Secondary Emission Monitor Research Articles

Secondary Emission Monitors at the Bevatron-Bevalac

Secondary Emission Monitors (SEM) are used for high intensity, high energy beam fluence monitoring of heavy ions. The improved electronics and autozeroing of background noise has extended the useful range of the SEM down to the region where the limiting factor is capacitive changes between the chamber foils, from acoustic and mechanical vibration. Usable levels are in the 104 particles per pulse range for C6 ions. The secondary electron yield is proportional to 1/ß2. This gives an increase in yield of about a factor of 25 for injection energies over peak energies at the Bevatron. This enhanced yield has been exploited in designing relative intensity and position monitors that give usable signal levels while intercepting only a small percentage of the injected beam. The detector in this case is a wire grid. The output can be: 1. Sum proportional to the beam intensity or as a time profile; 2. Split grids for a right-left monitor; 3. Single wire scan for a spatial profile. These monitors give usable signals down to a level of 0.01-0.1 #x003BC;A of injected beam.

A low-matter beam profile secondary emission monitor

We present a new technology of a secondary emission profile monitor based on a photographic process, which represents an improvement as far as beam disturbance and mechanical reliability are concerned. The amount of matter is 1.0 mg/cm 2 per grid-plane.

Abstract: Neutron sputtering from metal foils

Experiments to determine the material removed (sputtering) from the surfaces of metal foils during irradiation with 14.8 MeV (d, t) neutrons will be described. Neutron activation analysis was used to measure the sputtered material. This is an integral technique which determines quantitatively the number of atoms on the collector. Six experiments were done using the Lawrence Livermore Laboratories rotating target (d, t) neutron source and one experiment was done at the (d, Be) source at the University of California, Davis. In each experiment a large number of collector–target assemblies were located at various distances from the neutron source. The collector–target assemblies were housed in a stainless steel ultrahigh vacuum system and all irradiations were performed in a high-vacuum environment [∠1×10−7 torr to 5×10−9 torr (∠1.3×10−5 Pa to 6.5×10−7 Pa)]. For these experiments the mean (d, t) neutron energy was 14.8 MeV in the forward direction, and for the different beam-on times and various source to sample distances the target fluences ranged from 1×1014 to 2.6×1016 (d, t) n/cm2. A (d, Be) source is highly anisotropic and is peaked in the forward direction. For the 20μA, 40 MeV deuteron beam used at the University of California, Davis, the average (d, Be) neutron energy was about 15 MeV, and at the point of closest approach to the Be target fluxes of ∠2×1013 n/cm2 s are produced. Neutron sputtering yields were obtained for niobium and gold. Collectors for all of the niobium and gold targets were 2.54-cm diameter, single- or polycrystal wafers made from 9N pure silicon ingots. In all cases the gold targets consisted of Marz grade foils 2.54 cm in diameter and 0.0127 cm thick, vacuum annealed, and etched in aqua regia. The niobium targets were also prepared from high-purity commercial grade material but had a wide variety of metallurgically different surface preparations. Except for the single crystals all the Nb samples were in the form of foils. Some were completely cold worked while others were vacuum annealed. Surface preparations varied from just chemical etching, to mechanical polishing and etching, and mechanical polishing and electropolishing. In this way a large range of surface topographies was produced. In all, collectors from 25 niobium samples covering 7 irradiations were examined using slow neutron activation analyses. About one-third of the collectors had no measureable 93Nb. Analyses of the niobium yield data as a function of neutron dose gave a zero slope. This means that our data give upper bounds for the sputtering yields and our best estimate of the neutron sputtering ratio for niobium is ?1×10−5 atoms/neutron. Micrometer size niobium particles were not found even after extensive search with a scanning electron microscope equipped with energy dispersive x-ray analysis. Table I is a summary of the secondary-emission monitor (SEM) studies. Statistical studies of the neutron activation analyses of the gold data show a linear relationship between the gold yield and neutron dose. Also, there is a significant gold background on the collectors. The best estimates of (d, t) neutron gold sputtering ration are ∠1.5×10−5 atoms/neutron in the forward direction and ∠1.0×10−5 atoms/neutron in the backward direction. Details of these experiments and data analyses will be submitted for publication in the Journal of Applied Physics or Physical Review.

