Alpha-decay Events Research Articles

Method of detecting rare superheavy nuclei decay events—the active correlation technique

Recently, a successful cycle of experiments on the synthesis of superheavy elements with Z = 112–118 has been accomplished with 48Ca beam at the Flerov Laboratory of Nuclear Reactions (FLNR), JINR. From the viewpoint of detection of rare decay and background suppression this success was achieved owing to a new radical technique, the method of active correlations. It employs the search in a real-time mode for a pointer to probable correlation like recoil-alpha for switching the beam off. In the case of detection in the same detector strip an additional alpha-decay event of the “beam OFF” time interval is automatically prolonged. Reasonable scenarios of developing the method are considered.

Experimental Insight into the Radiation Resistance of Zirconia-Based Americium Ceramics

Our works shows that the americium pyrochlore (241)Am(2)Zr(2)O(7) undergoes a phase transition to a defect-fluorite structure along with an unusual volume contraction when subjected to internal radiation from alpha-emitting actinides. Disorder relaxation proceeds through the simultaneous formation of cation antisites and oxygen Frenkel pairs. X-ray absorption spectroscopy at the Am-L(II) and the Zr-K edges reveals that Am-O polyhedra show an increasing disorder with increasing exposure. In contrast, the Zr-O polyhedral units remain highly ordered, while rotating along edges and corners, thereby reducing the structural strain imposed by the growing disorder around americium. We believe it is this particular property of the compound that provides the remarkable resistance to radiation (>9.4 x 10(18) alpha-decay events g(-1) or 0.80 dpa).

Radiation-Stability of Smectite

The safety assessment of geological repositories for high-level nuclear waste and spent nuclear fuel requires an understanding of the response of materials to high temperatures and intense radiation fields. Clays, such as smectite, have been proposed as backfill material around waste packages, but their response to intense radiation from short-lived fission products and alpha decay of sorbed actinides remains poorly understood. Cumulative doses may amorphize clays and may alter their properties of sorption, swelling, or water retention. We describe the amorphization of smectites induced by electron and heavy ion irradiations to simulate ionizing radiation and alpha recoil nuclei, respectively. A new "bell-shaped" evolution of the amorphization dose with temperature has been determined. The maximum dose for amorphization occurs at about 300-400 degrees C, showing that temperature-induced dehydroxylation enhances amorphization. The exact shape of the bell-shaped curves depends on the interlayer cation. At ambient temperature, ionizing radiation and alpha-decay events do not show the same efficiency. The former results in amorphization at doses between 10(10)-10(11) Gy which are greater than the total radiation dose expected for radioactive waste over 10(6) years. In contrast, alpha-decay events amorphize clays at doses as low as 0.13-0.16 displacements per atom, i.e. doses consistent with nuclear waste accumulated over approximately 1000 yrs. However, the limited penetration of alpha particles and recoil nuclei, in the 100 nm - 20 microm range, will minimize damage. Clays will not be amorphized unless the waste package is breached and released actinides are heavily sorbed onto the clay overpack.

Micro-Raman and micro-infrared spectroscopic studies of Pb- and Au-irradiatedZrSiO4: Optical properties, structural damage, and amorphization

