252Cf Fission Fragments Research Articles

An experimental and theoretical description of the (NH 3) n−1{NH 3–H–H 2O} + cluster ions produced by fast ion bombardment

The (NH 3) n −1{NH 3–H–H 2O} + cluster series produced by 252Cf fission fragments (FF) impact onto a NH 3–H 2O ice target have been analyzed theoretically and experimentally. The ion desorption yields show a smooth dependence of the cluster population on its mass, with a relative higher abundance for n = 3. Results of DFT calculations show that two main series of cluster ions may be formed corresponding to the {H 3O} + and {NH 4H 2O} + cores. The theoretical analysis shows that the members of the (NH 3) n −1{NH 4–H 2O} + series are more stable than those of the (NH 3) n {H 3O} + series, the (NH 3) 2{NH 3–H–H 2O} + structure being the most stable one, in agreement with the experiments.

Frozen methanol bombarded by energetic particles: Relevance to solid state astrochemistry

Methanol is an important precursor of many complex prebiotic species and is found abundantly, in solid phase, in several astrophysical environments, such as comets and protostars. These environments are, in general, subjected to some form of ionizing particles as cosmic rays, solar wind particles and/or photons. To simulate physico chemical effects of cosmic rays on methanol and to investigate in particular its fragmentation and cluster emission from the solid phase, condensed methanol at 55 K was bombarded with ∼65 MeV heavy ions constituted by 252Cf fission fragments. Mass spectra of positive desorbed ions were obtained using a time-of-flight Plasma Desorption Mass Spectrometry (PDMS-TOF), giving information on the fragmentation pattern and abundance of the ionic species released from the frozen sample surface. Since CH 3 OH 2 + and CH 3O + are the most abundant species detected, the hydrogenation and de-hydrogenation of methanol result to be the predominant ionic processes in the methanol surface. Furthermore, the hydrogenation yields emission of the (CH 3OH) n CH 3 OH 2 + cluster series, while the CH 3O + species does not attach to methanol molecules. The production of other cluster ion series, some of them with mass/charge up to 300 u/e was also analyzed by PDMS.

Extended collective bands in neutron-rich 109Ru

Levels in the neutron-rich 109Ru have been studied by observing the prompt γ-rays following the spontaneous fission fragments of 252Cf. The ground state band and the negative parity bands have been confirmed and extended. A positive parity band with the band head level at 332.5 keV is newly identified and suggested as a single-neutron excitation band built on the 7/2+[404] Nilsson orbital. Some structural characteristics of these bands are discussed.

A Theoretical and Experimental Study of Positive and Neutral LiF Clusters Produced by Fast Ion Impact on a Polycrystalline LiF Target

The positive and neutral clusters produced by the impact of approximately 60 MeV (252)Cf fission fragments on a LiF polycrystalline target are analyzed. The positive ion spectrum is dominated by the (LiF)(n)Li(+) series, n = 0-7, exhibiting a total yield 2 orders of magnitude higher than that of the (LiF)(n)(+) series. The yield for the dominant (LiF)(n)Li(+) series decreases roughly as exp(-kn), where k approximately 0.9 for n = 0-3 and k approximately 0.6 for the heavier clusters (n = 4-9), while the yield of the (LiF)(n)(+) series also decreases exponentially as n increases with k approximately 0.6. Theoretical calculations were performed for the (LiF)(n)Li(0), (LiF)(n)Li(+), and (LiF)(n)(0) series for n up to 9. For the smaller clusters the structures first obtained with a genetic algorithm generator were further optimized at the DFT/B3LYP/6-311+G(3df), DFT/B3LYP/LACV3P*, and MP2/LACV3P* levels of theory. An energy criterion is used for a proper taxonomic description of the optimized cluster isomers. Cluster properties such as fragmentation energy and stability are discussed for the proposed configurations. The results show that for all three series the most stable isomers present a linear structure for small cluster size (n = 1-3), while cubic cells or polyhedral structures are preferred for larger cluster sizes (n = 4-9). Fragmentation energy results suggest that a desorbed excited (LiF)(n)Li(+) ion preferentially dissociates via a cascade of (LiF)(n)(0) units, in agreement with the slope modification in the exponential decay of the (LiF)(n)Li(+) ion abundances for n > or = 3.

