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Low-Background Germanium (HPGe)
Detectors
Factors Affecting Low-Background Gamma Spectroscopy
with Germanium (HPGe)
Detectors
The radiation background of standard cryostats used by ORTEC for
germanium detectors is lower than that required for the majority of users. Net area peak
counting rates ~0.1 counts/min are typical at energies of interest. Nonetheless, those
measuring environmental samples who require the lowest MDAs in the shortest possible
counting time will be best served by a large germanium detector in a low- or
extra-low-background cryostat. Coaxial detectors of efficiency from 80% to 175%, with
their exceptionally high peak-to-Compton ratios (approaching 100 to 1), are also
recommended (Ref. 12).ORTEC offers a variety of low-background detector options and cryostat
configurations to meet each customer’s specific requirements.
There are a number of naturally-occurring radionuclides that contribute
to the gamma-ray background observed by spectroscopists using a germanium detector system.
Contributions from the cosmic-ray induced background, 40K from building
structures, and radon, can be markedly reduced by appropriate shielding (including in some
cases an underground location) and flushing the shield with "aged" nitrogen. The
principal sources of activity from the cryostat are the primordial emitters, 238U,
232Th, 235U and their daughters, man-made radionuclides including 137Cs
fallout, and the activation product 60Co. There are both full-energy photopeaks
and associated Compton background from 46 to 2600 keV.In some materials, the natural emitter chains may not be in equilibrium.
Therefore, ORTEC reports the measured background at all the energies shown in Table 8.
Different spectroscopists have different "low-background"
requirements and energies to which the phrase "low background" applies.
Therefore, low-background cannot be rigorously defined as, for example, energy resolution
at the 1.33-MeV line of 60Co. For this reason various laboratories use
different nomenclature and different report formats when describing measured background.To satisfy the needs of spectroscopists, ORTEC does the following:
- Carefully selects low-background materials
- Characterizes completed HPGe detectors in a specialized low-background facility
- Produces the world’s largest detectors in the lowest-background cryostats —
the ultimate for gamma spectroscopy of low-level samples
- Provides with each detector an activity report of the 22 most common isotopes
- Remains current with technology for low-background materials
Environmental spectroscopists seeking to minimize MDA while maximizing
sample throughput should read: The Benefits of Using Super-Large Germanium Gamma-Ray
Detectors for the Quantitative Determination of Environmental Radionuclides.
Table 8. Environmental Radionuclide
Full-Energy
Photopeak Energies. |
Isotope
(Parent Nuclide) |
Energy in keV |
Isotope
(Parent Nuclide) |
Energy in keV |
| U x-rays |
13.0, 13.3* |
137Cs |
661.6 |
| 231U |
25.6* |
214Bi (238U) |
727.2 |
| 137Cs |
31.8, 32.2, 36.4* |
234mPa (238U) |
766.6 |
| 210Pb (238U) |
46.5 |
228Ac (232Th) |
911.0 |
| 234Th (238U) |
63.3 |
228Ac (232Th) |
969.0 |
| 234Th (238U) |
92.6 |
234mPa (238U) |
1001.0 |
| 235U, 226Ra |
>185.7, 186.2 |
214Bi (238U) |
1120.3 |
| 212Pb (232Th) |
238.6 |
60Co |
1173.0 |
| 214Pb (238U) |
295.2 |
214Bi (238U) |
1238.0 |
| 214Pb (238U) |
351.9 |
60Co |
1332.5 |
| Cosmic |
511.0 |
40K |
1460.8 |
| 208Tl (232Tl) |
583.1 |
214Bi (238U) |
1764.5 |
| 214Bi (238U) |
609.3 |
208Tl (232Tl) |
2614.5 |
| *The lines lower than 46 keV are reported only for LO-AX and
GMX detectors. |
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