Despite the tremendous importance of so-called ionizing radiations (X-rays, accelerated electrons and ions) in cancer treatment, most studies on their effects have focused on the ionization process itself, and neglect the excitation events the radiations can induce. Here, we show that the excited states of DNA exposed to accelerated electrons can be studied in the picosecond time domain using a recently developed cathodoluminescence system with high temporal resolution. Our study uses a table-top ultrafast, UV laser-triggered electron gun delivering picosecond electron bunches of keV energy. This scheme makes it possible to directly compare time-resolved cathodoluminescence with photoluminescence measurements. This comparison revealed qualitative differences, as well as quantitative similarities between excited states of DNA upon exposure to electrons or photons.

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Time-resolved cathodoluminescence of DNA triggered by picosecond electron bunches.

We show that the width of a gamma-line observed when stimulated amplification occurs is less than the natural linewidth emitted after spontaneous transition. The linewidths for a particular isotope ( 125m Te) are calculated for which the stimulated emission of gamma-rays is possible. If the amplification occurs between a long living upper state - via a short living lower state to the ground state - the width of the resonance line is also less than the natural width. In this case, the population of level n will be different (narrower) than the natural width of level n ( Γ n ). The Mossbauer spontaneous line Γ sp (transition n, o) will have the same linewidth as the amplified stimulated line ( Γ sp = Γ st Γ n ). Simultaneous experiments between resonance Mossbauer radiation and stimulated gamma-emission are proposed. These experiments are with better energy and time resolution.

Simultaneous Mössbauer experiments and gamma laser emission

HEITLER1 showed that in the absorption and emission of photons in resonance fluorescence, the scattering system “remembers” how it was excited; it always emits photons with the same energy as the absorbed photon. When a monochromatic line (Γ ≪ Γ0) is scattered in the manner shown in Fig. 1, the intensity of the scattered photons is given by the Breit–Wigner equation illustration Ir is the intensity of the scattered photons at the resonance Er Γ0 is the line width of the scattering system and E is the energy of the incident (and scattered) photons. When resonance scattering takes place, the photon absorbed by the scatterer is delayed for a certain time before being emitted again. Messiah2 showed that the delay depends on the energy of the incident photons illustration where ħ is Planck's constant. No matter how the nuclei are excited, the lifetime of the excited state Δt always depends on ∣E−Er∣. In this work I report another relation which also depends on ∣E−Er∣ illustration and this equation can be verified experimentally.
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P. Kamenov

Papers

Relation between the Lifetime of the Excited State and the Energy of Excitation