Daughter Atoms Research Articles

In vitro and in vivo study of 221Fr and 213Bi progeny release from the 225Ac-labelled TiO2 nanoparticles

BackgroundTargeted alpha therapy (TAT) is an effective option for cancer treatment. To maximize its efficacy and minimize side effects, carriers must deliver radionuclides to target tissues. Most of the nuclides used in TAT decay via the alpha cascade, producing several radioactive daughter nuclei with sufficient energy to escape from the original carrier. Therefore, studying these daughter atoms is crucial in the search for new carriers. Nanoparticles have potential as carriers due to their structure, which can prevent the escape of daughter atoms and reduce radiation exposure to non-target tissues. This work focuses on determining the released activity of 221Fr and 213Bi resulting from the decay of 225Ac labelled TiO2 nanoparticles. ResultsLabelling of TiO2 nanoparticles has shown high sorption rates of 225Ac and its progeny, 221Fr and 213Bi, with over 92 % of activities sorbed on the nanoparticle surface for all measured radionuclides. However, in the quasi-dynamic in vitro system, the released activity of 221Fr and 213Bi is strongly dependent on the nanoparticles concentration, ranging from 15 % for a concentration of 1 mg/mL to approximately 50 % for a nanoparticle concentration of 10 μg/mL in saline solution. The released activities of 213Bi were lower, with a maximum value of around 20 % for concentrations of 0.05, 0.025, and 0.01 mg/mL. The leakage of 225Ac and its progeny was tested in various biological matrices. Minimal released activity was measured in saline at around 10 % after 48 h, while the maximum activity was measured in blood serum and plasma at 20 %. The amount of 225Ac released into the media was minimal (<3 %). The in vitro results were confirmed in a healthy mouse model. The difference in %ID/g was clearly visible immediately after dissection and again after 6 h when 213Bi reached equilibrium with 225Ac. ConclusionThe study verified the potential release of 225Ac progeny from the labelled TiO2 nanoparticles. Experiments were performed to determine the dependence of released activity on nanoparticle concentration and the biological environment. The results demonstrated the high stability of the prepared 225Ac@TiO2 NPs and the potential release of progeny over time. In vivo studies confirmed our hypothesis. The data obtained suggest that the daughter atoms can escape from the original carrier and follow their own biological pathways in the organism.

DNA damage response in a 2D-culture model by diffusing alpha-emitters radiation therapy (Alpha-DaRT)

Diffusing alpha-emitters radiation therapy (Alpha-DaRT) is a unique method, in which interstitial sources carrying 224Ra release a chain of short-lived daughter atoms from their surface. Although DNA damage response (DDR) is crucial to inducing cell death after irradiation, how the DDR occurs during Alpha-DaRT treatment has not yet been explored. In this study, we temporo-spatially characterized DDR such as kinetics of DNA double-strand breaks (DSBs) and cell cycle, in two-dimensional (2D) culture conditions qualitatively mimicking Alpha-DaRT treatments, by employing HeLa cells expressing the Fucci cell cycle-visualizing system. The distribution of the alpha-particle pits detected by a plastic nuclear track detector, CR-39, strongly correlated with γH2AX staining, a marker of DSBs, around the 224Ra source, but the area of G2 arrested cells was more widely spread 24 h from the start of the exposure. Thereafter, close time-lapse observation revealed varying cell cycle kinetics, depending on the distance from the source. A medium containing daughter nuclides prepared from 224Ra sources allowed us to estimate the radiation dose after 24 h of exposure, and determine surviving fractions. The present experimental model revealed for the first time temporo-spatial information of DDR occurring around the source in its early stages.

External Radiation Assistance of Neutrinoless Double Electron Capture

The influence of electromagnetic radiation on nuclear processes is applied to an example of a neutrinoless double electron capture (0ν2ec). For cases with X-ray free-electron lasers (X-ray FELs) and/or inverse Compton X-ray sources, it was shown that such a decay can be significantly enhanced by tuning the system to the resonant conditions through the absorption and/or emission of a photon with the decay resonance defect energy Δ. In this case, the 0v2ec decay rate Γ2e of nuclide Z grew linearly with field intensity (S/Sz) up to the X-ray flux power Sm~Z6, while Sz~Z6 (Γ/Δ)2 with decay width Γ of a daughter atom. For the case of 78Kr → 78Se − 0ν2eL1L1 capture we find Sz~109 W cm−2 and Sm~1017 W cm−2 which indicate a possibility of increasing decay rate to eight orders of magnitude or even larger.