Theoretical Investigation on the Stability of Ionic Formic Acid Clusters

Recent experimental results on positive charged formic acid clusters generated by the impact of (252)Cf fission fragments (FF) on icy formic acid target are examined in this paper by quantum mechanical calculations. Structures for the clusters series, (HCOOH)(n)H(+) and (HCOOH)(n)H(3)O(+), where 2 < or = n < or = 4, are proposed based on ab initio electronic structure methods. Results show that cluster growth does not present a regular pattern of nucleation. A stability analysis was performed considering the commonly defined stability function, where E is the total electronic energy plus the zero point vibrational energy correction, including the BSSE correction. The stability analysis leads to a picture that is compatible with experimental observations, indicating a decay of the stability with the increase of cluster mass. Temporal behavior of the clusters was evaluated by Born-Oppenheimer molecular dynamics to check the mechanism that provides cluster stability. The evaluated temporal profiles indicate the importance of hydrogen atom migration between the formic acid moieties to maintain the stability of the structures.

Cluster emission and chemical reactions in oxygen and nitrogen ices induced by fast heavy‐ion impact

Two ices, O2 and a mixture of O2 and N2, are bombarded by 252Cf fission fragments (FF) (approximately 65 MeV at target surface); the emitted positive and negative secondary ions are analyzed by time-of-flight mass spectrometry (TOF-SIMS). These studies shall enlighten sputtering from planetary and interstellar ices. Three temperature regions in the 28-42-K range are analyzed: (1) before N2 sublimation, in which hybrid chemical species are formed, (2) before O2 sublimation, in which the TOF mass spectrum is dominated by low-mass (O2)p cluster ions and (3) after O2 sublimation, in which (N2)p or (O2)p cluster ions are practically inexistent. In the first region, four hybrid ion series are observed: NOn-1+, N2On-2(+/-), and N4On-4(-). In the second region, two positive and negative ion series are identified: (O2)pO(+/-) and (O2)pO2(+/-). Their yield distributions are fitted by the sum of two decreasing exponentials, whose decay constants are the same for all series. It is observed that the cluster ion desorption from solid oxygen is very similar to that of other frozen gases, but its yield distribution oscillates with a three- or six-atom periodicity, suggesting O3 or 3O2 units in the cluster structure, respectively.

Novel polysulphonates and poly(sulphonate-co-carbonate) polymers for nuclear track detection

Preparation of homopolymer from diethylene glycol bis(allyl sulphonate) (DEAS) and its several copolymers with allyl diglycol carbonate (ADC) and their preliminary evaluation as nuclear track detector is described. The monomer is synthesized by reacting diethylene glycol with allyl sulphonyl chloride in presence of pyridine as a base. Spectral characteristics of the monomer are also given for the first time. The designed polymers were successfully tested for detection of 239Pu alpha particles and 252Cf fission fragment tracks after etching with 6 N NaOH at 70 °C. Some of the polymers prepared showed even better sensitivity than (SR-86)(20) track detectors for track detection properties. The polymers are also compared with commercial CR-39 track detectors for its track detection properties.

Secondary ion emission induced by fission fragment impact in CONH3 and CONH3H2O ices: modification in the CONH3 ice structure