The optical properties of damaged periodic and aperiodic domains created by ${\mathrm{Pb}}^{+}$ $(280\phantom{\rule{0.3em}{0ex}}\mathrm{keV})$ and ${\mathrm{Au}}^{4+}$ $(10\phantom{\rule{0.3em}{0ex}}\mathrm{MeV})$ implantation of zircon were studied using micro-infrared (IR) and micro-Raman spectroscopy. The ${\mathrm{Pb}}^{+}$ and ${\mathrm{Au}}^{4+}$ irradiations caused a dramatic decrease in the IR reflectivity similar to that observed for metamict natural zircon. The irradiation with $10\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ ${\mathrm{Au}}^{4+}$ ions (to fluences of $1\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.3em}{0ex}}{\mathrm{Au}}^{4+}\phantom{\rule{0.3em}{0ex}}\mathrm{ions}∕{\mathrm{cm}}^{2}$) also results in the formation of an amorphized phase similar to that observed in metamict zircon. These results show that high-energy, heavy-ion irradiations provide a good simulation of the ballistic effects of the recoil nucleus of an alpha-decay event and, in both cases, the result is the creation of aperiodic domains. Additional IR and Raman features were recorded in samples irradiated with $280\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ ${\mathrm{Pb}}^{+}$ ions (to fluences of $1\ifmmode\times\else\texttimes\fi{}{10}^{14}$ and $1\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.3em}{0ex}}{\mathrm{Pb}}^{+}\phantom{\rule{0.3em}{0ex}}\mathrm{ions}∕{\mathrm{cm}}^{2}$), indicating the formation of an irradiation-induced additional phase(s). The frequencies of the features are consistent with lead silicates, $\mathrm{Zr}{\mathrm{O}}_{2}$, and $\mathrm{Si}{\mathrm{O}}_{2}$. The results show that spectral features of the ${\mathrm{Au}}^{4+}$- and ${\mathrm{Pb}}^{+}$-irradiated zircon are different from those of quenched $\mathrm{Zr}\mathrm{Si}{\mathrm{O}}_{4}$ melts, and the finding further confirms that the amorphous state produced by high-energy ion irradiations is structurally different from the glassy state that results from quenching a high temperature melt. In contrast to significant changes in the frequency and width of the Raman ${\ensuremath{\nu}}_{3}$ band observed in metamict zircon, the ${\mathrm{Pb}}^{+}$ and ${\mathrm{Au}}^{4+}$ irradiations do not cause similar variations, indicating that the remaining zircon crystalline domains in irradiated samples have a crystalline structure with fewer defects than those of metamict zircon.

Dissolution of radiation-damaged zircon in lateritic soils

Zircon crystals from lateritic soils at Nsimi, Cameroon, were investigated using electron microprobe analysis (EMPA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy to determine the extent of radiation damage from alpha-decay events. The soils belong to a small watershed developed on granitic rocks of the Congo craton (2.9 Ga). Interactions with fluids are evidenced by significant CaO (up to 1.5 wt%), Al2O3 (up to 2.9 wt%), and Fe2O3 (up to 2.9 wt%) concentrations in UO2 rich regions (0.05 to 1 wt%) of the zircon. Regional heating up to 500 °C, related to the Pan-African orogeny about 0.6 Ga ago, has lead to the recrystallization of the radiation-damaged grains and the formation of a nanoporous microstructure. The correlation observed between the presence of dissolution features and the actual damage state of zircon shows that zircon dissolution occurs under tropical weathering conditions and with preferential dissolution of the highly radiation-damaged regions. Congruent dissolution of zircon and the limited mobility of Zr are supported by the absence of zirconium oxide precipitates in the fractures of weathered grains of zircon

Ion beam irradiation of U-, Th- and Ce-doped pyrochlores

U-, Th- and Ce-doped pyrochlores were synthesized by solid state reaction: (Ca 0.62Gd 0.97U 0.23)(Zr 0.84Ti 1.34)O 6.9, (Ca 0.47Gd 0.95Th 0.40)(Zr 1.29Ti 0.89)O 7.05, (Ca 0.44GdTh 0.42)Zr 2.13O 7.05, Ca 0.91Th 0.84Zr 2.25O 7.09 and Ca 1.04Ce 0.97Ti 1.99O 6.96. Their response to the radiation damage from the recoils of the alpha-decay events has been simulated by 1 MeV Kr 2+ ion irradiation and studied by in situ transmission electron microscopy (TEM). An ion beam-induced pyrochlore-to-fluorite structural transition occurred for all compositions, and independent kinetics were observed for cation and anion disordering processes. Similar to the stoichiometric titanate and zirconate pyrochlores, the Ti-rich compositions are susceptible to ion beam-induced amorphization; while, the Zr-rich compositions are more radiation “resistant”. A close relation between the critical amorphization temperature of the U-, Th- and Ce-doped pyrochlores and the average cation ionic radius ratio of the A- and B-sites was observed.

