Angular distribution of coherent bremsstrahlung

The angular distribution of the linearly polarised photon beam produced by coherent bremsstrahlung from an aligned diamond radiator has been measured at the MAMI A2 tagged photon facility. The measurements were made with a prototype position sensitive photon detector which utilises the pair production process and a double sided silicon strip detector. This polarised photon beam is used for nuclear and hadronic experiments and in their analysis the polarisation is obtained from a calculation, which matches the experimental intensity spectrum. As the polarisation is related to the photon beam angular distribution, the present measurements can be used to test this calculation. The overall agreement is found to be good although there are some regions where significant discrepancies exist.     

X射线管出射谱源项及过滤谱的蒙特卡罗模拟

利用蒙特卡罗方法对X射线管出射谱及过滤谱进行模拟,研究了X射线管Be窗及靶角对X射线谱的影响,并模拟分析了不同靶角及管电压下X射线管阳极效应.将1 m处经过附加过滤后的模拟谱与德国联邦物理技术研究院(PTB)实测谱进行对比,结果表明,模拟谱与实测谱符合较好,验证了蒙特卡罗方法对模拟X射线管出射谱的有效性及所建模型的正确性.     

工业钼靶X射线光机能谱的蒙特卡罗模拟

利用蒙特卡罗模拟程序EGSnrc,构建出工业钼靶X射线光机模型,进行了28 keV的电子束经过钼靶产生的光子在光机各组件中的传输模拟,得到距源焦点50 cm处, 射野半径为5 cm平面上的粒子相空间文件,通过对相空间文件分析得到粒子注量、能谱分布、角分布、平均能量等信息,模拟计算了过滤材料和管电压对钼靶X射线谱分布的影响。钼过滤下钼靶X射线的平均能量小于铑过滤,但对较高能量部分的影响要大于铑过滤;随着管电压的升高,钼靶光机的光子产生效率呈上升趋势,平均能量增加。25,28,30,35 kV 4组辐射质条件下,钼靶X射线能谱的平均能量分别为16.0,16.6,17.0,17.8 keV,与实验测量值接近,相对误差在1%以内。     

Efficiency calibration of a Si(Li) detector in the low-energy region using electron bremsstrahlung

In the low-energy region the determination of the detector efficiency is complicated by the fact of nonavailability of certificated calibration sources. To solve this problem the use of electron bremsstrahlung for determining the relative photon detection efficiency of a Si(Li) detector in the low-energy range 0.5–19 keV is proposed. The calibration is based on energy dispersive measurements of electron bremsstrahlung emitted by an X-ray tube. Model spectra are computed from thick-target bremsstrahlung spectra in a semiclassical approximation and from a physical model for the detector response. Model spectra are fitted to the measured spectra using a parametric adaptor term. Thereby, the error of the determined detector efficiency is approximately 5%.     

Measurement of absorbed dose by 7-GeV bremsstrahlung in a PMMA phantom

High-energy electron storage rings generate energetic bremsstrahlung photons through radiative interaction of the particle beam with the residual gas molecules and other components inside the storage ring. At synchrotron radiation facilities, where beamlines are channeled out of the storage ring, a continuous bremsstrahlung spectrum, with a maximum energy of the stored particle beam, will be present. At the advanced photon source (APS), where the stored beam energy is 7 GeV, bremsstrahlung generated in the straight sections of the insertion device beamlines, which are a total of 15.38 m in length, can be significant. The contribution from each bremsstrahlung interaction adds up to produce a narrow mono-directional bremsstrahlung beam that comes down through the insertion device beamlines. The resulting absorbed dose distributions by this radiation in a 300 mm×300 mm×300 mm tissue substitute cube phantom were measured with LiF:Mg,Ti (TLD-700) thermoluminescent dosemeters. The normalized absorbed dose, in a cross-sectional area of 100 mm² at a depth of 150 mm of the PMMA phantom, was measured as 3.3×10⁶ mGy h⁻¹W⁻¹ for 7-GeV bremsstrahlung spectrum.     

Single photon response of photomultiplier tubes

Beta or gamma rays, when directly incident on the window of an optically shielded photomultiplier tube, yield a typical single photon spectrum. The single photons are possibly generated in the glass window of the photomultiplier tube through excitation of atoms in glass by electrons. The coincidence resolving time has also been measured with a ⁶⁰Co gamma source and a pair of optically shielded photomultiplier tubes detecting single photons.     

