Excellent timing resolution
  • High energy rate capability
  • Diode FET protection
  • Warm-up/HV shutdown
  • High rate indicator

Standard Electrode Coaxial Ge Detectors (SEGe)
Coaxial Ge Detector Configuration

Broad Energy Germanium Detectors (BEGe)

Description

The CANBERRA Broad Energy Ge (BEGe) Detector covers the energy range of 3 keV to 3 MeV like no other. The resolution at low energies is equivalent to that of our Low Energy Ge (LEGe) Detector and the resolution at high energy is comparable to that of good quality coaxial (SEGe) detectors.

Most importantly the BEGe has a short, fat shape which greatly enhances the efficiency below 1 MeV for typical sample geometries. This shape is chosen for optimum efficiency for real samples in the energy range that is most important for routine gamma analysis. This is in stark contrast to the traditional relative efficiency measurement – a 60Co point source at 25 cm which is hardly a relevant test condition for real samples. See the figures below comparing absolute detector efficiencies of a 5000 mm² and 6500 mm² BEGe Detector to coaxial detectors with approximately 60% relative efficiency.

In addition to higher efficiency for typical samples, the BEGe exhibits lower background than typical coaxial detectors because it is more transparent to high energy cosmogenic background radiation that permeates above ground laboratories and to high energy gammas from naturally occurring radioisotopes such as 40K and 208Tl (thorium). This aspect of thin detector performance has long been recognized in applications such as actinide lung burden analysis.

Most Low Energy Detectors are aptly named because they do not give good resolution at higher energies. In fact resolution is not usually specified above 122 keV. The BEGe represents a breakthrough in this respect. The BEGe is designed with an electrode structure that enhances low energy resolution and is fabricated from select germanium having an impurity profile that improves charge collection (thus resolution and peak shape) at high energies. Indeed, this ensures good resolution and peak shape over the entire mid-range which is particularly important in analysis of the complex spectra from uranium and plutonium.

Features

  • Energy range from 3 keV to 3 MeV combines the spectral advantages of Low Energy and Coaxial HPGe detectors
  • Detection efficiencies and energy resolutions are optimized in the 3 keV to 662 keV energy region where most tightly-grouped gamma lines of interest are located
  • Flat, non-bulletized crystals offer optimum efficiencies for samples counted close to the detector
  • Thin, stable entrance window allows the detector to be stored warm with no fear of low energy efficiency loss over time

Broad Energy Germanium Detectors (BEGe)}</

Reverse Electrode Coaxial Ge Detectors (REGe)

Description

The reverse-electrode detector (REGe) is similar in geometry to other coaxial germanium detectors with one important difference. The electrodes of the REGe are opposite from the conventional coaxial detector in that the p-type electrode, (ion-implanted boron) is on the outside, and the n-type contact (diffused lithium) is on the inside. There are two advantages to this electrode arrangement – window thickness and radiation damage resistance.

The ion-implanted outside contact is extremely thin (0.3 μm) compared to a lithium-diffused contact, enabling the REGe detector to cover a broad energy range from 3 keV up to several MeV. REGe detectors are normally equipped with a carbon composite window which is robust and provides excellent transmission to below 10 keV. Beryllium or aluminum windows are also available. Aluminum is preferred when there is no interest in energies below 20 keV and improved ruggedness is desired. Beryllium should be selected to take full advantage of the low energy capability (down to 3 keV) of the REGe detector.

It has been found that radiation damage, principally due to neutrons or charged particles, causes hole trapping in germanium. Unlike the case of the conventional coaxial detector, holes are collected by the outside electrode of the REGe detector. Since a much greater amount of the active detector volume is situated within a given distance, ∆ R, of the outside contact, than of the inside contact (Volume ≈ R2) it follows that, on average, holes have less distance to travel if they are attracted to the outside contact than if they are attracted to the inside contact. With less distance to travel, they are less likely to be trapped in radiation damaged material. The extent of the improved resistance to radiation damage depends on other facts, of course, but experimental evidence suggests that the REGe detector may be 10 times as resistant to damage as conventional coaxial germanium detectors.

Features

  • Spectroscopy from 3 keV to >10 MeV
  • Ultra-thin ion implanted contacts
  • Radiation damage resistant
  • Excellent timing resolution
  • High energy rate capability
  • Diode FET protection

Reverse Electrode Coaxial Ge Detectors (REGe)

Extended Range Coaxial Ge Detectors (XtRa)

Description

The CANBERRA XtRa is a coaxial germanium detector having a unique thin-window contact on the front surface which extends the useful energy range down to 3 keV. Conventional coaxial detectors have a lithium-diffused contact typically between 0.5 and 1.5 mm thick. This dead layer stops most photons below 40 keV or so rendering the detector virtually worthless at low energies. The XtRa detector, with its exclusive thin entrance window and with a Carbon Composite cryostat window, offers all the advantages of conventional standard coaxial detectors such as high efficiency, good resolution, and moderate cost along with the energy response of the more expensive Reverse Electrode Ge (REGe) detector.

The response curves (below) illustrate the efficiency of the XtRa detector compared to a conventional Ge detector. The effective window thickness can be determined experimentally by comparing the intensities of the 22 keV and 88 keV peaks from 109Cd. With the standard 0.6 mm Carbon Composite window, the XtRa detector is guaranteed to give a 22 to 88 keV intensity ratio of greater than 20:1. Beryllium and aluminum windows are also available.

