Silicon surface detector

In nonresonant profiling, the silicon surface barrier detectors that detect the products of the nuclear reaction may also detect signals from incident ions that have been backscattered from the sample. Figure 4 shows an a particle spectrum from the reaction (p, a) along with the signal produced by backscattered... 

As earlier discussed, the dominant factor in the near-surface region is the particle detection system. For a typical silicon surface barrier detector (15-keV FWHM resolution for Fle ions), this translates to a few hundred A for protons and 100— 150 A for Fle in most targets. When y rays induced by incident heavy ions are the detected species (as in FI profiling), resolutions in the near-surface region may be on order of tens of A. The exact value for depth resolution in a particular material depends on the rate of energy loss of incident ions in that material and therefore upon its composition and density. 

Early measurements of " Th were on seawater samples and Th was co-precipitated from 20-30 L of seawater with iron hydroxide (Bhat et al. 1969). This procedure may not recover all of the " Th in the sample, and an alpha emitting Th isotope (e g., °Th or Th) is added as a yield monitor. Following chemical purification of the Th fraction by ion exchange chromatography, the Th is electrodeposited onto platinum or stainless steel planchets. The planchets are then counted in a low background gas-flow beta detector to measure the beta activity and subsequently with a silicon surface barrier detector to determine the alpha activity of the yield monitor. The " Th activity is thus determined as ... 

The chamber may also be equipped at 180° to the beam with a (silicon surface barrier) detector for analysis of scattered protons, which provides the option of performing quantitative light element analysis by RBS (q.v.). In certain applications RBS can determine most of the matrix composition and PIXE the trace element contribution. 

In this example, as in many facilities, the final detector is simply used to count the particles. When this is the case, solid state detectors, like silicon surface barrier ones, can also be used. 

Radioactivity of uranium can be measured by alpha counters. The metal is digested in nitric acid. Alpha activity is measured by a counting instrument, such as an alpha scintillation counter or gas-flow proportional counter. Uranium may be separated from the other radioactive substances by radiochemical methods. The metal or its compound(s) is first dissolved. Uranium is coprecipitated with ferric hydroxide. Precipitate is dissolved in an acid and the solution passed through an anion exchange column. Uranium is eluted with dilute hydrochloric acid. The solution is evaporated to near dryness. Uranium is converted to its nitrate and alpha activity is counted. Alternatively, uranium is separated and electrodeposited onto a stainless steel disk and alpha particles counted by alpha pulse height analysis using a silicon surface barrier detector, a semiconductor particle-type detector. 

Sodium contamination and drift effects have traditionally been measured using static bias-temperature stress on metal-oxide-silicon (MOS) capacitors (7). This technique depends upon the perfection of the oxidized silicon interface to permit its use as a sensitive detector of charges induced in the silicon surface as a result of the density and distribution of mobile ions in the oxide above it. To measure the sodium ion barrier properties of another insulator by an analogous procedure, oxidized silicon samples would be coated with the film in question, a measured amount of sodium contamination would be placed on the surface, and a top electrode would be affixed to attempt to drift the sodium through the film with an applied dc bias voltage. Resulting inward motion of the sodium would be sensed by shifts in the MOS capacitance-voltage characteristic. 

The imager of JP-A-6089991 (Toshiba Corp., Japan, 29.03.94) comprises HgCdTe detector regions which have been grown on a silicon substrate. A method to clean the silicon surface before the HgCdTe regions are grown thereon is disclosed. 

For a spectrometry silicon surface-barrier detectors are most suitable. They are operated at room temperature in a vacuum chamber to avoid energy losses. The a particles are stopped within a thin depleted region of the detector and the number of electron-hole pairs is directly proportional to the energy of the a particles. The charge pulses are integrated in a charge-sensitive amplifier. Some a emitters used as a. sources for the purpose of calibration are listed in Table 7.4. 

A modem version of the charged particle detector is called PIPS, an acronym for Passivated Implanted Planar Silicon. This detector employs implanted rather than surface barrier contacts and is therefore more mgged and reliable than the Silicon Surface Barrier (SSB) detector it replaces. 

At 1 MeV/amu energies, the dE/dx and total energy measurements are made with either gas ionization detectors or silicon surface-barrier detectors or a combination of these. The time-of-flight detector serves as an additional positive-ion mass analysis stage. It is most useful for the heaviest (slowest) ion such as I and consists of two time-pickoff detectors with time resolution of a few hundred picoseconds. 

Spectra. The energy spectrum is collected from the particles emitted from all depths simultaneously using a silicon surface barrier detector, electronic amplifiers, an analog-to-digital converter and a multichannel analyzer. A reference pulse is fed into the electronics to monitor the stability of the system thus allowing corrections to be made should electronic drift occur during the course of the measurement. Specific systems are described in the references (1 -4,6,7,12-17). By using computer-based data acquisition systems, the depth profile can be displayed at the time of analysis. 

