HPGe Detector
Definition
A high-purity germanium semiconductor crystal cooled with liquid nitrogen to about 77 kelvin. Resolution at the 1,332 keV gamma line of cobalt-60 is around 1.8 keV, an order of magnitude better than a NaI(Tl) scintillator. The reference detector for neutron activation analysis and for any case where multiple gamma emitters have to be unscrambled in one spectrum.
- Operating temperature
- About 77 kelvin (liquid nitrogen)
- Energy resolution at Co-60 peak
- Approximately 1.8 keV
- Primary forensic application
- Neutron activation analysis (NAA)
- Key advantage
- Can separate gamma lines from dozens of elements in one spectrum
Common questions
Why do HPGe detectors need liquid nitrogen cooling?+
The germanium crystal must be cooled to about 77 kelvin (liquid nitrogen temperature) to operate effectively. This reduces thermal noise in the semiconductor, which is essential for achieving the high energy resolution that makes HPGe detectors so precise at distinguishing gamma rays from different elements.
How much better is an HPGe detector than other gamma detectors?+
HPGe detectors are about an order of magnitude better than sodium iodide scintillators. At the standard cobalt-60 gamma line (1,332 keV), HPGe achieves energy resolution around 1.8 keV, compared to much broader peaks from NaI(Tl) detectors. This superior resolution lets you identify multiple elements in the same sample simultaneously.
What makes HPGe the standard for neutron activation analysis?+
NAA works by bombarding a sample with neutrons and measuring the gamma rays emitted by activated nuclei. Because HPGe can separate gamma lines from dozens of elements at once with typical energy resolution under 2 keV, it's the reference detector for this technique, allowing forensic analysts to identify and quantify multiple trace elements in a single measurement.
Related terms
- Half-Life
- The time required for half the radioactive atoms in a sample to decay. Set by the decay constant lambda through t1/2 =...
- Accelerator Mass Spectrometry
- A mass spectrometer fronted by a high-energy particle accelerator (typically a 500 keV to 6 MeV tandem). Counts individual carbon-14 atoms directly...
- BARC Trombay
- Bhabha Atomic Research Centre, Mumbai. Hosts the Dhruva and Apsara-U research reactors that have historically supplied neutron flux for Indian NAA work.
- Becquerel and Curie
- The becquerel (Bq) is one nuclear disintegration per second, the SI unit of activity. The older curie (Ci) is 3.7 × 10^10...
- Gamma Spectrometry
- Energy-resolving detection of gamma rays. Each radioisotope emits a characteristic gamma at a known energy, which is how NAA identifies elements.
- Geiger-Muller Tube
- A gas-filled cylindrical tube with a thin entrance window and a high-voltage central wire. An ionising particle entering the tube triggers a...
- INAA
- Instrumental Neutron Activation Analysis. The whole sample is irradiated, cooled, and counted directly. No wet chemistry. Non-destructive.
- Neutron Capture (N, Gamma)
- The reaction in which a stable nucleus absorbs a neutron and becomes a radioactive isotope of the same element, emitting a prompt...
- RNAA
- Radiochemical Neutron Activation Analysis. After irradiation, the analyst chemically separates the isotope of interest to remove interfering activity. More sensitive, destructive.
- Scintillation Detector
- A crystal or liquid (sodium iodide doped with thallium for gamma, an organic cocktail for low-energy beta) that emits visible-light flashes when...
- Thermal Neutron
- A low-energy neutron (around 0.025 eV) that has been slowed by a moderator. The workhorse particle of NAA because slow neutrons are...
Explained in these topics
- Neutron Activation Analysis (NAA) in Forensic ScienceHigh-purity germanium semiconductor detector. The standard detector for NAA because its energy resolution (typically under 2 keV at 1332 keV) can separate gamm...
- Radiochemistry: Radioactivity, Carbon Dating, Neutron ActivationA high-purity germanium semiconductor crystal cooled with liquid nitrogen to about 77 kelvin. Resolution at the 1,332 keV gamma line of cobalt-60 is around 1.8...