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Neutron Spectrometry Measurements Of The Recoil Proton Track

Jun 9

By employing real-time imaging equipment to capture the optical tracks of recoil protons in scintillation gas, RPTI is a promising technology. An online neutron track detector for neutron spectrometry measurements (NSM) is investigated for the first time. Proton track imaging and neutron to proton recoil counters form the basis of a simple detector design based on NSM's RPTI technique. According to simulations of RPTI neutron spectroscopy, its performance is satisfactory. Preliminary studies have shown the detector's capacity to image proton tracks at the single-particle level in real-time utilizing well-defined line-proton sources from a tandem accelerator. The proton ranges could be clearly separated, and exact proton energy spectra could be unveiled thanks to the pictures' distinct single proton tracks, establishing a strong experimental basis for NSM's future deployment.

For decades, neutron spectrometry measurements (NSM) have been vital in the development of nuclear physics1 and other relevant fields, such as fusion plasma diagnostics, radiotherapy, and radiation protection.

Traditional detectors for NSM, such as nuclear emulsion10, CR-3911, and the fluorescent nuclear track detector (FNTD)12, rely on the recoil proton approach and unfold the neutron energy after recording several tracks of recoil protons. Nuclear tracks may be reconstructed via chemical processing or post-irradiation readings using a special microscope. To quickly calculate the neutron spectra for future data processing or other uses, the recoil tracks must sometimes be read out in real-time.

An online nuclear track methodology for NSM is lacking in this area, hence we've lately looked into recoil-proton track imaging (RPTI). Real-time imaging is combined with gas scintillation in RPTI. An advanced scientific camera, such as an image-intensified CCD/COMS camera, coupled with a transparent scintillation gas of high photon yield and visible light spectra (such as CF4), makes it possible to image the recoil-proton tracks in scintillation gas in real-time; analyze the track information online; and further unfold the neutron energy quickly.

The RPTI approach for NSM is described in detail here, and a simple RPTI detector system is designed in part 2. A Monte Carlo algorithm is used to simulate the performance of a new RPTI detector created in section 3. Preliminary testing of the track imaging system for proton tracks and proton spectra is reported in Part 4; in the last section, summary and future research are explored.


Proton recoiling

Hydrogen nuclei in a polyethylene (CH2) foil react with collimated neutrons to produce recoil protons. In the laboratory coordinate system, the recoil proton energy is linked to the impinging neutron energy in terms of the scattering angle.

To create scintillation photons, recoil protons deposit energy in scintillation gas, such as CF4 or a noble gas. Detection of produced photons shows the proton track based on their spatial distribution. So if photons can be captured using a real-time imaging system, one can see the proton track picture, from which the proton track range may be retrieved.

Specific energy loss (dE/dx) for charged particles in a particular absorber is stated using the Bethe formula.

For example, the atomic number and velocity of the primary particle, as well as the rest mass and atomic number density of the absorber atoms, are given in Equations 2 and 3 respectively. The charge of an electron, e, is also given in these equations. The average ionization and excitation potential of the absorber is represented by the parameter I, which is calculated experimentally for each individual element. The track range changes with the starting energy, as expected. The SRIM code14 was used to compute the connection between the proton range (R) and the initial energy (E) of protons in CF4 gas at 1 bar. There is a one-to-one correlation between the original proton energy and the proton range.

With optically recorded track pictures, the ranges of the optically recorded proton tracks may be directly retrieved, and then the energy spectrum of the recoil protons can be determined from the Energy-Range relationship in the given atmosphere (the Energy-Range method). Based on Eq., the neutron spectrum will eventually unfold.

Basic detection system

A rudimentary RPTI detector system shown schematically in Figure 2 follows the principles outlined above. The detector consists of two main parts. Nuclear recoil systems, such as this one, are made up of neutron collimators and shields, CH2 foils for converting the neutrons to recoil protons, and proton collimators for choosing the incident proton with the predetermined recoil angle. To capture a picture of the proton tracks, a gas scintillation chamber and a series of imaging equipment are used. The gas scintillation chamber generates photons that travel along the tracks.

In addition to its outstanding properties (such as wide spectral wavelength, relatively high yield, and fast emission)15,16,17–19 for imaging purposes and the fact that it is a heavy molecule that contains only low-Z atoms, CF4 is also an excellent scintillation gas because it is insensitive to gamma radiation and has a good neutron/gamma discrimination ratio. CF4 A 5 m-thickness Ti window may be used to enclose the gas chamber while still allowing recoil protons to pass through, according to technical standards. Installing an external imaging device's photons requires a quartz window. Image-intensified CCD/CMOS cameras may be used to capture the proton track pictures in real time, and the collected imaging data will be sent to the online analysis system for quick unfolding of energy spectra.

Simulations based on a person's abilities

This setup has been used in a series of Monte Carlo simulations20 to test the RPTI approach for NSM. The collimated neutron source is defined as a 14 MeV neutron beam in the context of DT neutron observations. In this case, the neutron beam's orientation is fixed at 45 degrees from the x-axis for the sake of ease of analysis. This angle should be carefully chosen in real applications in order to decrease the direct neutron emission. A CH2 foil is immediately struck by the neutrons, resulting in the generation of recoil protons. The CH2 foil has an effective area of 0.52 cm2 that is collimated by the neutron beam. The proton collimator is a 2 mm thick Fe disk with an inner radium of 3 mm, located 1 mm from the entry window (5 m-thickness Ti window). One bar of CF4 gas fills the gas chamber, which accepts only protons traveling in the xy direction. For this experiment, a 42 mm spacing was established between the CH2 foil and the proton collimator.

After passing through the CH2 foil, air gap, and Ti window, the simulation measures the remaining proton energy. The energy spectrum of recoil protons is also obtained by combining the Energy-Range relations in Ti and air (calculated using SRIM code) with the average energy loss in the CH2 foil. In the end, Eq. unfolds the neutron spectra from the recoil proton energy spectrum.