Determination of the uranium valence state in the brannerite structure using EELS, XPS, and EDX

In this study, the valence states of uranium in synthetic and natural brannerite samples were studied using a combination of transmission electron microscopy-electron energy loss spectroscopy, scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), and X-ray photoelectron spectroscopy (XPS) techniques. We used a set of five (UO2, CaUO4, SrCa2UO6, UTi2O6, and Y0.5U0.5Ti2O6) U standard samples, including two synthetic brannerites, to calibrate the EELS branching ratio, M5/(M4 +M5), against the number of f electrons. The EELS data were collected at liquid nitrogen temperature in order to minimise the effects of electron beam reduction of U6+ and U5+. Test samples consisted of three additional synthetic brannerites (Th0.7U0.3Ti2O6, Ca0.2U0.8Ti2O6, and Th0.55U0.3Ca0.15Ti2O6) and three natural brannerites from different localities. The natural brannerite samples are all completely amorphous, due to cumulative alpha decay events over geological time periods (24–508 Ma). Our U valence calibration results are in reasonable agreement with previous work, suggesting possibly a non-linear relationship between the branching ratio and the number of f electrons (and hence the average valence state) of U in solids. We found excellent agreement between the nominal valence states of U and the average valence states determined directly by EELS and estimated by EDX analysis (with assumptions regarding stoichiometry) in two of the three synthetic brannerite test samples. The average U oxidation states of the five synthetic brannerite samples, as derived from XPS analyses, are also in good agreement with those determined by other techniques. The average valence states of U in three amorphous (metamict) natural brannerite samples with alpha decay doses ranging from 3.6×1016 to 6.9×1017 α/mg were found to be 4.4, 4.7, and 4.8, consistent with the presence of U5+ and/or U6+ as well as U4+ in these samples. These results are in general agreement with previous wet chemical analyses of natural brannerite. However, the average valence states inferred by SEM-EDX for two of the natural brannerite samples do not show satisfactory agreement with the EELS determined valence. This may be due to the occurrence of OH− groups, cation vacancies, anion vacancies, or excess oxygen in the radiation-damaged structure of natural brannerite.

Thermally induced phase decomposition and nanocrystal formation in murataite ceramics

Synthetic murataite, an isometric derivative of the fluorite structure, has been proposed as a host phase for the immobilization of lanthanides and actinides. In this study, murataite polytypes have been synthesized in the system Ca–Ti–U–Mn–Al–Zr–Fe–Ce–O by melting oxide mixtures at 1400–1600 °C followed by crystallization under slightly oxidizing conditions. The phase and structural compositions of the synthetic murataite ceramics were characterized by energy dispersive spectroscopy and transmission electron microscopy (TEM). The thermal stability was studied by in-situ heat treatment under TEM observation. The influence of radiation damage as a result of alpha-decay events of the incorporated actinides on the thermal stability of the synthetic murataite ceramics was simulated by 1 MeV Kr+ irradiation. A thermally induced, Fe-rich phase decomposition and nanocrystal formation was observed in murataite ceramics under vacuum above 573 K. Ce was closely associated with the Fe-rich phase in the Ce-incorporated murataite ceramics. Ion beam irradiation-induced amorphization inhibited the process of phase decomposition. The effects of thermally induced phase decomposition on the radiation resistance of murataite ceramics utilized as potential nuclear waste forms for the immobilization of actinides and, particularly plutonium, were also analyzed.

Simultaneous determination of radionuclides separable into natural decay series by use of time-interval analysis.

A delayed coincidence method, time-interval analysis (TIA), has been applied to successive alpha- alpha decay events on the millisecond time-scale. Such decay events are part of the (220)Rn-->(216)Po ( T(1/2) 145 ms) (Th-series) and (219)Rn-->(215)Po ( T(1/2) 1.78 ms) (Ac-series). By using TIA in addition to measurement of (226)Ra (U-series) from alpha-spectrometry by liquid scintillation counting (LSC), two natural decay series could be identified and separated. The TIA detection efficiency was improved by using the pulse-shape discrimination technique (PSD) to reject beta-pulses, by solvent extraction of Ra combined with simple chemical separation, and by purging the scintillation solution with dry N(2) gas. The U- and Th-series together with the Ac-series were determined, respectively, from alpha spectra and TIA carried out immediately after Ra-extraction. Using the (221)Fr-->(217)At ( T(1/2) 32.3 ms) decay process as a tracer, overall yields were estimated from application of TIA to the (225)Ra (Np-decay series) at the time of maximum growth. The present method has proven useful for simultaneous determination of three radioactive decay series in environmental samples.

