PhysicsOpenLab Modern DIY Physics Laboratory for Science Enthusiasts
Abstract: in this article, we present the improved version of the alpha spectrometer described in the post DIY Alpha Spectrometer. The apparatus described here has enhanced the front-end electronics and signal acquisition. Specifically, a low-noise shaping amplifier with a shaping time of 1 μs has been adopted. Additionally, the electronic assembly and shielding have been improved, reducing noise and enhancing energy resolution. The signals were acquired using the Red Pitaya board, which, with a sample rate of 125 Ms/s, easily allows for the acquisition of signals with a duration on the order of a microsecond.
Introduction
Alpha spectrometry is a fascinating technique because it provides precise information on the radioactive decay of heavy nuclei and the physics of the interaction between charged particles and matter. However, it is quite a challenging technique, even more so than gamma spectrometry.
The difficulties of this method lie in the type of detector, typically a silicon solid-state detector (quite expensive). This detector produces a very weak signal that requires extremely low-noise amplifiers for analysis.
Additionally, the measurement must be carried out under vacuum conditions (though not high vacuum) to prevent alpha particles from being attenuated by air. The sources being measured also need to be carefully prepared to ensure a layer that is as thin and homogeneous as possible, so the alpha particles are not scattered or absorbed within the source itself.
Despite these challenges, DIY instrumentation capable of delivering great satisfaction can be set up with effort and patience.
Solid State Sensor
In a semiconductor, the equivalent of the ionization energy is the band-gap energy, which promotes a valence electron to the conduction band. In silicon at room temperature, Eg=1.1 eV, compared to approximately ~ 15 eV required to ionize a gas. A charged particle moving through silicon therefore creates more ionization and generates a stronger signal.
When p-type and n-type silicon are brought into contact, forming a p-n junction, the flow of free charges across the boundary creates a depletion zone—a region near the junction that is electrically neutral, where an internal electric field sweeps away any free charges.
By reverse-biasing the junction, the depletion zone can be expanded, even up to hundreds of microns. When a charged particle passes through the depletion zone, it causes ionization, producing several charges proportional to the particle’s initial energy. These charges are influenced by the strong electric field in the depletion zone and are driven toward the edges of the region.
By creating ohmic contacts on the external surfaces of the semiconductor crystal, it is possible to apply the reverse bias voltage and collect the free charges generated within the depletion zone. In this way, a high-gain particle detector is created, which is essentially the solid-state version of an ionization chamber.
In our project, we used the detector shown in the image on the side. It has the following technical specifications:
– Canberra PIPS SPD-100-12 (partially depleted)
– Active surface = 100mm2
– FWHM 12KeV at 5MeV
– Bias voltage = 40V
– Thickness = 100μm
Signal Processing
The signal produced by the detector has a very low amplitude and therefore requires appropriate amplification. Due to the very low signal level, ultra-low noise amplifiers must be used, and the bias voltage must be ripple-free. The signal preamplifier is based on a charge amplifier (CSP) type: the current pulse generated by the detector is converted into a voltage pulse by charging a capacitor. Fig. 1 shows the basic schematic of a charge preamplifier:
Fig.1 – Basic Schematic of a Charge Preamplifier
Response of a CSP amplifier
The output of the CSP preamplifier is the time integral of the current pulse produced by the detector. The rise time of the output signal is approximately equal to the current pulse’s duration, although the CSP’s speed imposes a lower limit on this rise time.
Since the CSP produces an output that is a step voltage proportional to the time integral of the input current, and recalling that
y a long exponential decay before returning to the baseline.The side shows an example of the pulse. The reason for the exponential decay is the resistance in parallel with the feedback capacitor. This solution is necessary for the CSP to respond to subsequent pulses.
Shaping of a Gaussian pulse
The output of the CSP (with its impulsive signal shape) should be considered only as an intermediate step in producing a measurable output. The long tail makes it impractical to digitize the pulses, as they often overlap with one another. Therefore, it is necessary to use a downstream amplifier (shaping amplifier) that transforms the exponential pulse into a Gaussian pulse suitable for digitization. Another important feature of the shaping amplifier is that much of the noise is filtered out, significantly improving the signal-to-noise ratio. Signals that might be “buried” in the noise at the output of the CSP become distinguishable above the noise threshold after the shaping phase. Fig. 2 shows the basic schematic of a shaping amplifier.
