Vertex/propagator model for least-scattered photons traversing a turbid medium

The least-scattered photons that arrive at a detector through highly scattering tissues have the potential to image internal structures, functions, and status with high imaging resolution. In contrast, optical diffusing tomography is based on the use of the late-arriving photons, which have been diffusely scattered, leading to very low imaging resolution. A good model of the early-arriving photons, i.e., the least-scattered photons, may have a significant effect on the development of imaging algorithms and a further understanding of imaging mechanisms within current high-resolution optical-imaging techniques. We describe a vertex/propagator approach that attempts to find the probabilities for least-scattered photons traversing a scattering medium, based on analytical expressions for photon histories. The basic mathematical derivations for the model are outlined, and the results are discussed and found to be in very good agreement with those from the Monte Carlo simulations.     

Analytic beam spread function for ocean optics applications

A discrete ordinates code is developed with which to compute the beam spread function (BSF) without invoking the small-angle scattering approximation or performing Monte Carlo calculations. The computed BSF is used to predict the response of a detector versus its distance to the origin of a highly collimated beam, its angle with respect to the beam, and the two local angles that specify the detector orientation. Numerical results have been obtained for water models that simulate a clear ocean, a coastal ocean, and a turbid harbor. Six orders of magnitude or more change in the detector response caused by scattered photons can be predicted for different detector locations while simultaneously obtaining small changes for different detector orientations. This capability is useful for assessment of the sensitivity of the detector response to the interpretation of time-independent underwater imaging systems or visibility models.     

Dosimetry of a nearly monoenergetic 6 to 7 MeV photon source by NaI(Tl) scintillation spectrometry

The dosimetry of a nearly-monoenergetic 6–7 MeV photon source developed at the National Bureau of Standards (NBS) for radiation protection instrument calibration has been carried out by NaI(Tl) scintillation spectrometry. This approach uses calculated 3 in. × 3 in. NaI(Tl) detector-response functions that have been shown to be reasonably accurate up to 20 MeV. A least-squares fit of the appropriate response functions to a selected region of the pulse-height distribution determines the primary 6–7 MeV photon fluence. The uncertainty in the fluence determination is based on the χ² of the fit, the statistics of the data, and the uncertainty in the response functions. The air kerma delivery due to the primary photons at a reference point in the photon field was calculated from the primary photon fluence. The uncertainty in the determination of air kerma delivery for primary photons was less than 5% (1 std. dev.). The primary high-energy photon contribution to the detector response was subtracted from the data and the remaining distribution due to lower-energy photons was evaluated by spectrum unfolding analyses. The spectrum-unfolded results indicate that a contribution of approximately 12% of the total air kerma was mostly from a continuous distribution of photons extending up to 4.5 MeV.     

低能过滤X射线神经网络解谱方法研究

利用神经网络算法实现对PIPS探测器测量得到的低能过滤X射线脉冲幅度谱的快速解谱。首先根据PIPS探测器的计算机断层扫描图像,在MCNP5中建立该探测器的蒙特卡罗(MC)模型。并通过实验、MC效率刻度以及能谱展宽,对该探测器模型进行验证。之后计算PIPS探测器对单能光子(5~30 keV)的响应函数,并将其作为单层线性神经网络的训练数据。使用训练后的神经网络和GRV_MC33程序分别对N10~N30和L10~L30辐射质的X射线脉冲幅度谱进行解谱。结果表明:除N25和N30辐射质外,二者解谱结果相符较好。其解谱结果的差异可能来源于探测器响应函数和GRV_MC33程序解谱方法的不确定度。训练好的神经网络可被移植到微型计算机中,帮助校准实验室实现对低能过滤X射线脉冲幅度谱的快速解谱。     

A detailed study of the interaction mechanisms for Si(Li) detector response functions by the direct Monte Carlo approach

A direct Monte Carlo (DMC) calculation of the complete response functions of Si(Li) detectors for low energy X-rays (keV) is proposed. Interaction mechanisms for X-rays in the sensitive volume of the detector and in the surrounding material are investigated in detail. Response functions at several energy values are calculated and compared with the corresponding experimental ones. The present approach uses a detailed simulation of individual interactions rather than the simple range formulation used by Gardner et al. [Nucl. Instr. and Meth. A242 (1986) 399] and gives better results in some cases. The physical model and calculation method presented here can be applied widely to low energy X-ray fluorescence analysis and other complicated photon spectrum analyses. In principle it is also suited for Ge detectors.     