Radiation Effects in Crystalline Oxide Host Phases for the Immobilization of Actinides

ABSTRACTRadiation effects from alpha-decay events in crystalline oxides, which are proposed for the immobilization of actinides, often lead to amorphization, macroscopic swelling and order-of-magnitude increases in dissolution rates for all of the phases currently under consideration. However, the results of systematic experimental studies using short-lived actinides and ion-beam irradiations, studies of radiation effects in U- and Th-bearing minerals, and the development of new models of the damage process over the past 20 years have led to a substantial increase in the understanding of the processes of damage accumulation in apatite, zircon, perovskite, zirconolite, and pyrochlore/fluorite structures. This fundamental scientific understanding now provides a basis for predicting the performance of nuclear waste forms in a radiation field. One of the recent successes of these studies has been the discovery of a class of radiation-resistant pyrochlore/fluorite structures that can serve as highly durable, radiation-resistant host phases for the immobilization of actinides.

Ion irradiation of rare-earth- and yttrium-titanate-pyrochlores

Pyrochlore, A 1–2B 2O 6(O,OH,F) 0–1, is an actinide-bearing phase in Synroc, a polyphase ceramic proposed for the immobilization of high level nuclear waste. Structural damage due to alpha-decay events can significantly affect the chemical and physical stability of the nuclear waste form. Pyrochlore can effectively incorporate a variety of actinides into its structure. Four titanate pyrochlores were synthesized with compositions of Gd 2Ti 2O 7, Sm 2Ti 2O 7, Eu 2Ti 2O 7 and Y 2Ti 2O 2. These samples were irradiated with 1 MeV Kr + in order to simulate alpha-decay damage and were observed by in situ electron microscopy. Irradiations were conducted from 25 K to 1023 K. At room temperature, Gd-, Sm- and Eu-pyrochlores amorphized at a dose of ∼2×10 14 ions/cm 2 (∼0.5 dpa) and Y-pyrochlore amorphized at 4×10 14 ions/cm 2 (∼0.8 dpa). The amorphization dose became higher at elevated temperatures with different rates of increase for each composition. The critical temperatures for amorphization are ∼1100 K for Gd-, Sm-, Eu-pyrochlore and ∼780 K for Y-pyrochlore. The rare-earth-pyrochlores are more susceptible to amorphization and have higher critical temperatures than Y-pyrochlore. The difference in amorphization dose and critical temperature is attributed to the different cascade sizes caused by the different cation masses of the target. Based on a model of cascade quenching, the larger cascade is related to a lower amorphization dose and higher critical temperature. The irradiated materials were studied by electron diffraction and high-resolution electron microscopy. All the pyrochlores transformed to a fluorite substructure prior to the completion of amorphization of the observed regions. This transformation was caused by the disordering between cations and between oxygen and oxygen vacancies. The concurrence of cation disordering with amorphization suggests the partial recrystallization of the displacement cascades. Isolated cascade damage regions were observed by high-resolution electron microscopy, and the cation disordering was associated with the damaged regions.

Determination of naturalα-radionuclides related to millisecond order lives in environmental samples using the time interval analysis (TIA) method

The time files due to radioactive decay events were analyzed for the selective extraction of successive alpha-decay pulse events in millisecond orders, such as216Po (145 ms, thorium series),217At (32.3 ms, neptunium series), and215Po (1.78 ms, actinium series) after respective preceding parents decay, using a multiple time interval analysis (TIA-MTA) technique from both theoretical and experimental viewpoints. In the theoretical treatments, the detection sensitivities of each correlated event in natural decay series was compared with more generalized conditions. It was noteworthy that the evaluated detection sensitivity had increased proportionally with shorter half-lives in the following order:214Po>215Po>217At>216Po.

Rietveld Analysis of Phase Separation in Annealed and Leach Tested Cm-Doped Perovskite

AbstractA quantitative powder X-ray diffraction study was made of actinide doped perovskite of bulk composition Ca0.98919An0.01081Ti0.98919Al0.01081O3 where An corresponded to approximately equimolar proportions of Cm-244 and Pu-240. Sections of this sample accumulated irradiation doses up to 7.51 × 1017 alpha decay events per gram (± g−1). The damaged samples were treated in two ways. First, to establish the critical temperature for structural recovery under the reducing conditions of geological repositories, isochronal annealing was carried out at 600, 800, 1000 and 1100˚C for 12 hours in graphite crucibles. Two groups of perovskites that had previously sustained doses of 4.5 × 1017 and 7.4 × 1017 ± g−1 were tested in this way. For the former group, these conditions resulted in up to 9 weight percent (wt%) of available actinide separating as a fluorite-type dioxide near the perovskite surface. In the latter group, calcium was reduced to the metal which vaporised, leaving an excess of refractory titanium that crystalised as rutile. Second, material which had sustained doses of 1.6–4.0 × 1017 alpha decays per gram was subjected to an MCC-1 leach test for two months at 90˚C using a pH ∼ 2 solution. Under these conditions surficial perovskite dissolved congruently to release calcium into solution while the titanium reprecipitated as anatase. The implications of these results for the ultimate disposal of perovskite-bearing polyphase nuclear waste ceramics are considered.

