Silicon surface barrier 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. 

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. 

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. 

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 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). 

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.

Since the energy resolution of silicon surface barrier detectors is bad for heavier ions, more complicated methods such as time of flight measurements or magnetic spectrometers are required. 

Detection of backscattered ions is normally performed by a silicon surface barrier detector at high scattering angles for increased surface sensitivity, as indicated in Figure 67. The detector position is often variable about the specimen position to allow various scattering angles to be chosen. 

The beam of accelerated and collimated He+ ions collides with the polymer sample, then the recoiled and forward-scattered particles pass through a thin carbon foil and enter a silicon surface barrier detector located at a distance of... 

An ORTEC Octete Plus high resolution a-spectrometry system was used to measure uranium isotopes and thorium a-emitters. The system consisted of eight independently-operated, 450 mm silicon surface barrier detectors located in separate chambers connected to a vacuum pump. The efficiency of the detectors ranged from 20 - 21.5%. The background count rate ranged from 0-2 counts within 18 hours under each of the energy... 

Name ( common name ) Silicon surface barrier detector (surface barrier, SSB) Lithium drilled silicon (Si-Li) High purity germaruum (HpGe)... 

In general the detector is a silicon surface barrier detector with a 100 % efficiency for the charged particles crossing the dead layer. The surface... 

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