A prototype study of the POLAR front-end electronics

POLAR is a multi-channel position-sensitive detector used for polarization property detection of gamma-ray burst (GRB) photons. We have designed a prototype of POLAR front-end electronics (PFEE) based on Application Specific Integrated Circuits (ASICs) with large dynamic range and low power consumption. The performances of PFEE are measured using a pulse generator and an alpha-ray source. The measured average Equivalent Noise Charge (ENC) and input signal dynamic range of the PFEE prototype are 6.7 fC (rms) and 9 pC, respectively. The results that are obtained show that the main functions of the PFEE have been achieved, making this prototype usable in future experiments.     

A High-Sensitivity Microwave-Single-Photon Detector with Rydberg Atoms at Low Temperature

A high-sensitivity microwave-single-photon detector was developed in Kyoto, in which microwave photons in a resonant cavity cooled at very low temperatures are absorbed by highly excited Rydberg atoms and the Rydberg atoms thereby promoted to a higher excited state are then selectively field-ionized and detected. This scheme allows us to count microwave photons one by one, thus provide a single-photon counting without the limit of standard quantum limit (SQL). The apparatus “CARRACK” for the single-photon detector was constructed based on this scheme, where the cavity was cooled down to 10 mK range to reduce the background of thermal blackbody photons from the cavity wall. The apparatus has served for years to search for dark matter axions in the 10 μeV (∼2.4 GHz) mass region. Thermal blackbody photons in a microwave resonant cavity at temperatures as low as 70 mK have been measured, the sensitivity being below the SQL limit. A number of improvements in the detection efficiency and sensitivity have been planned and will be reported. Applications of the detector to fundamental physics are also discussed shortly.      

A new synthetic formalism for true coincidence summing calculations

Simultaneous detection in one sensing crystal of photons emitted in cascade from the same radioactive atom gives rise to true coincidence summing effects (excess and deficit counting). Presented in this paper is a general formula to correct for true coincidence summing. This analytical formula enables calculation of the correction factor and its associated uncertainty for any gamma-ray placed in a decay scheme of any radionuclide (all decay modes treated) in any measuring configuration. It takes account of any cascade of photons either through the nature of the involved photons (γ-rays, X-rays, and annihilation photons) or the unlimited number of photons emitted in cascade. The requirement for this calculation is twofold. From the radionuclide part, theoretical knowledge of the decay scheme is necessary along with its characteristics (gamma-ray intensities and internal conversion coefficients for instances). Some atomic quantities such as fluorescence yields have also to be known. From the detector part, calibrations for full-energy peak efficiency and total efficiency in the measuring configuration have to be available. The demonstration of the formula is done with restrictive conditions (point source, no angular correlation, and no metastable state between the initial and final energy states) but adaptation can be implemented to partially or totally lift these limitations or approximations. Implementation of the formula in a computer code named Coincal is mentioned. Its successful application to point sources of ¹⁵²Eu placed 10 cm and 15 mm away from an n-type coaxial HPGe detector with 60% relative efficiency is also described.     

Fabrication and performance characteristics of high-speed ion-implanted Si metal-semiconductor-metal photodetectors

We have fabricated high-speed Si metal-semiconductor-metal photodetectors using(19) F(+) ion implantation in low-doped Si. Bandwidths in excess of 6 GHz have been obtained that represent more than an order-of-magnitude improvement over unimplanted counterparts. Measurements with short optical pulses show that the increase in bandwidth is due primarily to a shorter carrier lifetime in implanted devices. In the absence of implantation, the response under short optical pulse excitation has a long decay with a time constant of ~0.35 ns. We carried out an optical fiber transmission experiment using a GaAs (lambda ~ 0.85 mum) laser source and the implanted Si photodetector. Error-free transmission (bit error rate < 10(-11)) with good receiver sensitivity was obtained at 2 Gbits/s. These results demonstrate that implanted Si can be used as a detector for short-wavelength fiber-optic communication systems for speeds to a few gigabits per second. Monolithic integration of this detector technology with conventional Si processing offers the potential for low-cost receiver designs.     