Testing and calibration of a Faraday cup and other intensity monitors for the external proton beam at LAMPF

Three different methods of measuring beam intensities have been tested in the external proton beam at the Los Alamos Meson Physics Facility (LAMPF) over a wide range of beam currents. A 700 kg Faraday cup was constructed and has been demonstrated to be capable of absolute measurements with an error lower than 1%. A high gain ArCO 2 ion chamber was built for low beam intensity monitoring (down to a few pA) and provided an independent check of the Faraday cup calibration to better than 3%. A secondary emission monitor was also tested and calibrated relative to the Faraday cup. This secondary emission monitor provides reliable relative data for beams greater than about 50 pA.

Spill Control and Intensity Maonitoring for the Bevatron-Bevalac External Particle Beams

Time-intensity modulation in beam spill can be of primary concern in some experiments. The major source of this beam structure is from main-guide field-magnet power supply ripple. If the time constants are appropriate, then final control of beam structure can be accomplished by closed loop control of the intensity of beam spill. The response characteristics of the feedback system will determine the final structure. At high beam fluxes signal to noise ratio of beam detectors, in the feedback loop, can be improved by at least four orders of magnitude by using photomultiplier tubes and a water Cerenkov counter in place of the normal secondary emission monitor. At beam fluxes below 1010 particles per second (PPS), a plastic scintillator and photomultiplier tube are used in the feedback system. A plastic scintillator and photomultiplier are also used in the beam as intensity monitors. At intensities below about 107 PPS standard counting techniques are used. For intensities between 106 to 1109 PPS, the photomultiplier is used as a current source driving an integrating circuit which is then calibrated to read the number of particles per pulse.

Beam Monitoring in the Extraction Region of the Nevis Synchrocyclotron

A secondary emission monitor has been designed and tested for use in beam profile measurements of the extracted proton beam of the Nevis Synchrocyclotron. The monitor will provide a non-destructive simultaneous measurement of the vertical and horizontal position distributions, with a resolution of ~ .1 in. A residual gas ionization counter has been also designed. This counter will measure the radial profile of the accelerated beam at the parking radius.

Design and sensitivity of a secondary emission monitor of accelerated electron beam position

The design and sensitivity of a secondary emission monitor of accelerated electron beam position are considered in this paper. The emitting electrode-foils are cut into two equal halves, in two mutually perpendicular directions. The monitor sensitivity, measured at two values of beam pulsed current, 50 an d 100 mA appeared to be equal to 8.8 and 7.6 μA/mA mm respectively. At the given values of beam pulsed current the calculated sensitivity is respectively equal to 10.6 and 9.6 μA/mA mm.

Electroproduction of single positive pions above the resonance region

In this letter we report results of measurements of the reaction e § above the resonance region for k ~ (four-momentum transfer squared to the proton) equal to 0.7 (GeV/c) ~ and a 7:+n invariant mass s 89 equal to 1.9 GeV/c ~. These results are part of a preliminary programme of coincidence electroproduction measurements to establish the contribution of different hadronie states to the deep continuum crosssection. The results are fitted with fixed-t dispersion relation calculations of DEVENISH and LYTH (1) which have shown the cross-sections to be very sensitive to the electromagnetic charge form factor of the pion. From these fits a value is deduced for the pion form factor at this value of k 2. ENGELS et al. (3) have shown that similar fixed-t dispersion relation calculations fit measurements of y q p + n + r : + at our value of s, with their calculation demonstrating the importance of the Born terms, in particular the pionpole term. The importance of the pion-pole term in eleetroproduction has also been remarked on by BERENDS (3) and by MANWEILER and SCHMII)T (4). The experinient was performed at the Daresbury electron synchrotron using a 4.5 GeV extracted beam with a typical intensity of 2-1012 electrons/s and a duty ratio between 5 and 10%. The electron beam was focused into a 7.5 cm liquid-hydrogen target with a spot size typically {0.2x0.1)em 2, and divergence (2• 2)mrad% The beam intensity was continuously monitored with a 17-layer secondary emission monitor, which had been calibrated relative to a Faraday cup. An identical dummy target cell was used to determine the empty-target contribution to the scattering. The scattered electron and produce4 =+-meson were detected in two similar magnetic spectrometers, each consisting of two half-quadrupoles and a bending magnet. Scintillation counter hodoscopes were used to measure the momentum and production angles of the produced particles. The electron spectrometer had a momentum acceptance of =~ 5% and a