Double atomic-field bremsstrahlung

We have made the first observation of the fourth order quantum electrodynamic process of double atomic-field bremsstrahlung. The experiment observed two photons radiated in coincidence at 90° and 270° with respect to a 75 keV electron beam incident on targets of silver, terbium, gold and uranium. The measured absolute cross section differential in energy and solid angle of both photons disagrees with the prediction of the first Born approximation in both magnitude and photon energy dependence. The atomic number dependence, however, is consistent with Z².     

Radiation transport calculations for 50 MV photon therapy beam using the Monte Carlo code GEANT4

A new thin transmission target technique for fast dose delivery using narrow scanned photon beams has been developed. High-energy, 50-100 MeV, electron beams of low emittance incident on thin low-Z targets produce narrow and intense high-energy bremsstrahlung beams. However, electrons transmitted through the target are bent from the therapeutic beam by a purging magnet and have to be effectively absorbed in a dedicated electron collector. The electron-photon transport through a treatment head has been studied using the Monte Carlo simulation toolkit Geant4. The Geant4 electromagnetic physics processes have been compared with experimental data of radial dose profiles. The differences between calculated and measured radial dose distributions are approximately 2-10%. Preliminary investigations of the collector design have been carried out in order to minimise secondary electron and photon contamination of the therapeutic beam. The toolkit presented here is promising for further development of narrow photon beam therapy.     

A tagging system for linearly polarized photons

A bremsstrahlung tagging system was installed at the Mainz microtron facility and was successfully used for experiments. At a primary electron energy of 184 MeV photons from 14 to 144 MeV could simultaneously be tagged. The magnetic electron optics for the tagging spectrometer were chosen in such a way, that the linear polarization of an off-axis tagged photon beam was considerably enhanced. The setup is described and results of performance tests are discussed.     

Bremsstrahlung radiation from high-energy electrons in wide gas-filled gaps

Electric discharge in wide interelectrode gaps filled with air at atmospheric pressure have been experimentally studied using applied voltage pulses with amplitudes up to 800 kV and a front width within 150–200 ns. The discharge was accompanied by the emission of a 10–20-ns-long pulse of radiation with photon energies above 5 keV. It is established that this emission is related to the generation of runaway electrons from the heads of anode-directed streamers. The calculated spectrum of the observed bremsstrahlung radiation exhibits a maximum at photon energies about 15 kV, which is due to the absorption of photons by the molecules of a gas in which the discharge takes place.     

Single-photon generation by electron beams

We propose a drastically new method for generating single photons in a deterministic way by interaction of electron beams with optical waveguides. We find a single swift electron to produce a guided photon with large probability. The change in energy and propagation direction of the electron reveals the creation of a photon, with the photon energy directly read from the energy-loss spectrum or the beam displacement. Our study demonstrates the viability of deterministically creating single guided photons using electron beams with better than picosecond time uncertainty, thus opening a new avenue for making room temperature, heralded frequency-tunable sources affordable for scientific and commercial developments.     

Possible use of CdTe detectors in kVp monitoring of diagnostic x-ray tubes

It has been suggested that kVp of diagnostic X-ray devices (or maximal energy of x-ray photon spectra) should be monitored routinely; however a standardized noninvasive technique has yet to be developed and proposed. It is well known that the integral number of Compton scattered photons and the intensities of fluorescent x-ray lines registered after irradiation of some material by an x-ray beam are a function of the maximal beam energy. CdTe detectors have sufficient energy resolution to distinguish individual x-ray fluorescence lines and high efficiency for the photon energies in the diagnostic region. Our initial measurements have demonstrated that the different ratios of the integral number of Compton scattered photons and intensities of K and L fluorescent lines detected by CdTe detector are sensitive function of maximal photon energy and could be successfully applied for kVp monitoring.     

A tagged photon spectrometer for use with the mainz 180 MeV microtron

This paper describes a photon tagging system which has been installed on the 180 MeV electron microtron at the Institut für Kernphysik in Mainz for use in experiments on photonuclear reactions at intermediate energies. The system enables bremsstrahlung produced photons in the energy range 80–174 MeV to be tagged at rates up to 5×10⁷ s^?1.     