First measurements of radon-220 diffusion in mice tumors, towards treatment planning in diffusing alpha-emitters radiation therapy.

Diffusing alpha-emitters radiation therapy ("Alpha-DaRT") is a new method for treating solid tumors with alpha particles, relying on the release of the short-lived alpha-emitting daughter atoms of radium-224 from interstitial sources inserted into the tumor. Alpha-DaRT tumor dosimetry is governed by the spread of radium's progeny around the source, as described by an approximate framework called the "diffusion-leakage model". The most important model parameters are the diffusion lengths of radon-220 and lead-212, and their estimation is therefore essential for treatmentplanning. Previous works have provided initial estimates for the dominant diffusion length, by measuring the activity spread inside mice-borne tumors several days after the insertion of an Alpha-DaRT source. The measurements, taken when lead-212 was in secular equilibrium with radium-224, were interpreted as representing the lead-212 diffusion length. The aim of this work is to provide first experimental estimates for the diffusion length of radon-220, using a newmethodology. The diffusion length of radon-220 was estimated from autoradiography measurements of histological sectionstaken from 24 mice-borne subcutaneous tumors of five different types. Unlike previous studies, the source dwell time inside the tumor was limited to 30 min, to prevent the buildup of lead-212. To investigate the contribution of potential non-diffusive processes, experiments were done in two sets: fourteen in vivo tumors, where during the treatment the tumors were still carried by the mice with active blood supply, and 10 ex-vivo tumors, where the tumors were excised before source insertion and kept in a medium at with the sourceinside. The measured diffusion lengths of radon-220, extracted by fitting the recorded activity pattern up to 1.5mm from the source, lie in the range , with no significant difference between the average values measured in in-vivo and ex-vivo tumors: versus . However, in-vivo tumors display an enhanced spread of activity 2-3mm away from the source. This effect is not explained by the current model and is much less pronounced in ex-vivotumors. The average measured radon-220 diffusion lengths in both in-vivo and ex-vivo tumors are consistent with published data on the diffusion length of radon in water and lie close to the upper limit of the previously estimated range of . The observation that close to the source there is no apparent difference between in-vivo and ex-vivo tumors, and the good agreement with the theoretical model in this region suggest that the spread of radon-220 is predominantly diffusive in this region. The departure from the model prediction in in-vivo tumors at large radial distances may hint at potential vascular contribution, which will be the subject of futureworks.

Energy Spectrum of β Electrons in Neutrinoless Double-β Decay Including the Excitation of the Electron Shell of Atoms

Double-β decay is accompanied with a high probability by the excitation of the electron shell of the daughter atom; as a result, the energy carried away by β electrons decreases. The mean value and standard deviation of the excitation energy of the electron shell of the daughter atom in the double-β decay of germaniumhave been determined within the Thomas–Fermi and relativistic Dirac–Hartree–Fock methods. Using the estimates thus obtained, a two-parameter model of the energy spectrum of β electrons in the neutrinoless mode has been developed including the redistribution of the reaction energy between the decay products. The shift of the total energy of β electrons does not exceed 50 eV with a probability of 90%. However, the mean excitation energy is ~400 eV, i.e., an order of magnitude higher, whereas the standard deviation is ~2900 eV, which is apparently due to a significant contribution from inner electron levels to the energy characteristics of the process. The distortion of the shape of the peak of the 0ν2β decay should be taken into account when analyzing the data of detectors with a resolution of ~100 eV or higher.

Energy Spectrum of β Electrons in Neutrinoless Double-β Decay Including the Excitation of the Electron Shell of Atoms

Double-β decay is accompanied with a high probability by the excitation of the electron shell of the daughter atom; as a result, the energy carried away by β electrons decreases. The mean value and standard deviation of the excitation energy of the electron shell of the daughter atom in the double-β decay of germanium _{{32}}^{{76}}{text{Ge}} to ;_{{34}}^{{76}}{text{Se}}text{*} + ;2{{beta }^{ - }}( + 2{{bar {nu }}_{e}}) have been determined within the Thomas–Fermi and relativistic Dirac–Hartree–Fock methods. Using the estimates thus obtained, a two-parameter model of the energy spectrum of β electrons in the neutrinoless mode has been developed including the redistribution of the reaction energy between the decay products. The shift of the total energy of β electrons does not exceed 50 eV with a probability of 90%. However, the mean excitation energy is ~400 eV, i.e., an order of magnitude higher, whereas the standard deviation is ~2900 eV, which is apparently due to a significant contribution from inner electron levels to the energy characteristics of the process. The distortion of the shape of the peak of the 0ν2β decay should be taken into account when analyzing the data of detectors with a resolution of ~100 eV or higher.

