Beam Stoppers Research Articles

Reduction of contaminants originating from primary beam by improving the beam stoppers in GARIS-II

Two independent beam stoppers have been developed for improving the beam separation of the gas-filled recoil ion separator GARIS-II. Performance of these supplemental beam stoppers was investigated by using the 208Pb (18O,3n) 223Th reaction. A160-fold enhancement of the signal-to-noise ratio at the GARIS-II focal plane was observed.

Charged particle activation facility in NPI CAS and in future GANIL/SPIRAL2-NFS

The proton, deuteron and alpha induced reactions are of a great interest for the assessment of induced radioactivity of accelerator components, targets and beam stoppers as well as isotope production for medicine and also to nuclear astrophysics. We present a new irradiation chamber for activation measurements, that forms a prolongation of long-term experimental activities using stacked-foil activation technique in NPI CAS, Řež. The chamber is based on an airlock system and is coupled to a pneumatic transfer system delivered by KIT Karlsruhe. This system is installed in GANIL/SPIRAL2-NFS and will be used for proton, deuteron and alpha particle activation measurements with long- and short-lived isotopes.

Energy Calibration of the ReA3 Accelerator by Time-of- Flight Technique1

We report on a simple method to perform an absolute calibration of the magnetic beam analyzer of the reaccelerator ReA3 at the National Superconducting Cyclotron Laboratory. The method is based on the time of flight between two beam stoppers 7.65 m apart. Based on two independent time-of-flight measurements at three different beam energies, the beam analyzer magnet is calibrated with an accuracy of 0.12 %, corresponding to a beam energy accuracy of 0.24 %.

Characterization and core renovation of beam stoppers for personnel safety

To minimize the risk of accident on personnel accessing the PS complex at CERN transfer lines are equipped with beam stoppers. They immediately shut the beam aperture whenever members of personnel enter an accelerator or test facility. Most of them consist of cores out of stainless steel designed for a proton beam-pulse energy at 9.0 kJ during the 70's. Further data on the design history of the beam stoppers is scarce. With the current pulse energy at 35.0 kJ steel reaches its structural limits. Based on the current function of the core requirements on the structural and beam-interaction performance of beam stoppers for personnel safety are drawn. To meet those requirements a compact core compatible to all PS complex is developed, replacing five existing design types. The new core introduces high-density sliced materials which withstand upgraded beam-pulse energies up to 92.5 kJ.

Method for thermal and structural evaluation of shallow intense-beam deposition in matter

The projected range of high-intensity proton and heavy-ion beams at energies below a few tens of MeV/A in matter can be as short as a few micrometers. For the evaluation of temperature and stresses from a shallow beam energy deposition in matter conventional numerical 3D models require minuscule element sizes for acceptable element aspect ratio as well as extremely short time steps for numerical convergence. In order to simulate energy deposition using a manageable number of elements this article presents a method using layered elements. This method is applied to beam stoppers and accidental intense-beam impact onto UHV sector valves. In those cases the thermal results from the new method are congruent to those from conventional solid-element and adiabatic models.

Thermal performance and redesign of beam stoppers under superficial heat transfer from intense ion beams in matter

In the Linear accelerator 3 (Linac 3) and Low Energy Ion Ring (LEIR) at CERN dedicated beam stoppers intercept ion beams for machine and personnel protection whenever required. The interaction of accelerated ions and the stopper induces through-thickness heat deposition in the stopper material as deep as a few hundreds of micrometers. Following a multi-layered geometrical discretization this paper discusses the thermal and structural performance of the stoppers under different ion beams (Pb, Xe, Ar). Design considerations based on the projected range of different ions in target materials (aluminum, steel and copper alloys) are discussed. An improved design for ion stoppers based on thermo-structural performance and projected range is proposed.

The activation of W and Zr by deuterons at energies up to 20 MeV

The proton and deuteron induced reactions are of a great interest for the assessment of induced radioactivity of accelerator components, target and beam stoppers. In order to investigate the important nuclides, we have carried up the irradiation experiments with the variable-energy cyclotron U-120 M of the NPI CAS Řež. The production cross sections of the nuclides 179,181,182m,182,183,184m,184,186Re and 187W from reaction on natural W were investigated by deuteron beams of 20 MeV energy. A part of preliminary results of deuteron activation of natural Zr is also shown. The stacked-foil technique was utilized. The comparison of present results to data of other authors and to predictions of evaluated data libraries is discussed.