Generation of linearly polarized transition radiation X-ray beam

We have successfully produced almost linearly polarized X-ray beam by extracting Transition Radiation (TR) X-rays through a rectangular slit 3 mrad long and 0.2 mrad wide placed downstream of a 7.5-μm thick Kapton foil stack radiator bombarded with a 1-GeV electron beam. Our calculation predicted that the linearity of TR photon was 94% for both a vertical and a horizontal slit. The measured results for the vertical slit agreed with the calculation, and the linearity of obtained TR X-ray beam was more than 90%. For the horizontal slit, the measured results differed from our expectations due to a prolonged electron beam profile. In the polarization measurement of TR photon we used the Bragg reflection on a lithium fluoride crystal [LiF(200)] which acted as both a polarimeter and a spectrometer.     

Precise dose evaluation using a commercial phototransistor as a radiation detector

An experimental arrangement and a circuitry based on an NPN phototransistor-type silicon radiation detector have been used for evaluating the X-ray beam dose in the diagnostic range. The circuitry was built to allow alteration of the electric field in the phototransistor internal structure, with some devices that have an available base connection. By changing the transistor base bias it is possible to alter its operation point to obtain a response gain from the selected photon energy range. In this way we have made an electronic energy-domain discretisation and we are investigating a model to calculate the dose contribution from each energy discretised into 10 keV steps. The method has been tested in filtered radiation beams generated from an HF-160 Pantak X-ray unit and compared with the usual dosimetry method. Our results have demonstrated that it is possible to make such a dose deconvolution from 40 to 140 keV energies by controlling the phototransistor base bias properly.     

The neutron component of two high-energy photon reference fields

The 4.4 MeV photon reference field described in ISO 4037 is produced by the (12)C(p,p')(12)C (E(x) = 4.4389 MeV) reaction using a thick elemental carbon target and a proton beam with an energy of 5.7 MeV. The relative abundance of the isotope (13)C in elemental carbon is 1.10%. Therefore, the 4.4 MeV photon field is contaminated by neutrons produced by the (13)C(p,n) (13)N reaction (Q = -3.003 MeV). The ambient dose equivalent H*(10) produced by these neutrons is of the same order of magnitude as the ambient dose equivalent produced by the 4.4 MeV photons. For the calibration of dosemeters, especially those also sensitive to neutrons, the spectral fluence distribution of these neutrons has to be known in detail. On the other hand, a mixed photon/neutron field is very useful for the calibration of tissue-equivalent proportional counters (TEPC), if this field combines a high-linear energy transfer (LET) component produced by low-energy neutrons and a low-LET component resulting from photons with about the same ambient dose equivalent and energies up to 7 MeV. Such a mixed field was produced at the PTB accelerator facility using a thin CaF(2) + (nat)C target and a 5.7 MeV proton beam.     

Calibration of X-Ray Film Used in the Detection of Soft X-Rays from the CTR Device Sceptre

To assist in the interpretation of X-ray pinhole photographs in thermonuclear research with the toroidal pinch device Sceptre, with a view to applying such photographs to the study of the energy spectrum of the electron flux incident upon the torus walls (constructed from aluminium and copper), experiments have been performed to determine the photographic effect of the X-rays produced from a known flux of electrons of given energy. A demountable X-ray tube was made in which the target was alternately copper or aluminium foil, indirectly heated, and in which a beam of electrons of known energy was focused on the foil. X-rays from the target fell upon llford Industrial G type X-ray flim, after passing through one or more metal foils. By maintaining a known beam current for different times, a range of exposures was given to different parts of the same film specimen and density-log exposure curves were compiled. Results of such experiments are presented for target elements of aluminium and copper and various thicknesses of absorbing foil, for electron energies in the range 1.8-8.4 KeV. A method is suggested by which data accumulated in this way may be used in analysis of electron energy spectra.     

X-ray spectral determination by successive modifications of the beam intensity

A simple and practical indirect method to determine spectra from X-ray tubes has been developed on the basis of successive modifications of beam flux. The beam is either scattered at different angles or attenuated by metal foils and the intensity is measured by an ionization chamber. The data are analyzed by means of an iterative program that solves a system of equations whose unknowns define a parametric fit of the X-ray spectrum. We have checked this method with simulated measurements of tabulated spectra, as well as with our own measurements on available X-ray apparatus and for several different applied voltages.     

Design of a fast compton spectrometer for measuring megavoltage bremsstrahlung spectra

Measurements of bremsstrahlung X-ray spectra produced by high energy electron linacs (such as those found in many cancer therapy centers) pose special problems. These machines typically operate at very low duty factors, have very high photon fluences, and produce X-rays of energy too high to be measured accurately with a single detector. A compton pair spectrometer utilizing fast plastic scintillators and nanosecond timing techniques suitable for these measurements is described.