Pulsed source of ultra low energy positive muons for near-surface μSR studies

• Published in: Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms Beam interactions with materials and atoms, 2008-01, Vol.266 (2), p.335-346
• Main Authors: Bakule, Pavel, Matsuda, Yasuyuki, Miyake, Yasuhiro, Nagamine, Kanetada, Iwasaki, Masahiko, Ikedo, Yutaka, Shimomura, Koichiro, Strasser, Patrick, Makimura, Shunshuke
• Format: Article
• Language: English
• Subjects: 07.77.Ka; 32.80.Rm; 36.10.Dr; 41.75.–i; 42.65.Ky; 76.75.+i; Low energy muons; Lyman-α; Muon spin rotation; Muonium generation; Muonium ionization; Thin film; Ultra slow muons; Vacuum-ultraviolet laser
• ISSN: 0168-583X; 1872-9584
• DOI: 10.1016/j.nimb.2007.11.009

We have produced a pulsed beam of low energy (ultra slow) polarized positive muons (LE-μ+) and performed several demonstration muon spin rotation/relaxation (μSR) experiments at ISIS RIKEN-RAL muon facility in UK. The energy of the muons implanted into a sample is tuneable between 0.1keV and 18keV. This allows us to use muons as local magnetic microprobes on a nanometre scale. The control over the implantation depth is from several nanometres to hundreds of nanometres depending on the sample density and muon energy. The LE-μ+ are produced by two-photon resonant laser ionization of thermal muonium atoms. Currently ∼15 LE-μ+/s with 50% spin polarization are transported to the μSR sample position, where they are focused to a small spot with a diameter of only 4mm. The overall LE-μ+ generation efficiency of 3×10−5 is comparable to that obtained when moderating the muon beam to epithermal energies in simple van der Waals bound solids. In contrast to other methods of LE-μ+ generation, the implantation of the muons into the sample can be externally triggered with the duration of the LE-μ+ pulse being only 7.5ns. This allows us to measure spin rotation frequencies of up to 40MHz.