The structure of aperiodic, metamict (Ca, Th)ZrTi2O7(zirconolite): An EXAFS study of the Zr, Th, and U sites

The structural environments of Zr, Th, and U in aperiodic (metamict) (Ca, Th)ZrTi2O7were examined using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Samples are aperiodic due to a radiation-induced transformation caused by alpha-decay event damage. In the aperiodic samples, Zr is mainly 7-coordinated [d(Zr−O) ≍ 2.14–2.17 ≍ 0.02 Å]; whereas, Th is mainly 8-coordinated [d(Th–O) ≍ 2.40−2.41 ≍ 0.03 Å]. Nearly identical bond lengths and coordination numbers for these elements were determined for an annealed, crystalline sample. The radiation-induced transition from the periodic to the aperiodic state is characterized by a significant broadening of the distribution of (Zr, Th)–O distances. In one metamict sample with ≍1.9 wt.% U3O8, U is essentially tetravalent. The absence of higher oxidation states (U6+) is consistent with the lack of evidence for alteration (samples are over 500 million years old). The reduced medium-range order around Zr, Th, and U is related to the increase of alpha-decay event damage and precludes decomposition of zirconolite into simple oxides of Zr, Th, or U. Comparison with other metamict (Zr, Th, U)-bearing phases (e.g., ZrSiO4and ThSiO4) suggests that Zr4+, Th4+, and U4+prefer 7-, 8-, and 6-coordinated sites, respectively, in aperiodic phases at ambient temperatures and pressures. Examination of the structure of crystalline (Ca, Th)ZrTi2O7demonstrates that M–O–M angles (M = Ca, Ti, Zr, and Th) are relatively small (≍100–120° for edge-sharing polyhedra). A limited relaxation of the constraints of periodicity around M cations caused by radiation damage (e.g., tilting of polyhedra) dramatically affects the distribution of these angles. This type of structural relaxation may be the mechanism by which long-range periodicity is lost and medium-range order is reduced with increasing radiation damage, while the major cations retain their nearest-neighbor environments. This relaxation may also help explain the lattice expansion observed in zirkelites when they undergo radiation damage.

Radiation-induced amorphization in complex silicates

The macroscopic swelling from alpha-decay events in Pu-doped ZrSiO4, natural ZrSiO4 minerals, and Cm-doped Ca2Nd8(SiO4)6O2 is interpreted in terms of both unit-cell expansion from defect accumulation and the volume change that occurs as the local crystal structure and long-range order collapse during amorphization. The derived amorphous fraction in ZrSiO4 as a function of cumulative alpha-decay events is consistent with models based on the multiple overlap of displacement cascades, suggesting that amorphization in ZrSiO4 occurs as a result of the local accumulation of high-defect concentrations. In Ca2Nd8(SiO4)6O2, on the other hand, the rate of amorphization is higher and the derived amorphous fraction as a function of cumulative alpha-decay events is consistent with amorphization occurring directly within the displacement cascade.

Amorphization of zirconolite: alpha-decay event damage versus krypton ion irradiation

Natural zirconolite (CaZrTi2O7) can receive α-decay event doses of > 1026 alpha-decay events/m3 (~2 dpa), and can thus become amorphous (metamict) due to the radioactive decay of 238U, 235U and 232Th and their daughter products which occur as substitutional impurities (the zirconolite contains approximately 20 wt% ThO2 and is 550 million years old). In this study, the zirconolite sample was recrystallized by thermal annealing at 1130°C for 8 hours in air. Amorphization was induced with 1.5 MeV Kr+ ions using the HVEM-Tandem Facility at Argonne National Laboratory. The Kr dose rate used during the irradiation was 3.4 × 1011 ions/cm2s. which is a damage rate 2 × 1012 times higher than that which has occurred in the natural sample due to decay of the actinides. In-situ transmission electron microscopy was completed during the ion irradiation. The recrystallized zirconolite also formed isolated thorianite (ThO2) crystals, 25–100 nm in size, in the matrix of the polycrystalline zirconolite. After a Kr+ dose of 4 × 1014 ions/cm2 (= 0.3 dpa). the zirconolite grains were reamorphized: however, the thorianite grains remained crystalline even after a Kr+ dose of 6 × 1014 ions/cm2.