The implication of the atomic effects in neutrinoless double beta decay

In the planned ton-scale neutrinoless double beta [Formula: see text] decay experiments, the used [Formula: see text] value is the mass-energy difference between the ground states of mother and daughter atoms, without correcting the atomic energy difference due to the alteration in the nuclear charge before and after the decay process. We show that the atomic energy change between the two ground states can potentially reduce the [Formula: see text] value by 5.502, 12.103, and 12.372 keV for the decays from [Formula: see text], [Formula: see text], and [Formula: see text], respectively. The available kinetic energy for two electrons can be 2033.51 keV for [Formula: see text] decay, 2515.41 keV for [Formula: see text] decay, and 2445.45 keV for [Formula: see text] decay after correcting for atomic effects. This implies that the planned experiments could potentially miss the decay signature if a narrow energy window is designated to reduce background events including [Formula: see text] decay events.

The “Reverse Magnet Effect” of Alpha Emitters and the Health Costs of Depleted Uranium in the Balkans: The Root of the Debt Crisis in Greece, Italy, Portugal, Spain and Ireland

Very large scale health effects of the depleted uranium (DU) spreading from the Balkans can be observed through analysis of WHO DALYs data and explain the budgetary deficits at the roots of the 2010s’ debt crisis in Southern Europe (the “PIGS”, Portugal, Italy, Greece and Spain) and Ireland. The countries around the Balkans suffered from DU exposure through patterns of “positive push”, the brain being constrained in its decisions by pre-existing internal contamination with alpha emitters and their positive charge repelled by the new alpha emitters of the DU strikes in the Balkans (“reverse magnet effect” from +/+ repelling). The subjects are constrained to move in certain patterns by the positive resistance coming from the area of the Balkans, clogs emerge in their bodies leading for instance to cardiomyopathies, kidney and urinary diseases, mesothelioma and other neoplasms, along with immune deficiencies and mutations causing rises in infectious diseases, and alcoholism. The slow rise in alpha activity of depleted uranium, with its decay chain pattern leading to more and more alpha-emitting atoms over time (because daughter atoms are shorter-lived) causes a “timebomb” pattern (as usual in veterans of wars with DU) with an increasing “push” effect and explains the later public debt crisis. Beyond the so-called “PIGS”, Austria suffered as well a lot from the DU fallout, with health costs and environmental damage altogether and this is shown analytically. More direct endocrine effects on the brain are also shown, with behavioural consequences. The case for more epidemiological effects if nothing is done is clear and comes together with strong societal issues resulting directly from the intertwinning of the neurological impairments affecting decisionmaking and divergence in the answers that can be offered. NATO refunding of the ECB’s public bond-buying programs with depleted uranium to allow it to emit depleted uranium coins is suggested as final fixture to the root problems of the debt crisis.

Limits on the Existence of sub-MeV Sterile Neutrinos from the Decay of Be7 in Superconducting Quantum Sensors

Sterile neutrinos are natural extensions to the standard model of particle physics and provide a possible portal to the dark sector. We report a new search for the existence of sub-MeV sterile neutrinos using the decay-momentum reconstruction technique in the decay of $^{7}\mathrm{Be}$. The experiment measures the total energy of the $^{7}\mathrm{Li}$ daughter atom from the electron capture decay of $^{7}\mathrm{Be}$ implanted into sensitive superconducting tunnel junction (STJ) quantum sensors. This first experiment presents data from a single STJ operated at a low count rate for a net total of 28 days, and provides exclusion limits on sterile neutrinos in the mass range from 100 to 850 keV that improve upon previous work by up to an order of magnitude.

