Photonuclear reactions with monoenergetic electron beams

Energetic electron beams produced by laser wakefield acceleration have an array of applications in radiography, radioisotope production, nuclear physics, and possibly the transmutation of nuclear waste. In particular, the multi-MeV bremsstrahlung produced using high-energy laser wakefield accelerated electrons incident upon a high Z converter is an effective technique to induce photonuclear reactions and photo-fission on a table-top [1,2] and may have additional applications in dynamic gamma-ray radiography and nuclear resonance fluorescence to detect explosives or other materials.  Activation of nuclei by gamma-induced reactions requires gamma-ray energies corresponding to the giant dipole resonances of the nuclei, which typically lie in the 10-20 MeV energy range for (gamma,n) reactions and 5-10 MeV for (gamma,fission) reactions. Generation of gamma-rays lying within the giant resonance energy range can be made very efficiently with a high energy monoenergetic electron source.  We optimized the electron beam's spatial profile and its total charge, and consistently generated a quasi-monoenergetic electron beam with a peak energy ranging between 100 and 150 MeV by adjusting plasma background density. After characterizing the electron beam and establishing its reproducibility, a 2.9 mm thick, 11 mm diameter natural uranium target was placed 15 cm behind the gas jet, where the electron beam was approximately 3 mm in diameter. Bremsstrahlung yield scales as the atomic number squared Z² thus the uranium target was used both as a converter and as a fissionable target (Fig. 1). 

Fig. 1. Experimental setup for photo-fission using high-energy electrons from laser wakefield.

The uranium fission process produces, among other fission fragments, ¹³⁴Iodine and ⁹²Strontium  with 53 min and 2.7 h half-lives, respectively. The laser was fired 72 times over 75 min, corresponding to roughly 1.5 ¹³⁴IodineI half-lives, and an additional 18 min delay was experienced between removing the sample and the counting process. The emitted gamma spectrum was recorded every 10 min for the first 1.5 h, every 20 min for the next hour, and finally every 60 min for the next 7 h. The ¹³⁴Iodine and ⁹²Strontium fission products were identified, using an ORTEC GAMMX Ge(Li) gamma-ray detector, through their signature gamma decays of 0.847 MeV and 1.384 MeV gamma rays (Figure 2).  

Fig.2. The gamma spectra taken from the irradiated uranium target. The signature gamma-emission peaks from ¹³⁴Iodine (847 keV) and ⁹²Strontium (1384 keV) decay are shown.

The presence and populations of ¹³⁴Iodine and ⁹²Strontium  were confirmed by measuring the numbers of the emitted gamma rays as functions of time, as shown in Fig. 3. Primary fission to the ¹³⁴Iodine fragment accounts for about one-third of the integrated total reactions. Feeding of the ¹³⁴Iodine population through the decays of other ²³⁸Uranium fission fragment pathways causes the deviation observed during the first 2 h of the measured ¹³⁴Iodine decay. The secondary fission fragment pathways decaying to ⁹²Strontium have shorter and decayed during the 18 min delay before counting began, thus showing little deviation from the single exponential decay model. By fitting the data after several expected lifetimes, 1.0 h and 2.64 h half-lives were determined for ¹³⁴Iodine and ⁹²Strontium, respectively, in relatively good agreement with the literature values of 53 min and 2.7 h.

Fig. 3. Measured count rates for 847 keV and 1384 keV gamma rays detected, as a function of time after the last laser shot for ²³⁸U(gamma,fission)¹³⁴I and ²³⁸U(gamma,fission)⁹²Sr processes.

To derive the fission yields the tabulated lifetimes and relative fission fractions for fragments contributing to ¹³⁴Iodine (and ⁹²Strontium) were used. The apparent total ¹³⁴Iodine and ⁹²Strontium isotope production yields after the last laser pulse derived from fitting the data with this model are 1.5x10⁷ and 1.0x10⁷, respectively. Dividing the totals by the appropriate effective number of laser shots, and taking into account the decays between shots, gives a resultant total average fission yield of 3.3x10⁵ fissions per laser shot for both the ¹³⁴Iodine and ⁹²Strontium data sets, which is more than an  order of magnitude greater than previously achieved.

More detailed information on efficient initiation of photo-nuclear reactions and photo-fission can be found in the poster here.

References

1. S. A. Reed, V. Chvykov, G. Kalintchenko, T. Matsuoka, P. Rousseau, V. Yanovsky, C. R. Vane, J. R. Beene, D. Stracener, D. R. Schultz, and A. Maksimchuk, “Photonuclear fission with quasi-monoenergetic electron beams from laser wakefields,” Appl. Phys. Lett. 89, 231107-1-3 (2006).

2. S. A. Reed, V. Chvykov, G. Kalintchenko, T. Matsuoka, P. Rousseau, V. Yanovsky, C. R. Vane, J. R. Beene, D. Stracener, D. R. Schultz and A. Maksimchuk, “Efficient initiation of photonuclear reactions using quasimonoenergetic electron beams from laser wakefield acceleration,” Journal of Applied Physics  102, 073103 (2007).