Proton and deuteron activation measurements at the NPI and future plans in SPIRAL2/NFS

The proton- and deuteron-induced reactions are of a great interest for the assessment of induced radioactivity of accelerator components, target and beam stoppers as well as isotope production for medicine. In the present work, the deuteron-induced reaction cross sections on zinc were investigated by stacked-foil activation technique with deuteron beam of 20 MeV energy from the cyclotron U-120M of NPI CAS Řež. Also the proton activation cross section measurement of iron is presented. The comparison of present results to data of other authors and to predictions of evaluated data libraries is discussed. The investigation shall continue for higher proton and deuteron energy interval 20–35 MeV at SPIRAL2/NFS facility using a charged particle irradiation chamber with pneumatic transport system to measure isotopes and isomers with half-lives in minutes-regions.

SU‐E‐J‐43: Scatter and Beam Hardening Corrections for MV‐CBCT Using a Few MLC Leaf‐Pairs and a Cylindrical Water Equivalent Phantom

Purpose: MV‐CBCT has multiple advantages over KV‐CBCT for online IGRT, but its imaging contrast is lower than that of KV‐CBCT, partly because of the scatter and the beam hardening effects. 1. To correct the scatter artifacts in megavoltage CBCT using a few MLC leaf‐pairs as beam stoppers and after that; 2. To correct the beam hardening effect using fan beam projections acquired from a water equivalent cylindrical phantom. Methods: Two or three evenly spaced MLC leaf pairs were used as beam stoppers when acquiring the MV CBCT projections, and then the pixel readings under the leaf‐blocked area was largely due to scatter. These pixel readings, plus the pixel readings just outside the field edges, can help to estimate the 2D scatter map by interpolation, which can then be subtracted from the original projections. After the scatter correction, the beam hardening correction was made by using the fan beam projections taken from a water equivalent cylindrical phantom with little scatter. In this study, 180 full MV‐projections for a Catphan500 phantom with a bone equivalent annulus were acquired over 360 degree using 6 MV photon beams, with 0–180 and 180‐360 degree scans using different leaf‐pair as beam stoppers. The fan beam projections for beam hardening correction were acquired from a phantom of uniform density with a diameter of 25 cm. Results: Scatter and beam hardening induced cupping and shading artifacts were substantially reduced in MV‐CBCT. Also, the reconstructed Catphan images were complete because we used different leaf‐pairs as beam stoppers for 0‐180 and 180‐360 degree scans. Conclusion: An efficient and effective scatter correction scheme was proposed and applied for MV‐CBCT. After the scatter correction, the MV‐CBCT image quality matched that from fan‐beam MVCT. Beam hardening correction further improved the imaging contrast of MV‐CBCT.

Use of beam stoppers to correct random and scatter coincidence in PET: A Monte Carlo simulation

3D acquisition of positron emission tomography (PET) produce data with improved signal-to-noise ratios compared with conventional 2D PET. However, the sensitivity increase is accompanied by an increase in the number of scattered photons and random coincidences detected. Scatter and random coincidence lead to a loss in image contrast and degrade the accuracy of quantitative analysis. This work examines the feasibility of using beam stoppers (BS) for correcting scatter and random coincidence simultaneously. The origins of the photons are not on the path of non-true event. Therefore, a BS placed on the line of response (LOR) that passes through the source position absorbs a particular fraction of the true events but has little effect on the scatter and random events. The subtraction of the two scanned data, with and without BS, can be employed to estimate the non-true events at the LOR. Monte Carlo (MC) simulations of 3D PET on an EEC phantom and a Zubal Phantom are conducted to validate the proposed approach. Both scattered and random coincidences can be estimated and corrected using the proposed method. The mean squared errors measured on the random+scatter sinogram of the phantom obtained by the proposed method are much less than those obtained using the conventional correction method (the delayed coincidence subtraction for random correction combined with single scatter simulation for scatter correction). Preliminary results indicate that the proposed method is feasible for clinical application.