Real-time in-vivo dosimetry for DaRT

Diffusing alpha-emitters radiation therapy (DaRT) is a revolutionary brachytherapy technique used to treat solid tumours. Implant seeds are coated with 224Ra which, along its shortlived daughter atoms, emits alpha particles of high linear energy transfer (LET) and of high relative biological efficiency (RBE), creating a tumour-killing dose distribution a few mm wide. Those alpha particles are of energy between 5.67 and 8.78 MeV. DaRT is under investigation in clinical trials, but there currently is no obvious solution for dosimetry aimed at quality assurance of treatment. This study introduces alpha-RAD, a dosimeter based on a metal-oxide-semiconductor (MOS) sensor technology. Alpha-RAD was characterized with 241Am, which emits alpha particles of energy 5.49 MeV. The results showed that alpha-RAD had good linearity with dose, with the signal increasing linearly in the range from 0 to 6.84 Gy. Also, an external bias in the range between 15 and 60 V, applied on the gate of alpha-RAD during irradiation, would optimize sensitivity to alpha particles of energies typical of DaRT. Alpha-RAD, owing to its compactness, can fit into a brachytherapy needle, to be placed next to 224Ra seed implants in the tumour, for real-time in vivo dosimetry.

Fluorescent bicolour sensor for low-background neutrinoless double β decay experiments.

Observation of the neutrinoless double β decay is the only practical way to establish that neutrinos are their own antiparticles1. Because of the small masses of neutrinos, the lifetime of neutrinoless double β decay is expected to be at least ten orders of magnitude greater than the typical lifetimes of natural radioactive chains, which can mimic the experimental signature of neutrinoless double β decay2. The most robust identification of neutrinoless double β decay requires the definition of a signature signal-such as the observation of the daughter atom in the decay-that cannot be generated by radioactive backgrounds, as well as excellent energy resolution. In particular, the neutrinoless double β decay of 136Xe could be established by detecting the daughter atom, 136Ba2+, in its doubly ionized state3-8. Here we demonstrate an important step towards a 'barium-tagging' experiment, which identifies double β decay through the detection of a single Ba2+ ion. We propose a fluorescent bicolour indicator as the core of a sensor that can detect single Ba2+ ions in a high-pressure xenon gas detector. In a sensor made of a monolayer of such indicators, the Ba2+ dication would be captured by one of the molecules and generate a Ba2+-coordinated species with distinct photophysical properties. The presence of such a single Ba2+-coordinated indicator would be revealed by its response to repeated interrogation with a laser system, enabling the development of a sensor able to detect single Ba2+ ions in high-pressure xenon gas detectors for barium-tagging experiments.

Atom Motion in Solids Following Nuclear Transmutation

Following nuclear decay, a daughter atom in a solid will "stay in place" if the recoil energy is less than the threshold for displacement. At high temperature, it may subsequently undergo long-range diffusion or some other kind of atomic motion. In this paper, motion of 111Cd tracer probe atoms is reconsidered following electron-capture decay of 111In in the series of In3R phases (R= rare-earth). The motion produces nuclear relaxation that was measured using the method of perturbed angular correlation. Previous measurements along the entire series of In3R phases appeared to show a crossover between two diffusional regimes. While relaxation for R= Lu-Tb is consistent with a simple vacancy diffusion mechanism, relaxation for R= Nd-La is not. More recent measurements in Pd3R phases demonstrate that the site-preference of the parent In-probe changes along the series and suggests that the same behavior occurs for daughter Cd-probes. The anomalous motion observed for R= Nd-La is attributed to "lanthanide expansion" occurring towards La end-member phases. For In3La, the Cd-tracer is found to jump away from its original location on the In-sublattice in an extremely short time, of order 0.5 ns at 1000 K and 1.2 ms at room temperature, a residence time too short to be consistent with defect-mediated diffusion. Several scenarios that can explain the relaxation are presented based on the hypothesis that daughter Cd-probes first jump to neighboring interstitial sites and then are either trapped and immobilized, undergo long-range diffusion, or persist in a localized motion in a cage.