Features

  • Spectroscopy from 3 keV to >10 MeV
  • Wide range of efficiencies
  • High resolution - good peak shape
  • Excellent timing resolution
  • High energy rate capability
  • Diode FET protection

xtra-detector

Small Anode Germanium Well Detectors (SAGe Well)

Description

The CANBERRA SAGe™ Well Detector combines excellent energy resolution at low and high energies with maximum efficiency for small samples. Like Traditional Well Detectors, the SAGe Well is fabricated with a blind hole, leaving at least 20 mm of active detector thickness at the bottom of the well. The counting geometry therefore approaches 4p.

The low detector capacitance associated with the small anode technology (similar to what is used on CANBERRA's BEGe detectors) gives the SAGe Well superior low and medium-energy resolution performance compared to Traditional Well or Coaxial Detectors, as well as excellent resolution for higher energy gamma rays.

Furthermore, the detector is manufactured with an aspect ratio of a coaxial detector to allow excellent efficiency performance for standard laboratory geometries such as Marinelli beakers or other large sample containers. The result is a versatile detector that can deliver reductions in count time, through improvements in Minimum Detectable Concentration/Activity (MDC/MDA), for a range of sample sizes and geometries counted inside the well, on the end cap or in Marinelli beakers.

Features

  • Blind well approaches 4p counting geometry yielding high absolute efficiency
  • Superior resolution compared to Traditional Well Detectors at both low and high energies
  • Larger well diameter (28 mm) available with the same excellent resolution as the standard (16 mm) well sizes
  • Thin lithium diffused contact inside well allows spectroscopy from 20 keV up to 10 MeV
  • Full LabSOCS™ characterization available, allowing True Coincidence Summing correction

germanium-well

Traditional Germanium Well Detectors

Description

The CANBERRA High-Purity Germanium (HPGe) Well Detector provides maximum efficiency for small samples because the sample is virtually surrounded by active detector material. The CANBERRA Well detector is fabricated with a blind hole rather than a through hole, leaving at least 15 mm of active detector thickness at the bottom of the well. The counting geometry therefore approaches 4π.

The Well insert in the endcap is made of aluminum with a side-wall thickness of 0.5 mm and a 1 mm thick bottom. The ion implanted contact on the detector element is negligibly thin compared to 0.5 mm of aluminum so these detectors have intrinsically good low energy response, allowing spectroscopy down to 20 keV.

Features

  • Blind well approaches 4π counting geometry yielding high absolute efficiency
  • Large variety of models available allowing to select the optimum Well detector for your application
  • Thin, ion-implanted contact inside Well allows spectroscopy from 20 keV up to 10 MeV

germanium-well

Low Energy Germanium Detectors (LEGe)

Description

The Low Energy Germanium Detector (LEGe) is in all aspects optimized for performance at low and moderate energies and has specific advantages over conventional planar or coaxial detectors. The LEGe detector is fabricated with a thin front and side contact. The rear contact is of less than full area which gives a lower detector capacitance compared to a planar device of similar size. Since preamplifier noise increases with detector capacitance, the LEGe affords lower noise and consequently better resolution at low and moderate energies than any other detector geometry. Unlike grooved planar detectors, there is little dead germanium beyond the active region. This, and the fact that the side surface is charge collecting rather than insulating, results in fewer long-rise time pulses with improved count rate performance and peak-to-background ratios.

The LEGe detector is available with active areas from 50 mm2 to 2000 mm2 and with thicknesses ranging from 5 to 20 mm. For applications involving moderate gamma-ray energies, the LEGe may well outperform a more expensive large volume coaxial detector. The efficiency curve given below illustrates the performance of a typical LEGe detector.

Features

  • Thin front and side contact, allowing spectroscopy from 3 keV up
  • Wide range of sizes allows selecting the best detector for your application
  • Low noise and consequently high resolution at low and moderate energies

germanium-well

Ultra-LEGe Detectors (GUL)

Description

The CANBERRA Ultra-LEGe detector extends the performance range of Ge detectors down to a few hundred electron volts, providing resolution and peak-to-background ratios once thought to be unattainable with semiconductor detectors. The Ultra-LEGe retains the high-energy efficiency intrinsic to germanium detectors because of the high atomic number (Z), combined with a relatively high thickness (5-10 mm), and thus covers an extremely wide range of energies. The graph in Figure 2 below compares the efficiency on the high-energy side of the X-ray spectrum of a 5 mm thick germanium detector to typical silicon based detectors.

Conventional Ge detectors, including those made especially for low energies, suffer from poor peak shape and efficiency below 3 keV. This characteristic, once thought to be fundamental to Ge, prohibited use of Ge detectors in most analytical x-ray applications. CANBERRA has developed detector fabrication techniques which have eliminated these problems. The resulting detector, the Ultra-LEGe, delivers the intrinsic efficiency and resolution advantages of germanium without the disadvantages of the conventional germanium detector.

Features

  • Spectroscopy from 300 eV to 300 keV
  • High efficiency compared to Si(Li) and SDD
  • Excellent resolution up to very high count rates
  • High peak/background ratio

Ultra-LEGe

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