Design concept for a novel SQUID-based microdosemeter

The interest in proton and ion-beam therapy has increased substantially in recent years, leading the Bureau International des Poids et Mesures to recommend that a new quantity be defined to account for the biological effect of treatment modalities used in the radiotherapy. In response, we have begun work on the design of a new microbolometer based on the inductive superconducting transition-edge detector (ISTED) designed at the National Physical Laboratory. Our work aims to expand the use of the ISTED from its current use as an infra-red detector to the measurement of energy deposition of photons, hadrons and ions by incorporating a tissue-equivalent absorber. We present here the work carried out till date, including the detection of single visible photons, with an energy resolution of 0.2 eV and a time response of a few microseconds, using a 15-μm Superconducting QUantum Interference Device operating at 9 K. We also describe thermal computational modelling carried out for a simple ISTED absorber. Finally, we discuss the work currently in progress and that to be carried out before the detector is realised.     

Depth profile analysis of porous Si film by ERDA using a ΔE — E detector telescope

A complete analytical study as well as depth profiling of the constituent elements of a porous Si layer have been carried out by elastic recoil detection analysis using 100 MeV Ag ions and a ΔE — E detector telescope. Quantitative estimates of elements H, C, N, O, F, Mg and Si in a porous Si film have been made. The analysis of data indicates that the lowest detection limit of the technique is 8 × 10⁻⁴ at% with an inaccuracy of 15%.     

Corrections to β-continua measured with solid state spectrometers

The end point energies of β⁺-spectra show a systematic deviation from the actual value when measured with solid state particle detectors. This deviation is caused by the effect of summation of annihilation photons. The summation has been examined in detail by a simulation of the behavior of 511 keV photons in the detector, using a Monte Carlo computer program. The functioning of this program has been checked by careful measurement of the detector response to gamma rays. Conclusions could be drawn concerning the effective crystal diameter. A method is described to calculate the effect of the summation on a beta spectrum, using the results of the simulation program. The procedure has been used to analyze a measurement of the ⁶⁸Geβ⁺-spectrum.     

Simulation and measurements of the response of an air ionisation chamber exposed to a mixed high-energy radiation field

CERN's radiation protection group operates a network of simple and robust ionisation chambers that are installed inside CERN's accelerator tunnels. These ionisation chambers are used for the remote reading of ambient dose rate equivalents inside the machines during beam-off periods. This Radiation Protection Monitor for dose rates due to Induced Radioactivity ('PMI', trade name: PTW, Type 34031) is a non-confined air ionisation plastic chamber which is operated under atmospheric pressure. Besides its current field of operation it is planned to extend the use of this detector in the Large Hadron Collider to measure radiation under beam operation conditions to obtain an indication of the machine performance. Until now, studies of the PMI detector have been limited to the response to photons. In order to evaluate its response to other radiation components, this chamber type was tested at CERF, the high-energy reference field facility at CERN. Six PMI detectors were installed around a copper target being irradiated by a mixed hadron beam with a momentum of 120 GeV c(-1). Each of the chosen detector positions was defined by a different radiation field, varying in type and energy of the incident particles. For all positions, detailed measurements and FLUKA simulations of the detector response were performed. This paper presents the promising comparison between the measurements and simulations and analyses the influence of the different particle types on the resulting detector response.     

Comparative study of X-ray fluorescence signal collection by collimators of two types

The response signal collection functions in a confocal setup of the X-ray fluorescence (XRF) tomography have been modeled for a polycapillary collimator and a multichannel collimator fabricated by means of microelectronic technology (MT). The dimensions of the effective collection region for the two types of collimators are compared. For the MT-fabricated device, a fraction of X-ray photons transmitted due to the total external reflection from channel walls has been evaluated. It is shown that, for a proper choice of parameters, the contribution of these photons to the collection function is negligibly small.     