SWING detection vacuum tunnel

The SWING beamline is dedicated to the study of the small-angle X-ray scattering. In order to have the possibility to detect scattered intensity very close to the incident beam, it is absolutely necessary to install the detector at a long distance from the sample. In addition, it is easy to change the detector's position to access a wider angular range. A long and large vacuum chamber, the ‘tunnel’, has been designed with specific mechanisms inside to control the detector's position with micrometre resolution. Special attention has been given so as to offer a very useful device to the users. The paper will present the general design of the tunnel equipped with ancillary devices such as very narrow and stiff beam stoppers, diode holders and beam attenuators.

Excitation functions for some W, Ta and Hf radionuclides obtained by deuteron irradiation of 181Ta up to 40 MeV

Experimental excitation functions for deuteron induced reactions up to 40 MeV on mono-isotopic Ta ( 181Ta) were measured with the activation method using a stacked foil irradiation technique. From high resolution gamma spectrometry and X-ray analysis cross-section data for the production of 181W, 177,178g,180g,182m+gTa, and 179m2,180mHf were determined. Comparison with the scarce earlier published data are presented and results for values predicted by different theoretical codes, adapted for more reliable calculations for d-induced reactions, are included. Thick target yields for 182m+g,180g,178gTa and 181W were calculated from a fit to our experimental excitation curves. Using dose conversion factors and irradiation scenarios, possible occupational doses to maintenance or scientific personnel around high power accelerators where Ta based structural elements (collimators, beam stoppers, shielding) are present could be derived.

Bremsstrahlung and Photoneutron Leakage from Steel Shielding Board Impinged by 12-24 MeV Electrons Beams

Many medical linear accelerators generate not only high-energy photons, but also high-energy electrons, and they are no longer equipped with beam stoppers. Therefore, shields might be necessary against bremsstrahlung and photoneutron generated by high-energy electron beams. However, there are few physical studies, and no recommendations are made about shields nowadays. In this report, the leakage doses of bremsstrahlung and photoneutron were calculated by the use of Monte Carlo simulation. To verify the calculated results, the photoneutron leakage dose was measured with a rem counter. The results clearly show that the bremsstrahlung and photoneutron leakage dose generated by electron beams of 24 MeV or below is negligible.

Optimisation of scatter correction based on the use of partially transparent beam stoppers in PET

To improve the counting efficiency, modern positron emission tomography scanners generally perform 3D data acquisition with the septa retracted. As a consequence, the scatter fraction can surge to as high as 40%. Our previous method proposed the use of a beam stoppers (BS) device to directly measure the scatter and to remove it from the sinogram. The BS partially attenuate the primary radiation of certain lines of response (LOR) without affecting the scattering events. To measure the scatter, dual scans are performed with and without the device in place. This study proposes an improved method. The dual scans are performed with the device placed at two different locations. The proposed method reduces the effective sampling distance without the need to actually increase the stopper's number. The scatter component at the LOR blocked by each stopper can be estimated. Assuming that the scattered radiation has a spatially slow-varying distribution, the whole scattered sinogram can be recovered using interpolation from these local measurements. Once the scatter component is estimated, the sinogram of the primary radiation can be obtained by subtracting the scatter component from the original data. The scatter free image is then reconstructed from the primary sinogram. This method was applied to scanned data of two digital phantoms using the Simulation System for Emission Tomography simulation package to demonstrate its effectiveness. Preliminary results showed that this method has slightly better performance than the previous BS method and the conventional single scatter simulation technique with smaller mean squared error and less noise in the corrected image.

Scatter correction for 3D PET using beam stoppers combined with dual-energy window acquisition: a feasibility study

Fully three-dimensional (3D) positron emission tomography (PET) can achieve high sensitivity of coincidence events, but the absence of inter-slice septa inevitably leads to increased scattered events. The scattered events can represent as much as 50% of the total detected events. In this research, we proposed a scatter correction method for 3D PET based on beam stoppers and dual-energy window acquisition. The beam stoppers were placed surrounding the object to attenuate primary beams. The scatter fractions were directly estimated at those blocked lines of response and then the entire scatter fraction distribution was recovered using the dual-energy window ratio as reference. The performance was evaluated by using Monte Carlo simulations of various digital phantoms. For the Utah phantom study, the proposed method accurately estimated the scatter fraction distribution, and improved image contrast and quantification based on four different quality indices as performance measures. For the non-homogeneous Zubal phantom, the simulated results also demonstrated that the proposed method achieved a better restoration of image contrast than the dual-energy window method. We conclude that the proposed scatter correction method could effectively suppress various kinds of scattered events, including multiple scatter and scatter from outside the field of view.