CO-NH(3) and CO-NH(3)-H(2)O ices at 25-130 K were bombarded by (252)Cf fission fragments ( approximately 65 MeV at the target surface) and the emitted secondary ions were analyzed by time-of-flight mass spectrometry (TOF-SIMS). It is observed that the mass spectra obtained from both ices have similar patterns. The production of hybrid ions (formed from CO and NH(3) molecules) emitted from CO-NH(3) ice has already been reported by R. Martinez et al., Int. J. Mass. Spectrom. 262 (2006) 195; here, the secondary ion emission and the modifications of the CO--NH(3) ice structure during the temperature increase of the ice are addressed. These studies are expected to throw light on the sputtering from planetary and interstellar ices and the possible formation of new organic molecules in CO-NH(3)-H(2)O ice by megaelectronvolt ion bombardment. The presence of water in the CO-NH(3) ice mixture generates molecular ion series such as (NH(3))(p-q)(H(2)O)(q)CO(+) and replaces the cluster series (NH(3))(n)NH(4) (+) emission by the hybrid series (NH(3))(I-i)(H(2)O)(i=1, 2...I)H(+). The distribution of NH(3) and H(2)O molecules within the cluster groups indicates that ammonia and water mix homogeneously in the icy condensate at T = 25 K. The desorption yield distribution of the cluster series (NH(3))(n)NH(4) (+) is described by the sum of two exponential functions: one, slow-decreasing, attributed to the fragmentation of the solid target into clusters; and another, fast-decreasing, due to a local sublimation followed by recombination of ammonia molecules. The analysis of the time-temperature dependence of these two yield components gives information on the formation process of molecular ions, the transient composition of the ice target and structural changes of the ice. Data suggest that the amorphous and porous structure of the NH(3) ice, formed by the condensation of the CO--NH(3) gas at T = 25 K, survives CO sublimation until the occurrence of a phase transition around 80 K, which produces a more fragile ice structure.

Characterization of [formula omitted] clusters produced by 252Cf fission fragment impact on a N 2 ice target

The structures and abundances of N n = 2 - 18 + clusters produced by impact of 252Cf fission fragments (FF) on N 2 ice target are analyzed. A systematic search for the more stable conformers at the DFT/B3LYP level and a recently proposed methodology for a proper taxonomic description ( D-plot) of the cluster isomers in series are presented. Several new members of the previously reported linear series have been identified. Besides that, three new series of clusters and some other stable structures have also been found. The clusters stability and ionization potential analysis showed that the 252Cf FF exponential dependence of the cluster yield with its mass is rather a consequence of the production mechanism than that of the cluster structure.

Characterization of (NH3)n=1-6NH+ Clusters Produced by 252Cf Fragments Impact onto a NH3 Condensed Target

This paper reports the first characterization of the (NH(3))(n)NH+ cluster series produced by a 252Cf fission fragments (FF) impact onto a NH(3) ice target. The (NH(3))(n=1-6)NH+ members of this series have been analyzed theoretically and experimentally. Their ion desorption yields show an exponential dependence of the cluster population on its mass, presenting a relative higher abundance at n = 5. The results of DFT/B3LYP calculations show that two main series of ammonium clusters may be formed. Both series follow a clear pattern: each additional NH(3) group makes a new hydrogen bond with one of the hydrogen atoms of the respective {NH(3)NH}+ and {NH(2)NH(2)}+ cores. The energy analysis (i.e., D-plot and stability analysis) shows that the calculated members of the (NH(3))(n-1){NH(2)NH(2)}+ series are more stable than those of the (NH(3))(n-1){NH(3)NH}+ series. The trend on the relative stability of the members of more stable series, (NH(3))(n-1){NH(2)NH(2)}+, shows excellent agreement with the experimental distribution of cluster abundances. In particular, the (NH(3))4{NH(2)NH(2)}+ structure is the most stable one, in agreement with the experiments.

Radioactive beams from 252Cf fission using a gas catcher and an ECR charge breeder at ATLAS

The Californium Rare Ion Breeder Upgrade (CARIBU) for the ATLAS facility is under construction. The facility will use 252Cf fission fragments thermalized and collected into a low-energy particle beam by a helium gas catcher. In order to reaccelerate these beams an existing ATLAS ECR ion source is being reconfigured as a charge breeder source. A 1Ci 252Cf source will provide sufficient yield to deliver beams of up to ∼106 far-from-stability ions per second on target. The facility design, expected performance and the project status will be presented in this paper.