Multitube proportional chambers for the localization of electromagnetic showers in the CERN UA2 detector

24 multitube proportional chambers with a total number of 5424 wires have been constructed for the CERN UA2 detector. They serve to localize the electromagnetic showers produced by electrons and photons in a lead-iron converter in front of each chamber, and to reject hadrons. Details of the construction and the operation parameters are given. The response to electrons and to hadrons in the energy range 20–60 GeV is discussed. The chambers and their electronics proved to be very reliable and stable.     

Developing instruments for measurement of plutonium activity in the environmental samples in the case of radiation accident

According to the Russian norms, the permissible specific activity of 239Pu in the air and water are the lowest. Still the problem of low (compared to permissible) Pu activity measurement for environmental samples arises, for instance, in the case of radiation accident during transportation or storage of excess weapons-grade plutonium transferred to the civil sphere of utilisation. The base method of Pu activity measurement is alpha spectrometry by using silicon detector. A new model of alpha spectrometer, based on silicon surface-barrier detector with background level less than 1-3 pulses per day has been developed. A simple unfolding method for the determination of Pu activity of samples with various thickness based on alpha spectrum measurement has been also developed. L-X photon detection is also used for reducing the measurement threshold of Pu activity determination. A method based on detecting L-X photons of Pu, excited by X rays and beta particles, is applied for further reducing the threshold.     

A method to register hidden photons with the aid of a multi-cathode counter

We propose to detect hidden photons (HPs) by using single electrons emitted from the surface of a metal cathode under the action of HPs of cold dark matter. A multiwire gas-discharge proportional counter of special design with four cathodes has been developed. Calibration measurements were performed for determining the efficiency of counting single electrons. Single-electron pulse counting rate in various detector configurations was measured. The upper limits of mixing parameter χ, which characterizes the kinetic mixing of HPs and ordinary photons, are estimated.

     

A simple method of estimating the position of a germanium gamma-ray detector crystal within its spectrometer housing using common gamma-ray calibration sources requiring the suppression of coincidence summing losses

A simple method of estimating the distance between the front face of the spectrometer housing and the germanium crystal of a gamma-ray detector is described. The activity (A) of small (10 μCi) gamma-ray point sources is measured over a range of distances (d) from the detector housing front face. A graph of d versus $\text{(A)}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$ gives the effective penetration depth into the crystal. Results of measurements on both planar and coaxial detectors using sources emitting photons over a range of energies (14–1332 keV) are presented. The success of the method depends on suppressing counting losses due to true coincidence summing of gamma and/or X-ray photons emitted in cascade by many gamma-ray sources. To achieve this, a 1.5 mm brass plate was placed in front of the detector to give high attenuation of all radiation below about 40 keV. The results of this study are relevant both to an understanding of the effective penetration depth of photons in germanium crystals and to the elimination of bias in detector efficiency measurements using common laboratory sources.     

Analytical approximate solutions of the time-domain diffusion equation in layered slabs.

Time-domain analytical solutions of the diffusion equation for photon migration through highly scattering two- and three-layered slabs have been obtained. The effect of the refractive-index mismatch with the external medium is taken into account, and approximate boundary conditions at the interface between the diffusive layers have been considered. A Monte Carlo code for photon migration through a layered slab has also been developed. Comparisons with the results of Monte Carlo simulations showed that the analytical solutions correctly describe the mean path length followed by photons inside each diffusive layer and the shape of the temporal profile of received photons, while discrepancies are observed for the continuous-wave reflectance or transmittance.     

Quasiparticle Edge Losses in Double STJs Strip X-Rays Detectors

The 2D diffusion model of the strip X-ray detector was developed. The detector consists of a long superconducting strip, which is ended by the trapping layers and superconducting tunnel junctions at each end. The model takes into account the diffusion of the excess quasiparticles, quasiparticle trapping at the tunnel junctions and quasiparticle losses in the volume of the strip and at the strip boundaries. The analytical solution was obtained. It has been shown that quasiparticle losses at the strip boundaries caused the dependence of the signals on the photon absorption site in transverse direction. The latter worsens the energy resolution and transforms the spectral line of the detector to nongaussian shape.