Fig.2 – Basic Schematic of a Shaping Amplifier
In Fig.3, the basic schematic of a signal processing chain is presented, consisting of bias, detector, CSP preamplifier, and pulse shaper:
Fig.3 – Basic Schematic of a Front-End Electronics System
Implementation of the Amplifier
For our alpha spectrometer, we chose to use a commercial CSP, specifically the CR-110 model from Cremat. It is a hybrid CSP preamplifier with a feedback capacitance of 1.4 pF and a feedback resistance of 100 MΩ, with an amplifier time constant of 140 μs. We decided to adopt a commercial component (at a low cost) because the CSP is a critical component, and the proper functioning of the system largely depends on the good performance of the CSP in terms of gain and low noise. The schematic in Fig. 4 shows the connections of the components:
Fig.4 – Wiring Diagram with CSP Cremat
In particular, the bias and filter resistances were chosen to be 10 MΩ. These values, taking into account a dark current of less than 100 nA, ensure a good compromise between the need to limit the bias voltage drop and the requirement to have a signal of adequate amplitude on the coupling capacitor to the CSP. For the shaping module, we initially built a fully homemade version, but then we opted for the Cremat shaping module to optimize performance and achieve a better signal-to-noise ratio. The schematic in Fig. 5 shows the equivalent circuit of the “shaping amplifier.” The input components Cin and Rin form a differentiation circuit. The circuit consists of two Sallen-Key filters, providing 4 poles of integration and signal gain. These integration stages produce an output pulse that closely approximates a Gaussian function. Our shaping module has a characteristic time constant of 1 μs so that the output pulses have a duration on the order of microseconds.
Fig.5 – Basic Schematic of Cremat Shaping Amplifier
Fig. 6 schematizes the signal processing chain. In addition to the preamplifier and shaping modules, a wideband amplifier has been added to increase the system’s final gain, if necessary.
Fig.6 – Signal Processing Chain Diagram, from CSP to Shaping Amplifier
In Fig. 7, the various modules are shown enclosed in separate, shielded metal boxes to reduce potential interference.
Fig.7 – Charge Preamplifier and Detector (on the left) – Shaping Amplifier (on the right)
Fig.8 also shows the small vacuum chamber in which the detector was placed.
Fig.8 – Detail of Front-End Electronics and Detector
Red Pitaya and Multichannel Analyzer Application
For the acquisition and processing of the pulses produced by the amplifier, we decided to use the Red Pitaya board. This electronic board is a Linux-based microcomputer and has two RF analog inputs at 125 MS/s and two RF analog outputs at 125 MS/s, with an analog bandwidth of 50 MHz. The analog I/Os are coupled to 14-bit analog-to-digital (ADC) and digital-to-analog converters. The preloaded software includes several applications, such as an oscilloscope, spectrum analyzer, and signal generator. The advantage of the board is that it can be reprogrammed by the user to perform other tasks, as all I/O ports are connected to a common FPGA. Fig.9 shows the board with its interfaces.
Fig.9 – Details of the Red Pitaya Board
The signal produced by the shaping amplifier is sent to one of the inputs of the Red Pitaya board, where it is acquired and processed using MCA (multichannel analyzer) software. This software is not part of the Red Pitaya “suite” and must be downloaded and preloaded onto a separate SD card. All the information can be found at the link https://pavel-demin.github.io/red-pitaya-notes/mcpha/.
The MCA application allows obtaining the energy spectrum of the radiation detected by the sensor. The GUI of the MCA application, with the spectrum of an alpha source, is shown in Fig. 10.
Fig.10 – MCA Application Interface
Conclusions
In this post, we describe the conceptual design and implementation of an alpha spectrometer based on a PIPS solid-state sensor. The readout electronics were built using Cremat’s “charge preamplifier” and “shaping amplifier” modules. The sensor was placed inside a small vacuum chamber. The pulse signal generated by the amplification chain was acquired using the Red Pitaya board and processed with Pavel Demin’s open-source MCA software. The instrument proved to be fully functional and capable of producing energy spectra of alpha radiation sources with excellent resolution.
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