DIY阿尔法谱仪（一）
Alpha谱仪系列产品，可以满足您各式各样的应用需求，不管是大样品还是小样品的测量，大样品量测量还是小样品测量，或者是原有系统升级还是全新购置。本设计采用模块化结构，不管是在机械性能方面还是在测量性能上都拥有了极大的提升，同时也具备了灵活性。完全支持在原有系统上的升级。
伽太科技提供ORTEC公司全系列Alpha谱仪系列产品，可以满足您各式各样的应用需求，不管是大样品还是小样品的测量，大样品量测量还是小样品测量，或者是原有系统升级还是全新购置。总有一款可以满足您的需求。最新设计采用模块化结构，不管是在机械性能方面还是在测量性能上都拥有了极大的提升，同时也具备了原有产品所不具备的灵活性。完全支持在原有系统上的升级。 产品联系：sales@gamtic.com，021-5197 0121.
Alpha Aria：
单通道Alpha谱仪系统，占用NIM机箱的两个标准宽度。可直接插入NIM机箱，通过机箱供电，与计算机通过USB进行连接。前面板设置机械旋钮，可以极为方便的实现抽气/保持/放气操作。用户只需将插件插入NIM机箱。
Alpha Duo：
桌面式双路Alpha谱仪拥有两个Alpha谱测量通道。每个测量通道包含100%的全电脑控制真空系统，内置可程控高压系统，前置放大器，可变幅度脉冲发生器，以及漏电流监测系统。Alpha Duo模块每一个通道均拥有独立的数字MCA，每个测量通道可进行完全独立的操作。
Alpha Mega：
桌面式单通道Alpha谱仪，拥有目前最大的测量舱室，可测量直径106mm的样品。外形尺寸与Alpha Duo一致。同时该型号的的Alpha谱仪可以配置更大的探测器，最大可到3000mm2。具有与Alpha Duo同样的特性，并且完全计算机程序控制。可作为单独的设备采购，也可作为Alpha Ensemble的部件采购。
Alpha Ensemble：
一套Alpha谱仪系统可以随意配置多达4个模块（Alpha Duo和Alpha Mega），每个Alpha模块中均包含独立的真空电磁阀，探测器偏压，前置放大器，可调节脉冲发生器，反冲抑制模块和漏电流监测器等。
内置的USB集线器，可以将不同模块之间的连接通过一根USB电缆直接连接到计算机。每个探测通道可以在计算机中单独设置转换增益等参数。提供了极大的灵活性。
Alpha Spectrometry is a fascinating technique because it allows you to have accurate information about the radioactive decay of heavy nuclei and about the physics of the interaction of charged particles with matter. But this is a rather difficult technique, even more difficult than gamma spectrometry. The difficulties of this technique lie in the type of detector, usually a solid state silicon detector (rather expensive) that produces a very weak signal which requires, to be analyzed, very low noise amplifiers.
The measurement has to be made in vacuum conditions (however not high vacuum) so that the alpha particles are not shielded from the air. The sources that are measured have to be carefully prepared so as to have a layer as thin and uniform as possible so that alpha particles are not diffused and absorbed within the source itself.
Despite these difficulties it is possible, with a fair amount of work and patience, prepare a DIY instrument that can give a lot of satisfaction.
Introduction
The alpha radioactive nuclei (typically heavy nuclei) can decay by emitting alpha particles (helium nuclei) with energies of the order of a few MeV, with spectra with lines, corresponding to the energy levels of involved nuclei .
In the figure aside it is an example of energy spectrum of alpha emissions of U-238.
The alpha-active nuclei are heavy nuclei with atomic number greater than 82 (lead). Examples are Polonium, Radium, Thorium, Uranium, etc …
The alpha decay has been explained theoretically by G. Gamow in the first half of the previous century
making use of the tunnel effect in quantum mechanics. In the figure is a graph which shows the wave function of the alpha particle inside the nucleus and outside, beyond the Coulomb barrier. Although the alpha particle does not have enough energy to overcome the barrier it is seen as outside the nucleus the wave function is not zero and thus there is a non-zero probability that the alpha particle is ejected from the nucleus. Using this model it is possible to explain with good accuracy the characteristics of alpha decay.
Solid State Detector
In a semiconductor, the equivalent of the ionization energy is the band-gap energy to promote an electron from the valence to the conduction band. In Si at room temperature, Eg = 1.1 eV, compared to ~15 eV to ionize a gas. A charged particle moving through Si therefore creates more ionization and a larger signal.
When n-type and p-type silicon are put in contact, creating a p-n junction, the flow of the two different free charges across the boundary creates a depletion zone, an electrically neutral area near the junction where an internal electric field sweeps out any free charge. By reverse biasing the junction, the depletion zone can be made large, ~hundreds of microns. If an energetic charged particle ranges out in the depletion zone, an amount of ionization proportional to the particle’s initial energy will be created there, and swept out. By plating metallic ohmic contacts on the outer surfaces of the crystal, it is possible to both apply the bias and collect the free charge from the depletion zone, so that the whole assembly is a high gain, solid state version of the capacitive ionization chamber.
In our project we have used the detector shown in the image aside (thanks to Professor John Bland).
It features the following technical data :
– Canberra PIPS SPD-100-12 (partially depleted)
– Active area = 100mm2
– FWHM 12KeV at 5MeV
– Bias Voltage = 40V
– Thickness = 100μm
Signal Processing
The signal produced by the detector has very low amplitude and therefore it requires an appropriate amplification. Given the very low level of the signal you must use very low noise amplifiers, also the bias voltage must be free of ripple, which is why we have adopted a power-based batteries. The preamplifier of the signal is based on a charge sensitive preamplifier type (CSP): the current pulse generated by the detector is converted into a voltage pulse by means of the charge of a capacitor. In the scheme below it is presented ta basic diagram of a charge preamplifier :
Response of a CSP
At time domains lasting up to a few microseconds, the CSP output is the time integral of the current pulse from the PIPS/Surface barrier detector. The output rise time is approximately equal to the duration of the current pulse, although the speed of the CSP sets a lower limit to this rise time.
Because the CSP Produces an output voltage step that is proportional to the time integral of the current input and remembering that :
the CSP output is proportional to the total charge (Q) from the PIPS detector. At much longer time domains the response of a CSP to a fast current pulse from a PIPS detectors is in the form of a tail pulse. A tail pulse has a fast initial rise time followed by a very long exponential decay back to the baseline. A tail pulse response from a CSP module is shown below.
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