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Introduction to
Charged-Particle Detectors
The detectors of choice for alpha spectroscopy are
ULTRAs with a
depletion depth of ³100 microns and
ULTRA-AS detectors for
ultra-low background applications. Many established installations are equipped with
reliable Ruggedized (R-Series) Surface Barrier Detectors. As these require negative bias,
the U Series are not a direct replacement in alpha spectrometer units. (All other ORTEC
charged-particle detectors require positive bias.) The reasons why the ULTRA and ULTRA-AS lines are widely used in alpha spectroscopy are the following: • Alpha spectroscopists with low activity samples often position samples as close as possible to the front detector contact. As noted above, the thin (500 Å silicon equivalent) window results in optimal energy resolution.• The front contact is cleanable. (This is also true of R-Series Surface Barrier Detectors, but not of other surface barrier detectors.) • The type of edge passivation used with ULTRA Series Detectors permits positioning the sample as close as 1 mm from the detector entrance contact; the minimum distance with surface barrier detectors is 2.5 mm. As, in many cases, the efficiency of the detector depends strictly on geometrical factors, ULTRA detectors provide higher efficiency than surface barrier detectors.• The low leakage current results in low noise, also contributing to good energy resolution. An issue of particular importance in alpha spectroscopy is the need to perform low-background measurements. As health physics regulations become more stringent, it is becoming increasingly important to be able to analyze samples with extremely low activity. Measurements performed at ORTEC have confirmed that the ultimate limit to the low- background performance of silicon detectors, when manufactured and packaged with special materials and following strict cleaning procedures, is associated with the omnipresent cosmic radiation. This limit in the energy range from 3–8 MeV is 0.05 counts/hr/10–2 cc of active volume. This means that for a 450 mm2 active area, 100-µm thick, low- background ULTRA-AS (AS denotes low background), a background counting rate of about 6 counts/day is expected. To achieve such a low level, one must take exquisite care both concerning previous or present vacuum chamber contamination and in detector handling procedures.To minimize cosmic ray interactions, ULTRA-AS series are made as thin as possible, consistent with achieving good resolution. As natural alpha particles have a range not exceeding 30 microns in silicon, it would seem that a depletion depth not exceeding 30 microns should be sufficient. It would be were it not for the fact that such a high-capacitance detector would display excessive noise. A depletion depth close to 100 microns provides the best tradeoff. Some alpha spectroscopists employ an alpha recoil avoidance package to reduce the tendency for a gradual increase of background contamination on the detector surface. Information on this package is contained in the description of the ORTEC RCAP-2 system.For rough spectroscopy and for simple counting applications (as in continuous air monitors), ORTEC offers the ruggedized ULTRA CAM line. ULTRA CAM detectors are light tight and moisture resistant. Beta Spectroscopy and CountingThe interaction of beta rays (electrons) is described in the Review of the Physics of Semiconductor Detectors. A key concern when selecting a silicon detector for room temperature beta spectroscopy or counting is the generation of a sufficiently large signal to well exceed the detector beta resolution. For example, 1 MeV electrons, which are minimum ionizing particles, deposit only 0.4 keV/micron of silicon. The average energy loss in a 100-µm thick detector is 40 keV. As the threshold of the discriminator must be set 2.5 times above the beta resolution (noise), the beta resolution of the detector must be well below 15 keV FWHM to obtain meaningful data.High quality beta spectroscopy cannot be obtained with room temperature silicon detectors. ORTEC offers a number of solutions: • A complete Si(Li) detector-cryostat-preamplifier package, the BETA-X, for optimum energy resolution• Thick, coolable silicon detectors (A- or L-Series) Nuclear and Atomic PhysicsThe selection of appropriate detectors for nuclear and atomic physics is experiment dependent. Here are responses to frequently asked questions on this subject: Q. Which detectors should be used for heavy-ion spectroscopy?Because of the short, highly-ionized track of heavy ions, detectors with high electric field at the front contact are best. The F-Series Detectors have a warranted minimum electric field of 20,000 V/cm at the front contact. (See also ORTEC Application Note 40, "Heavy Ion Spectroscopy With Silicon Surface Barrier Detectors.") Q. Which detectors and what techniques should be applied for low-energy ion and charged-particle spectroscopy?For ions or particles in the energy range from 0 to 50 keV, one should cool both the detector and the first stage of the preamplifier. See "Detection Of Low Energy Heavy Particles With Silicon Barrier Detectors" by J.A. Ray and C.F. Barnett, IEEE Trans on Nuc. Sci. Vol NS-16, N1 (1969), pp. 82–86. An example of the results given in this paper is the spectrum shown in Fig. 1. Q. Which detectors and what techniques should be applied for fast timing?A silicon detector used for fast timing must have a high and uniform electric field throughout the depletion depth. Totally depleted detectors, such as an ULTRA or a high field partially depleted detector, capable of withstanding overbias should be used. With particles in the MeV range and above, subnanosecond FWHM timing values are achievable (Ref: T.J. Paulus, et. al., IEEE NS-24, N1- 1977).
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