Radiation safety system of the B-Factory at the Stanford Linear Accelerator Center.

The radiation safety system of the B-Factory accelerator facility at the Stanford Linear Accelerator Center is described. The radiation safety system, which is designed to protect people from prompt radiation exposure due to beam operation, consists of the access control system and the radiation containment system. The access control system prevents people from being exposed to the very high radiation levels inside a beamline shielding enclosure. The access control system consists of barriers, a standard entry module at every entrance, and beam stoppers. The radiation containment system prevents people from being exposed to the radiation outside a shielding enclosure due to either normal or abnormal operation. The radiation containment system consists of power limiting devices, shielding, dump and collimator, and an active radiation monitoring system. The inter-related elements for the access control system and radiation containment system, as well as the associated interlock network, are described. The policies and practices used in establishing the radiation safety system are also compared with the regulatory requirements.

Symmetrization of radiation in heavy ion ICF targets with realistic beam stoppers

The symmetrization of radiation inside an indirectly driven axially symmetric target with two converters and an ellipsoidal casing is studied with the model of Murakami and Meyer-ter-Vehn, but with more realistic assumptions for the geometry. It is shown that with a useful value for the radius of the cylindrical converters, which transform beam energy into thermal radiation, it becomes much more difficult to achieve the required symmetrization, and this was not possible by varying the converter positions and radii

Beam Extraction System of the Synchrophasotron

The creation of a beam extraction system of the Synchrophasotron (SPT) began in the seventies. The high efficiency of the extraction system met the usual requirements - receiving an intense beam and lowering the induced radiation of accelerator parts. But besides that we had to have high efficiency because the radiation around the SPT had to be reduced as there is no biological shield and experiments with internal beam was limited to 10 ppp. The alternative of an intricate and high cost radiation shield was to extract the beam with an efficiency of more than 90% and to transport it into the external experimental halls with suitable beam stoppers, which allowed the accelerator beam intensity to be increased at least by a factor of ten. A large transverse size of the SPT vacuum chamber permitted the extraction system to be constructed with such a high coefficient of extraction, in principle, without great complications. On the other hand, such a large volume of the magnetic field complicates considerably its stabilization at a level of 10 Wdivided10 X which is necessary for uniform beam extraction. As the power consumption of the SPT magnet during a flat top exceeds 10 MW, the problemmore » of suppression of guide field ripples to the above level has not been solved completely until now.« less

MEA Computer Controlled Personal Access System

The computer based personal access system (PAS) overlooks eight separate entrances of the MEA linac and experimental areas, it eliminates the interference of the safety staff and and demands limited discipline of the users. The PAS is roughly based on: 1. Machine conditions. The status of magnets, beam stoppers, Etc., controls the access to the rooms by means of conventional electronic door control-units. 2. Room conditions. The result of the automatic measurement of the levels of the gamma radiation or the ozone contents of the air dominates the door controls. 3. Personal registration, based on the use of one lock-cabin per entrance and linked to a central computer. On permitted passage through the lock-cabin the computer authorizes and registers each passerby individually. In addition to keeping book of the total number of people present in the rooms an independent electromechanical device provides authorization at each lock-cabin. A weighing system guarantees passage of only one person at the time. Safe access of either the beam or a person is secured by this purely instrumental system.

Operational Experience with SLAC's Beam Containment Electronics

Considerable operating experience has been accumulated at SLAC with an extensive electronic system for the containment of high power accelerated beams. Average beam power at SLAC can approach 900 kilowatts with the potential for burning through beam stoppers, protection collimators, and other power absorbers within a few seconds. Fast, reliable, and redundant electronic monitoring circuits have been employed to provide some of the safeguards necessary for minimizing the risk to personnel. This paper describes the electronic systems and discusses the design philosophy and operating experience.