Radiation-induced defects and amorphization in zircon

The effects of self-radiation damage as a function of cumulative alpha-decay events in synthetic zircon doped with 238Pu and natural zircons damaged over geologic time are compared and interpreted in terms of the accumulation of both defects and amorphousness. The radiation-induced unit-cell expansion and amorphization result in macroscopic swelling that increases sigmoidally with cumulative decay events and saturates at a fully amorphous state. The derived amorphous fraction as a function of cumulative dose is consistent with models based on the multiple overlap of displacement cascades, indicating that amorphization in zircon occurs as a result of the local accumulation of high defect concentrations rather than directly within a displacement cascade. Annealing of point defects in the natural zircons suppresses initial swelling and delays the onset of amorphization. Full recrystallization of the zircon structure from the amorphous state occurs in two stages, with kinetics and activation energies consistent with the reported thermal stability of the amorphous state. This study further confirms that actinide doping is a viable accelerated technique to study or simulate radiation effects from alpha decay on geologic time scales.

Preferential leaching and natural annealing of alpha-recoil tracks in metamict betaflte and samarskite

Leaching experiments on naturally occurring, metamict betafite and samarskite minerals in a bicarbonate-carbonate solution show strongly enhanced release to the solution of short-lived 228Th relative to its parent isotope 232Th (by a factor of 3 to 6), but only slightly enhanced dissolution (by a factor of 1.2 to 2) of long-lived 234U and 230Th relative to 238U and 232Th, respectively. The betafite (a complex Ca–U–Ti–Nb oxide of the pyrochlore group, A2−mB2X6Y0−1.H2O) and samarskite (a complex Y–Fe–U–Nb oxide with varying stoichiometry between AB O4 and AB2O6) are x-ray and electron diffraction amorphous having experienced doses of 2.2–2.8 × 1017 alpha-decay events/mg (24–31 dpa) and 3.8–4.4 × 1017 alpha-decay events/mg (39–45 dpa), respectively. The isotopic fractionation is attributed to the radiation damage created by the alpha-decay events. Individual alpha-recoil tracks are preserved for some time as disordered regions of higher chemical reactivity in already fully damaged, aperiodic areas that result from the alpha-decay events. The annealing time of the alpha-recoil track within the aperiodic atomic array of the metamict state is calculated to be 29 300 ± 8100 years for samarskite and 42 700 ± 13 700 years for the betafite. These data plus data on an earlier studied betafite sample (annealing time = 2000 ± 1300 years) give an average annealing time of 25 000 ± 21 000 years. The variation in calculated annealing time is due, in part, to post-metamict alteration of the sample, particularly for the betafite. The wider range of values for the betafite is attributed to its greater degree of alteration. These results demonstrate that recoil nuclei of alpha-decay events may be selectively leached from damaged, aperiodic phases, but that these tracks are subject to low-temperature annealing in short periods of time relative to the age of the samples (∼ 500 m.y.). The same phenomena are expected in actinide-containing nuclear waste form glasses.

Half-lives ofRn199andRnm199

The nuclide $^{199}\mathrm{Rn}$ and the isomer $^{199}\mathrm{Rn}^{m}$ were produced in the reaction $^{169}\mathrm{Tm}(^{35}\mathrm{Cl},5n)^{199}\mathrm{Rn}$. An electromagnetic velocity filter separated the recoiling residual nuclei from the beam at zero degree. The nuclei were embedded into a surface-barrier detector recording both the arrival pulse and the alpha-decay event. The half-life for $^{199}\mathrm{Rn}$ in its presumed ground state (${E}_{a}=6.99$ MeV) has been determined as 0.5 \ifmmode\pm\else\textpm\fi{} 0.2 sec and the half-life of $^{199}\mathrm{Rm}^{m}$ (${E}_{a}=7.06$ MeV) is 0.29 \ifmmode\pm\else\textpm\fi{} 0.05 sec.[RADIOACTIVITY $^{199}\mathrm{Rn}$ and $^{199}\mathrm{Rn}^{m}$; measured half-lives.]