Measurement of excited Np237 ceday via the alpha-gamma coincidence method

Abstract: In this article, we present the design of a DIY apparatus for the coincidence detection of alpha and gamma emissions produced by the decay of the Americium-241 isotope. We aim to acquire the pulses and measure the time interval between them, with the goal of precisely measuring the decay time of the excited Neptunium-237 nucleus.

Introduction

The project proposed here involves constructing a setup to detect, in coincidence, the alpha and gamma emissions produced by the decay of the Americium-241 isotope. As shown in Fig. 1, the decay scheme begins with an alpha decay that produces an excited Neptunium-237 nucleus. This isotope subsequently undergoes gamma emission, with a primary emission at 59.5 keV. The gamma decay time (half-life) of the Neptunium nucleus is 67ns, a very short interval, yet one that can be readily measured with appropriate instrumentation. At PhysicsOpenLab, we use DIY equipment; while such time resolutions are somewhat “out of range” for our setup, we still intend to attempt this measurement.

Figure 1 – Decay scheme of Am 241

Detectors

To detect alpha particles, we used a plastic film with a ZnS(Ag) layer. This material produces luminescence upon interaction with alpha particles. The decay time of the light pulse is approximately 60 ns, making it quite fast. The plastic film is coupled to the SiPM so that the generated light pulses are detected by our device. For gamma radiation detection, we used a CsI(Tl) scintillator, also coupled to a SiPM. The detector is described in the article Gamma Spectroscopy with SiPM & Redpitaya. The decay time of the CsI(Tl) is approximately 1 µs, making it slower than the alpha detector. The detectors are shown in Figs. 2, 3, and 4.

Figure 2 – Plastic film with scintillator layer ZnS(Ag) sensitive to alpha particles

Figure 3 – “Dark box” containing the gamma radiation detector (in the small wooden block) and the alpha radiation detector (small black plastic block). The Am-241 source is the small white disc between the two detectors.

Figure 4 – SiPM used in conjunction with a plastic film containing ZnS(Ag) for alpha radiation detection.

SiPM Front-End

To read the signal produced by the SiPMs, we used the transimpedance amplifier described in the Trans-Impedance Amplifier project. This solution works well, though care must be taken with the connections between the SiPMs and the amplifier, ensuring they are not too long.

Figure 5 – SiPM front-end electronics

Edge Detector

The pulses generated by the two SiPMs—one coupled to a plastic film coated with a ZnS(Ag) layer and the other to a CsI(Tl) scintillator—correspond to the detection of the alpha particle and the gamma photon emitted during the decay of an Americium-241 atom. Since we are interested in the timing of these pulses, we employed an “edge detector” circuit that generates very narrow rectangular pulses at the rising edge of the original pulse. The edge detector is based on the PSoC 5LP microcontroller, which provides multiple analog inputs. Two of these inputs are used to acquire the signals generated by the SiPMs, while two others are connected to precision multi-turn potentiometers. The SiPM and potentiometer signals are fed into a comparator with hysteresis, which produces a step signal when the SiPM signal exceeds the threshold set by the potentiometer. This step signal is sent to a flip-flop that generates a rectangular pulse signal with a duration of approximately 20 ns (the pulse duration is determined by the values ​​of resistor Rp and capacitor Cp; in our case, 200 Ω and 100 pF). Fig. 8 shows the firmware schematic implemented in the PSoC.

Figure 6 – PSoC 5LP for SiPM analog signal acquisition and edge-detection pulse generation.

Figure 7 – Settings of thresholds in the “Edge Detector”

Figure 8 – Diagram of the firmware implemented in the PSoC for generating edge-detection pulses

Red Pitaya

For pulse acquisition and processing, we decided to use the Red Pitaya board. This electronic board is a Linux-based microcomputer featuring two 125 MS/s RF analog inputs and two 125 MS/s RF analog outputs, 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 pre-loaded software includes various applications, such as an oscilloscope, a spectrum analyzer, and a signal generator. A key advantage of the board is that it can be reprogrammed by the user for other tasks, as all I/O ports are connected to a common FPGA. Fig. 10 shows the board and its interfaces.

Figure 9 – Red Pitaya board

Data Processing

The pulses produced by the detectors—pre-processed by the PSoC to ensure constant, repeatable duration (effectively acting as an edge detector)—are acquired via the Red Pitaya board’s ADC channels. The board features two input channels; both are sampled and digitized to identify coincidence events. Trigger logic is implemented in software to detect alpha and gamma events occurring in coincidence. There is a high probability that such coincidence events originate from the same decay process, allowing us to measure the time interval between the detection of the alpha particle (emitted first) and the subsequent detection of the gamma emission.

The following listing shows an excerpt of the event acquisition and processing code.

...
// Acquire data channel 1
rp_AcqGetOldestDataV(RP_CH_1, &buff_size_1, buff_1);
printf("Buffer channel 1 Acquired\n");

// Acquire data channel 2
rp_AcqGetOldestDataV(RP_CH_2, &buff_size_2, buff_2);
printf("Buffer channel 2 Acquired\n");

// Scan the buffer sample
printf("Scan buffer channel 1\n");
int i;
for(i = 0; i < buff_size_1; i++){
      // Pulse gamma
      if(!gamma_found && buff_1[i] > 1){
           time_peak = i;
           printf("Above th %d\n", time_peak);
           time_peak_list[list_index] = time_peak;
           list_index++;
           gamma_found = true;
       }
}
...

Results

The aim of this work is to measure the decay of the excited state of Neptunium-237. Prior to this measurement, we acquired the gamma and alpha emission spectra of Americium. These experimental results are shown in Figs. 10 and 11. The gamma radiation exhibits the characteristic peak at 59.5 keV, while the alpha radiation shows a peak at 4.8 MeV. Compared to the intrinsic alpha particle emission energy, the alpha peak is shifted to a lower energy and broadened due to the thin metal safety foil enclosing the Americium source. Although the alpha particles pass through this foil because it is very thin, they lose some energy; this energy loss follows a Landau distribution, resulting in a Gaussian-like peak with a low-energy tail.

Figure 10 – Gamma emission spectrum – peak at 59 keV

Figure 11 – Alpha radiation spectrum

The decay curve obtained from the experimental data is shown in Fig. 12. The data were obtained by selecting gamma and alpha events with a short time separation, thereby attributing them to the same Americium decay event. The time difference is plotted on a linear scale alongside the exponential fit. The fit approximates the data well; the measured time constant is 120 ns, whereas the exact value is approximately 100 ns. This discrepancy can be easily explained by the relatively low time resolution of our measurement system.

Figure 12 – Alpha – Gamma pulse delay time 

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