Status of the HOLMES Experiment

The absolute neutrino mass is still an unknown parameter in the modern landscape of particle physics. The HOLMES experiment aims at exploiting the calorimetric approach to directly measure the neutrino mass through the kinematic measurement of the decay products of the weak process decay of $$^{163}$$Ho. This low energy decaying isotope, in fact, undergoes electron capture emitting a neutrino and leaving the daughter atom, $$^{163}$$Dy$$^*$$, in an atomic excited state. This, in turn, relaxes by emitting electrons and, to a considerably lesser extent, photons. The high-energy portion of the calorimetric spectrum of this decay is affected by the non-vanishing neutrino mass value. Given the small fraction of events falling within the region of interest, to achieve a high experimental sensitivity on the neutrino mass, it is important to have a high activity combined with a very small undetected pileup contribution. To achieve these targets, the final configuration of HOLMES foresees the deployment of a large number of $$^{163}$$Ho ion-implanted TESs characterized by an ambitiously high activity of 300 Hz each. In this paper, we outline the status of the major tasks that will bring HOLMES to achieve a statistical sensitivity on the neutrino mass as low as 2 eV/c$$^2$$.

Stimulated X-rays in resonant atom Majorana mixing

Massive neutrinos demand to ask whether they are Dirac or Majorana particles. Majorana neutrinos are an irrefutable proof of physics beyond the Standard Model. Neutrinoless double electron capture is not a process but a virtual ΔL = 2 mixing between a parent AZ atom and a daughter A(Z — 2) excited atom with two electron holes. As a mixing between two neutral atoms and the observable signal in terms of emitted two-hole X-rays, the strategy, experimental signature and background are different from neutrinoless double beta decay. The mixing is resonantly enhanced for almost degeneracy and, under these conditions, there is no irreducible background from the standard two-neutrino channel. We reconstruct the natural time history of a nominally stable parent atom since its production either by nature or in the laboratory. After the time periods of atom oscillations and the decay of the short-lived daughter atom, at observable times the relevant “stationary” states are the mixed metastable long-lived state and the short-lived excited state, as well as the ground state of the daughter atom. Their natural population inversion is most appropriate for exploiting the bosonic nature of the observed X-rays by means of stimulating X-ray beams. Among different observables of the atom Majorana mixing, we include the enhanced rate of stimulated X-ray emission from the long-lived metastable state by a high-intensity X-ray beam. A gain factor of 100 can be envisaged in a facility like European XFEL.

Overlap of electron shells in $$\beta $$ and double-$$\beta $$ decays

The $\beta$ and double-$\beta$ decay channels, which are not accompanied by excitation of the electron shells, are suppressed due to the nonorthogonality of the electron wave functions of the parent and daughter atoms. The effect is sensitive to the contribution of the outer electron shells. Since valence electrons participate in chemical bonding and collectivize in metals, the decay rates of the unstable nuclides are modified when they are embedded in a host material. Core electrons are less affected by the environment, and their overlap amplitudes are more stable. The suppression effect is estimated for $ \beta^- $ decay of $^{87}$Kr, electron capture in $^{163}$Ho, and $2\beta^-$ decays of $^{76}$Ge, $^{100}$Mo, $^{130}$Te, and $^{136}$Xe. The overlap amplitude of the electron shells enters the relationship between the half-life of neutrinoless $2\beta$ decay and the effective electron neutrino Majorana mass.

Imaging individual barium atoms in solid xenon for barium tagging in nEXO.

Double-β-decay involves the simultaneous conversion of two neutrons into two protons, and the emission of two electrons and two neutrinos; the neutrinoless process, although not yet observed, is thought to involve the emission of the two electrons but no neutrinos. The search for neutrinoless-double-β-decay probes fundamental properties of neutrinos, including whether or not the neutrino and antineutrino are distinct particles. Double-β-decay detectors are large and expensive, so it is essential to achieve the highest possible sensitivity with each study, and removing spurious contributions ('background') from detected signals is crucial. In the nEXO neutrinoless-double-β-decay experiment, the identification, or 'tagging', of the 136Ba daughter atom resulting from the double-β decay of 136Xeprovides a technique for discriminating background. The tagging scheme studied here uses a cryogenic probe to trap the barium atom in a solid xenon matrix, where the barium atom is tagged through fluorescence imaging. Here we demonstrate the imaging and counting of individual barium atoms in solid xenon by scanning a focused laser across a solid xenon matrix deposited on a sapphire window. When the laser irradiates an individual atom, the fluorescence persists for about 30 seconds before dropping abruptly to the background level-a clear confirmation of one-atom imaging. Following evaporation of a barium deposit, the residual barium fluorescence is 0.16 per cent or less. Our technique achieves the imaging of single atoms in a solid noble element, establishing the basic principle of barium tagging for nEXO.