Evaluation of High Stability Secondary Emission Monitors

A stable, remotely operated low scattering (? = 0.246 mrad) secondary emission monitor (SEM) which was developed at Argonne National Laboratory (ANL) is described. Experimental results for secondary yield vs. energy of incident protons of 3.6 GeV/c to 12.33 GeV/c indicate that the yield increase is about 0.03%/GeV/c/A1 surface. Theoretical yield increase in the same energy range is about 0.02%/GeV/c/A1 surface. The overall yield/proton/A1 surface as observed at ANL increases from 2.7%/A1 surface at 3.6 GeV/c to 2.96%/A1 surface at 12.33 GeV/c.

A secondary emission monitor for electron beams of high energy and intensity

A secondary emission device is described. It is able to detect position and profile of an electron beam of high intensity and energy in its transport magnetic channel. Prototype characteristics with associated electronic circuits for signal pick-up and experimental results obtained with the 400 MeV electron and positron linac of the Laboratori Nazionali di Frascati del CNEN (LNF) are also reported. Experimental results at a primary's energy of 400 MeV are compared with a theoretical calculation based on the Møller cross-section for the electron-electron scattering, keeping into account, at the thicknesses involved in the device, other contributions like those from electromagnetic showers and tertiary electrons.

199. Investigations of a secondary emission monitor by electron beam: Yu M Arkatov et al,Pribory Tekh Eksper, 11(4), July–Aug 1966, 27–28 (in Russian)

199. Investigations of a secondary emission monitor by electron beam: Yu M Arkatov et al,Pribory Tekh Eksper, 11(4), July–Aug 1966, 27–28 (in Russian)

Faraday Cup and Secondary-Emission Monitor Calibration

Two small Faraday cups, one shielded and one unshielded, have been built and calibrated for efficiency vs electron energy. Measurements with a pulse beam transformer gave the absolute efficiency by means of a unique method: direct measurement of the ratio of the "lost" current to the incident beam current. Efficiencies for the unshielded cup are in the range 93 to 99 percent with uncertainties of 0.5 percent for electron energies from 10 to 50 MeV. The shielded Faraday cup has a measured efficiency of 97.8 ± 0.2 percent over the electron energy range from 10 to 60 MeV. This cup was used to calibrate the secondary emission produced by electrons or positrons incident on a 0.4 mm Be target. The resulting positive current was 4.3 ± 0.3 percent of the incident beam current over the energy range of 15 to 40 MeV.

The Secondary Emission Monitor as a Linear Accelerator Electron Beam Dose Monitor

Summary form only, as given. Describes the secondary electron emission monitor (SEM) and explains is use for monitoring electron or bremsstruhlung beams at high dose rates.

Servo System for Automatic Dose Control

Circuitry has been designed and constructed such that conveyor motion past a scanned Linear Accelerator beam is automatically regulated as a function of average beam current. Using this, the ratio of average current to conveyor speed can be held constant, thereby eliminating beam current variation as a source of error in irradiation processing. In the system, the ratio of current to conveyor speed is computed, multiplied by a factor inversely proportional to the scan height, and displayed on a digital voltmeter. The current signal is derived from a secondary emission monitor within the accelerator, while the conveyor speed is sensed by a tachometer coupled with the conveyor drive assembly. The beam scan is detected by a pair of adjustable sensor bars which are perpendicular to the plane of scan.

Secondary Emission Monitor as a Linear Accelerator Electron Beam Dose Monitor

The secondary electron emission monitor (SEM) is useful for monitoring electron beams at high dose rates since it is a vacuum device and free of the saturation problem encountered with ionization chambers. In the SEM, the current associated with secondary electrons ejected from a thin foil is collected and used to monitor the electron beam. A description is given of a SEM unit which exhibits a desirable voltage saturation characteristic and good linearity with beam current. The SEM current is about 0.035 of the transmitted beam current for a single aluminum collecting foil located between adjacent polarizing foils. The SEM is basically a current monitor, but for a scattered electron beam of constant incident energy a unique relationship exists between SEM current and the absorbed dose rate at a specified point in the radiation field. However, if the energy varies by only a few percent, the relationship between SEM current and the dose rate is greatly altered. Errors of 20 to 50% were encountered with a particular linac before the beam energy stability was improved. These errors are primarily due to a variation in scattered electron intensity stemming from the energy dependent nature of the scattering process, but also to the dependence of secondary electron yield on beam energy. The change in dose rate arising from the energy dependent aspect of scattering is similar to that occurring in bremsstrahlung production under appropriate conditions. Over a beam energy range of about 1 to 600 MeV, the variation of yield with beam energy is similar to that for electron stopping power. By improving the energy stability of a particular linear accelerator the variation in dose monitor calibration was reduced to a few percent; an improvement by a factor of about 10. The effects of voltage changes, beam loading, and temperature are discussed. In the accelerator described, the microwave power was recirculated. By servo controlling the phase of the feedback power, an important improvement in energy stability was achieved.