Figure 16 shows a pulse height spectrum recorded in an ion-implanted-silicon surface barrier detector mounted in the zero degree direction of the electron cooler in CRYRING. A stored beam of 4.4 MeV D30" ions interacts with a beam of velocity matched electrons, thus the collision energy... 

Another apparatus, which permits the recording of simultaneous ETA. DTA. and TG DTG. is shown in Figure 8.50 (192). The system consists of a commercial DTA apparatus and thermobalance manufactured by Netzsch-Geratebau, Selb, West Germany. For ETA measurements, an inert carrier gas is passed over the sample S and the standard material I situated in the isothermal region of the furnace F. The radioactive emanation released from the sample is carried into a measuring cell. The alpha-activity of the emanation E is counted by means of a silicon surface barrier detector D connected... 

Figure 8.50. Apparatus for simultaneous ETA, DTA, and TG/DTG measurements (194). A, Amplifier D, silicon surface barrier detector F. furnace 1, standard material S. sample FM. flow meter ST, flow stabilizer RM. count-rate meter.

Figure 9.13 Four examples of response functions (a) 5-MeV Alpha particles detected by a silicon surface barrier detector (Chap. 13), or 20-keV X-rays detected by a Si(Li) reactor (Chap. 12). ib) 1-MeV Gamma ray detected by a NaI(Tl) crystal (Chap. 12). (c) 1-MeV Electrons detected by a plastic scintillator (Chap. 13). (

The best energy resolution for electrons is obtained using silicon semiconductor detectors, with the possible exception of magnetic spectrometers. Silicon detectors may be surface-barrier or Si(Li) detectors. The surface-barrier detectors operate at room temperature, while the Si(Li) detectors give best results when cooled to liquid nitrogen temperatures. The energy resolution of semiconductor detectors is determined by the electronic noise alone. It deteriorates as the area and the sensitive depth of the detector increase. For commercial detectors the full width at half maximum (FWHM) ranges from about 7 to 30 keV. 

The best energy resolution is obtained with silicon surface-barrier detectors. Most detector manufacturers quote the resolution obtained for the 5.486-MeV alphas of A typical spectrum obtained with a detector having 25 mm ... 

Figure 13.14 The Am alpha spectrum obtained with a silicon surface-barrier detector (from Canberra).

The uncertainty AE/E is the resolution of the detector measuring the energy of the ion. The best energy resolution that can be achieved with silicon surface-barrier detectors is about 1.5-2 percent. The resolution can be improved with magnetic or electrostatic analyzers (Dilorio and Wehring achieved 0.3 percent energy resolution using an electrostatic analyzer). 

Beta spectrometry with silicon semiconductor detectors (surface barriers or Si/Li type). 

The radiation sensitive depleted layer is available in various thicknesses, < 5 mm, enough to stop electrons of 2.2 MeV, p of 32 MeV, and a of 120 MeV. A typical silicon surface barrier detector for a-spectroscopy has a sensitive area of 300 mm, 300 fim depletion depth, 20 keV FWHM (full width at half maximum) and operates at 100 V reverse bias. The resolving time is about 10 s. Special "rugged" detectors are available which have an acid resistant Si02 surface layer permitting cleaning and contact with liquids. Detailed information for detector selection is available from various detector manufacturers. 

Figure 9. View of the essential parts of the crossed beam apparatus using short-lived radioactive labeling and detection (23) Ay radioactive beam source By scrubber-furnace C, LN -cooled collimator D, shut-off plug Ey nozzle beam furnace and cryopump F, gate valve G, hodoscope H, LN -coohd beam trap 7, calibrated beam monitor /, silicon surface barrier detectors K, halogen crossed beam L, radioactive beam M, rotary feed-through used to close the source stopcock.

Semiconductor nanowires provide very sensitive detectors, where adsorbate-induced charge separation can convert an entire silicon nanowire into an accumulation or depletion zone. The same adsorbate on a macroscale silicon surface would induce only a surface layer but leave the bulk unaffected. The main problems to date with nanowires and their assembly are quality control a number of research endeavors are currently aimed at exploiting nanowires for detection and overcoming these technical challenges. Groups at Caltech and the Naval Research Laboratory have independently been pursuing innovative technical solutions to these problems. ... 

Silicon semiconductor detectors for nuclear radiation monitors of neutron rays have been developed by Kitaguchi et al. (1995,1996). These are diffused p-n junction-type devices with low leakage current coated on the surface of the B-containing sensor element. Neutrons were detected as recoil protons by interaction of the proton radiator and a-particles generated by the nuclear reaction °B (n, a) Li. The energy response of this radiation detector meets the standard recommendations and is suited as an area monitor and a personal dosimeter as well. 

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