Identification and investigation of possible ultra-low Q value β decay candidates

Among the wide energy range of β decays, there can exist decays with Q values as low as a few hundred eV. These decays can occur when the parent decays to a excited state in the daughter nucleus. Such decays have been called “ultra-low” Q value β decays. Their application is mainly twofold: (1) they are of interest as potential candidates for neutrino mass determination experiments, and (2) they provide a testing ground for theoretical studies of atomic interference effects in the nuclear decay process. In this work we have identified a number of such potential candidates by analyzing the most recent atomic mass and nuclear energy level data. To determine if an ultra-low Q value β decay branch is energetically allowed for these candidates, more precise and accurate data for the Q value of the ultra-low decay branch is needed. In most cases, this requires more precise atomic mass measurements for the parent and/or daughter atoms. These requirements can be met using Penning trap mass spectrometry.

On the Structure of the Mössbauer Spectra of 119mSn Impurity Atoms in Lead Chalcogenides under Conditions of the Radioactive Equilibrium of 119mTe/119Sb Isotopes

The method of Mossbauer emission spectroscopy with 119mTe and 119Sb parent isotopes in the radioactive-equilibrium state is used to obtain information on the localization and valence state of 119mSn daughter atoms formed from the 119Sb and 119mTe parent atoms at cation and anion sites of the lattice of PbSe and PbTe lead chalcogenides.

The therapeutic potential of polymersomes loaded with 225Ac evaluated in 2D and 3D in vitro glioma models

Alpha emitters have great potential in targeted tumour therapy, especially in destroying micrometastases, due to their high linear energy transfer (LET). To prevent toxicity caused by recoiled daughter atoms in healthy tissue, alpha emitters like 225Ac can be encapsulated in polymeric nanocarriers (polymersomes), which are capable of retaining the daughter atoms to a large degree. In the translation to a (pre-)clinical setting, it is essential to evaluate their therapeutic potential. As multicellular tumour spheroids mimic a tumour microenvironment more closely than a two-dimensional cellular monolayer, this study has focussed on the interaction of the polymersomes with U87 human glioma spheroids. We have found that polymersomes distribute themselves throughout the spheroid after 4 days which, considering the long half-life of 225Ac (9.9 d) (Vaidyanathan and Zalutsky, 1996), allows for irradiation of the entire spheroid. A decrease in spheroidal growth has been observed upon the addition of only 0.1 kBq 225Ac, an effect which was more pronounced for the 225Ac in polymersomes than when only coupled to DTPA. At higher activities (5 kBq), the spheroids have been found to be destroyed completely after two days. We have thus demonstrated that 225Ac containing polymersomes effectively inhibit tumour spheroid growth, making them very promising candidates for future in vivo testing.

Stimulated transitions in resonant atom Majorana mixing

Massive neutrinos demand to ask whether they are Dirac or Majorana particles. Majorana neutrinos are an irrefutable proof of physics beyond the Standard Model. Neutrinoless double electron capture is not a process but a virtual ΔL = 2 mixing between a parent AZ atom and a daughter A(Z − 2) excited atom with two electron holes. As a mixing between two neutral atoms and the observable signal in terms of emitted two-hole X-rays, the strategy, experimental signature and background are different from neutrinoless double beta decay. The mixing is resonantly enhanced for almost degeneracy and, under these conditions, there is no irreducible background from the standard two-neutrino channel. We reconstruct the natural time history of a nominally stable parent atom since its production either by nature or in the laboratory. After the time periods of atom oscillations and the decay of the short-lived daughter atom, at observable times the relevant “stationary” states are the mixed metastable long-lived state and the non-orthogonal short-lived excited state, as well as the ground state of the daughter atom. We find that they have a natural population inversion which is most appropriate for exploiting the bosonic nature of the observed atomic transitions radiation. Among different observables of the atom Majorana mixing, we include the enhanced rate of stimulated X-ray emission from the long-lived metastable state by a high-intensity X-ray beam: a gain factor of 100 can be envisaged at current XFEL facilities. On the other hand, the historical population of the daughter atom ground state can be probed by exciting it with a current pulsed optical laser, showing the characteristic absorption lines: the whole population can be excited in a shorter time than typical pulse duration.