The Universal Fermi Interaction and the Conserved Vector Current in Beta Decay

The theoretical formulation of the (V-- A) fermi interac tion is shown. The requirements of the continuity equatio of the electromagnetic current are discussed in order to lead to the formulation of the conserved vector current theory and the unique properties arising from it. The present status of the question of equality between the vector coupling constant g/sub v/ of beta decay and the Fermi constant g/sub u/ of mu decay is examined. Four types of experiments to test the CVC hypothesis are discussed. (auth) O H21308 A procedure was developed for measuring photoneutron yields from targets of grent thickness by the method of induced activity. The dependence of the photoneutron yields on the electron energy, on /sup 16/O nuclei over an energy range of 19 to 66 Mev, and on /sup 63/Cu nuclei over an energy range of 13 to 50 Mev was investigated. The electron beam from a linear accelerator was monitored by means of a secondary emission monitor. The measured values of the photoneutron yields agree with the data of others; they proved to be haif the calculated values obtained by means of the Belenky-Tamm equilibrium spectrum. (auth)

A thin foil secondary emission monitor

A thin foil secondary emission monitor

Radiation therapy with high-energy electrons. I. Physical considerations, 10 to 60 Mev.

Deep radiation therapy with electron beams is now being performed at a number of institutions. Physical studies were first made with a 24-Mev betatron (18, 23, 35, 37–40, 67–69), but have subsequently been reported with the electron beam of an 18-Mev betatron (3–7, 12, 13, 21, 22, 25, 46–55, 64, 75, 78–80), a 100-Mev synchrotron (14, 62), a 35-Mev betatron (57, 61, 63, 65, 66, 82–87), a 45-Mev linear accelerator (8, 26, 27, 59, 70, 71, 76, 77), and 8- and 15-Mev linear accelerators (2, 15, 16, 20, 24, 81). Detailed reviews of electron beam therapy and dosimetry have been published by Laughlin (40–42) and by Becker et al. (6). An investigative program directed toward electron-beam radiation therapy was started at Stanford University in 1954 with the Mark IV 70-Mev microwave linear accelerator (1A) originally designed and built by the Microwave Laboratory of Stanford University for microwave and accelerator design studies. The present paper describes those studies on the physical properties of the electron beam which are of radiological significance. An accompanying paper reports the clinical aspects of the program (86). The Physical Arrangement Figure 1 shows the general layout of the accelerator, treatment room, and therapy control room. (The accelerator control console was at another level, not shown.) The electrons were accelerated in two sections, after which they passed through a double deflection magnetic system. The first (analyzer) magnet and the slit served to define the electron energy and the energy spread. The second (deflection) magnet separated the electron beam from neutrons and x-rays created at the slit. The magnetic field was monitored at the first magnet by a rotating coil fluxmeter, which was calibrated in terms of electron energy by photoneutron activation thresholds. The collimation system and accessories are shown in position in the treatment room in Figure 1 and in greater detail in Figure 2. The electrons emerged from the accelerator vacuum through an aluminum window, traversed the transmission ion-chamber monitor, and then were scattered to the desired angular width. After passing through a secondary emission monitor, the beam traversed a collimator made of heavy Presdwood. The walls of the upstream end of the collimator were 5 cm. thick, while those of the downstream end were 2.5 cm. thick, as shown in Figure 2. At the downstream end of the collimator were located a double, cylindrical chamber, a block of the Presdwood sufficiently thick to stop electrons of the highest energy in use at a given time, and a transparent plastic treatment cone. Forward and backward alignment lights aided in the precise set-up of patients and measuring equipment. A photograph of the collimator set-up is shown in Figure 3, and a mock patient set-up in Figure 4.