The paper by Belyaev et al. [Phys. Rev. E {\bf 72}, 026406 (2005)] reported the first experimental observation of alpha particles produced in the thermonuclear reaction $^{11}$B($p,\alpha$)$^{8}$Be induced by laser-irradiation on a $^{11}$B polyethylene (CH$_2$) composite target. The laser used in the experiment is characterized by a picosecond pulse duration and a peak of intensity of 2$\times10^{18}$ W/cm$^2$. We suggest that both the background-reduction method adopted in their detection system and the choice of the detection energy region of the reaction products are possibly inadequate. Consequently the total yield reported underestimates the true yield. Based on their observation, we give an estimation of the total yield to be higher than their conclusion, i.e., of the order of 10$^5 \alpha$ per shot.
Deep Dive into Comment on "Observation of neutronless fusion reactions in picosecond laser plasmas".
The paper by Belyaev et al. [Phys. Rev. E {\bf 72}, 026406 (2005)] reported the first experimental observation of alpha particles produced in the thermonuclear reaction $^{11}$B($p,\alpha$)$^{8}$Be induced by laser-irradiation on a $^{11}$B polyethylene (CH$_2$) composite target. The laser used in the experiment is characterized by a picosecond pulse duration and a peak of intensity of 2$\times10^{18}$ W/cm$^2$. We suggest that both the background-reduction method adopted in their detection system and the choice of the detection energy region of the reaction products are possibly inadequate. Consequently the total yield reported underestimates the true yield. Based on their observation, we give an estimation of the total yield to be higher than their conclusion, i.e., of the order of 10$^5 \alpha$ per shot.
The observations of the thermonuclear reactions in a high-power laser pulse irradiated target is one of the hottest topics [1,2,3,4,5,6,7]. The most investigated reaction is D(d, n) 3 He with a Q-value of 3.26 MeV. There have been studies using different characteristics of lasers irradiation on a wide variety of targets, solid CD 2 plastic [2,5,6], D 2 -gas [4] and deuterium-clusters [1]. Since the reactions produce monochromatic neutrons, the spectroscopy of these neutrons gives important information on the ion acceleration mechanism in the laser-induced plasma.
In the experiment recently carried out by a Russian group the yield of 10 3 α-particles has been reported [7], for the first time, in the laser-irradiation of a 11 B+CH 2 composite target. Their experiment is important for a deep understanding of the ion acceleration mechanism in the laser-matter interaction. The experiment has been carried out by using a “Neodymium” laser facility with the pulse energy of up to 15 J, a laser wave length of 1.055 µm, and a pulse duration of 1.5 ps. Before the main pulse, there are three pre-pulses with relative intensities 10 -4 , 10 -3 and 10 -8 , with ps durations for the former two and with 4 ns duration for the last one.
The laser beam has been focused on the solid target at an oblique incidence of 40 degrees to the target normal. CR-39 track detectors covered with 11 and 22 µm thick aluminum foils have been used to count the yield of αparticles from the reaction 11 B(p, α) 8 Be. The reaction induces three-particles decay. Either through the 8 Be ground state (α 0 ):
with the reaction Q-value = 8.59 MeV or through the 8 Be excited state (α 1 ):
(2) * Electronic address: kimura@lns.infn.it; Also at Dipartimento di Fisica e Astronomia dell’Universita’ di Catania, via Santa Sofia, 64, 95123 Catania, Italy with the reaction Q-value = 5.65 MeV and a large width of 1.5 MeV [8,9,10]. This is followed by the decay of the excited state (α 12 ):
and a reaction Q-value = 3.028 MeV. It is known that the main channel of the reaction is the second [11,12] and only 1 % of the reaction products are α 0 from the reaction (1). Using energy and momentum conservation laws, the α 0 and α 1 have kinetic energies:
where E is the center-of-mass incident energy in the case of the conventional beam-target experiment. But in the laser-induced plasma, the incident energy of the reactions is characterized by some energy distributions, which are not known clearly. If we assume a thermal equilibrium state for the plasma, the energy distribution is given by a Maxwellian. The temperature of the plasma is estimated [13,14] to be of the order of 67 keV for a background electron temperature T c = 0.5 keV and 84 keV for T c = 1. keV at the given laser intensity and the wavelength of the experiment. We mention that Ref. [15] gives an estimate of the nuclear temperature of 33 keV, lower than our estimation. The ions, therefore, can be accelerated up to the energies of the order of hundreds of keV at most. At such low energies, the α 0 and α 1 are estimated to have energies 5.7 MeV and 3.76 MeV, respectively, in the exit channel. However, the energy spectrum of α 1 has a large width, Γ= 1.5 MeV, consequently the α 12 spectra spread from 0 to higher than 5 MeV [16]. An α energy spectrum obtained experimentally in Ref. [10] shows clearly these characteristics of the reaction 11 B(p, α) 8 Be.
The full squares connected by the thick line in Fig. 1 reproduce the data reported in Ref. [10]. The two peaks at 3.76 MeV and 5.7 MeV are clearly visible.
In the experiment in Ref. [7] the CR-39 track detectors have been placed at angles of 0, 45 and 85 degrees to the target normal. The 11 B(p, α) 8 Be reaction yield has been estimated by subtracting the background obtained in the irradiation of the pure CH 2 target. The detectors are covered with Aluminum foils 11 or 22 µm thick. The reason for covering the plastic detectors is that the alpha tracks get confused with energetic ions coming from the high momentum tail of the plasma distributions. Cutting off the track diameter below 7 µm as in Ref. [7] eliminates all the protons but not heavier ions (B and C) of the plasma which leave bigger tracks. The authors of Ref. [7] observed that these ‘strange ions’ were still dominant when a 6 µm Al foil is used. This fact prompted them to increase the thickness of the foil which caused blocking lower energy α-particles as well. However this shielding of background is efficient, only if the energy of the detected ions is well specified as in the case of the reaction with a two-body exit-channel. By contrast in the reaction 11 B(p, α) 8 Be, the energy spectrum of reaction products spreads from 0 up to 5.7 MeV, as it is shown in Fig. 1. In such a case the Al foil will remove the major part of the reaction products. A 11 µm thick Al foil shields α-particles with energies lower than 3 MeV. If one uses a 22 µm thick Al foil, α-particles with energies lower than 5 MeV wi
…(Full text truncated)…
This content is AI-processed based on ArXiv data.
