基于电离室阵列测量X射线半值层实验的研究

目前测量X诊断医用辐射源的半值层的方法基本采用人工叠加吸收片,测量出射线穿透不同厚度吸收片后的照射量,从而采用绘图方法或插入法计算X诊断医用辐射源的半值层[1],该方法曝光次数多,光机损伤大,操作不方便。为了准确测量诊断X射线的辐射质,减少工作量,设计一种电离室阵列测量方案,通过单次测量不同厚度吸收片下的剂量值,根据衰减曲线插值计算得到给定千伏下的半值层。通过测试比较,电离室阵列测量方案操作简单,数据准确,曝光次数少,能减少光机的损耗,具有较高的应用前景。     

Arrhenius and the temperature dependence of non-constant failure rate

This paper examines the temperature dependence of component hazard rate for the cases of log-normal and Weibull failure-time distributions and shows that the common belief that the temperature variation of component failure rate follows the Arrhenius rule can be substantially in error. Although most failures in present-day equipment are not due to defective components, the paper also examines the temperature dependence of equipment rate of occurrence of failure having a power-law or negative exponential variation with time for the temperature range where the majority of failures are due to rate processes obeying the Arrhenius equation. The consequences of a Gaussian distribution of failure-mechanism activation energy in a device population are also considered. Although the temperature dependence of failure rate can be very high, in most situations it is much less than that of the Arrhenius acceleration factor. It is very improbable that the temperature dependence of component failure rate can be meaningfully modelled for reliability prediction purposes or for the purpose of optimizing thermal design component layout. Attention is drawn to the invalidity of determining the failure activation energy from the average failure rates in accelerated high-temperature time-terminated life tests.     

Surface tension of pure liquid bismuth and its temperature dependence: Theoretical calculations

A theoretical method for calculating the surface tension of liquid metals as a function of temperature is proposed. A mathematical equation, based on statistical thermodynamics, is applied to calculate the surface tension of pure liquid bismuth, in the temperature range 545-620 K. The calculated surface tension of liquid bismuth was found to be 388 mJ/m², which is in excellent agreement with the reported experimental values (374-417 mJ/m²). The surface tension of bismuth decreases linearly with temperature, confirming a negative slope.     

Temperature dependence of pure CsI: scintillation light yield and decay time

The temperature dependence of the light emission for pure CsI crystals has been measured with photomultipliers, and photodiodes with wavelength shifters from 80–300 K. The light yield at 80 K is N_γ=50,000±5000 photons/MeV. This number was deduced from the number of electron–hole pairs produced in the photodiode, N_eh=39,600±1200. The light yield at room temperature is lower by a factor of 15.8±1.0, giving 3200±400 photons/MeV. Decay times were measured with a photomultiplier. At room temperature two fast decay components were observed with decay times of 6±1 and 28±2 ns. Below 180 K only one component is observed and at 80 K the decay time is 1015±17 ns.     

The temperature dependence of luminescence from a long-lasting phosphor exposed to ionizing radiation

The temperature dependence of luminescence from a long-lasting phosphor (LLP), SrAl₂O₄ : Eu²⁺,Dy³⁺, exposed to ionizing radiation has been measured to understand the LLP luminescence mechanism. Evaluation of the decay constants of the LLP exposed to -, β- or γ-rays at temperatures from 200 to 390 K showed that the decay constant is divided into four components ranging from 10⁻⁴ to 10⁻¹ s⁻¹ with activation energies of 0.02–0.35 eV.

Total luminous intensity from the LLP with changing irradiation temperature has its maximum value around the room temperature. Irradiation at elevated temperature (390 K) has the total luminescence pattern with monotonous decrease as temperature rises. As a result of evaluating the temperature dependence of luminescence, the luminescence mechanism is considered as follows:

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Secondary ion emission of Fe-Ni alloys in the temperature range including the Curie point

The temperature dependence of secondary ion emission was investigated for Fe-Ni ferromagnetic alloys with different Curie points T_c and elemental composition: 35% Ni 65% Fe (T_c=240°C), 40% Ni 60% Fe (T_c=360°C), and 50% Ni 50% Fe (T_c=530°C). The alloy 79% Ni 16% Fe 5% Mo (T_c=345°C) was also studied. The spatial distribution of Ni⁺ and Fe⁺ secondary ions emitted from the (1 1 1) face of invar and permalloy single crystals was shown to be anisotropic with pronounced ion-yield maximum for both components in the 〈1 1 0〉 directions. The shape of the energy distribution of Ni⁺ and Fe⁺ ions was found to be virtually identical for all the alloys under investigation with a most probable energy at 7 eV and a width at half-maximum of 12 eV. The temperature dependence of the Ni⁺ and Fe⁺ emission has a maximum near the Curie point of the investigated alloys and another maximum at the Curie point of nickel which may indicate the precipitation of nickel into microscopic islands on the surface as a result of heating and sputtering. Auger analysis of the surface composition in the surface layers showed a variation in concentration of oxygen and carbon atoms when Fe-Ni alloys pass from the ferromagnetic to the paramagnetic state and this must affect also the secondary ion emission of alloy components.