Theoretical and Experimental Analysis of Ammonia Ionic Clusters Produced by 252Cf Fragment Impact on an NH3 Ice Target

Positive and negatively charged ammonia clusters produced by the impact of (252)Cf fission fragments (FF) on an NH(3) ice target have been examined theoretical and experimentally. The ammonia clusters generated by (252)Cf FF show an exponential dependence of the cluster population on its mass, and the desorption yields for the positive (NH(3))(n)NH(4)(+) clusters are 1 order of magnitude higher than those for the negative (NH(3))(n)NH(2)(-) clusters. The experimental population analysis of (NH(3))(n)NH(4)(+) (n = 0-18) and (NH(3))(n)NH(2)(-) (n = 0-8) cluster series show a special stability at n = 4 and 16 and n = 2, 4, and 6, respectively. DFT/B3LYP calculations of the (NH(3))(0)(-)(8)NH(4)(+) clusters show that the structures of the more stable conformers follow a clear pattern: each additional NH(3) group makes a new hydrogen bond with one of the hydrogen atoms of an NH(3) unit already bound to the NH(4)(+) core. For the (NH(3))(0)(-)(8)NH(2)(-) clusters, the DFT/B3LYP calculations show that, within the calculation error, the more stable conformers follow a clear pattern for n = 1-6: each additional NH(3) group makes a new hydrogen bond to the NH(2)(-) core. For n = 7 and 8, the additional NH(3) groups bind to other NH(3) groups, probably because of the saturation of the NH(2)(-) core. Similar results were obtained at the MP2 level of calculation. A stability analysis was performed using the commonly defined stability function E(n)(-)(1) + E(n)(+1) - 2E(n), where E is the total energy of the cluster, including the zero point correction energy (E = E(t) + ZPE). The trend on the relative stability of the clusters presents an excellent agreement with the distribution of experimental cluster abundances. Moreover, the stability analysis predicts that the (NH(3))(4)NH(4)(+) and the even negative clusters [(NH(3))(n)NH(2)(-), n = 2, 4, and 6] should be the most stable ones, in perfect agreement with the experimental results.

Energy losses of 252Cf fission fragments in thin foils

The stopping power of fission fragments from 252Cf(sf) has been measured in thin foils of C, Ni, Au, Al 2O 3 and Mylar as a function of fragment mass and energy. The chosen stopping materials are frequently used as the backing of sources and targets or entrance windows of gas detectors. The experimental setup allowed registration of velocities of two fission fragments and energy of one of them. In this way fragment masses could be unambiguously determined and the pulse-height defect of the silicon detector verified. The measured stopping power data for fission fragments cover masses between A = 101 and A = 148. The results are compared with semi-empirical predictions of SRIM 2003 code, LSS theory and theoretical calculation by Sigmund (PASS code). The best representations of the data are achieved by a semi-empirical formula based on classical Bohr theory. The formula was successfully tested also on the available data for Ar, Ca, Kr, Xe and Au ions in the energy range 0.2–6 MeV/u slowing down in carbon, nickel and gold.

Electronic Sputtering Produced by Fission Fragments on Condensed CO and CO 2

Condensed CO and CO2 are bombarded by approximately 65 MeV 252Cf fission fragments and the desorbed ions are analyzed by time-of-flight mass spectrometry as a function of target temperature, in the ranges 25-33 K and 75-91 K, respectively. Absolute desorption yields are measured up to complete ice sublimation. The mass spectra of both ice targets reveal the emission of: (1) low mass ions, produced by direct Coulomb interaction of the highly charged projectiles and delta-electrons with CO and CO2, and (2) pronounced series of cluster ions. The basic ice cluster structures (CO)n and (CO2)n are present in the emitted cluster series such as (CO)nCO+, (CO2)nCO2+, or (CO2)nCO3-. In the case of CO ice, however, the intense production of the series Cn+, Cn-, and (CO)mCn+ shows that Cn is the main cluster structure, consequence of a higher concentration of free carbon atoms in the nuclear track plasma of CO ice than in that of CO2 ice. Ion cluster abundance is observed to decrease exponentially with cluster mass. The decay constant is k(n) congruent with 0.13, about the same for series based on (CO)n and (CO2)n, but a factor 3.3 higher for the Cn series. The Cn clusters are formed by gas-phase condensation, but the (CO)n and (CO2)n clusters are produced by fracturing of the highly excited solid around the nuclear track. A dramatic reduction of the ion desorption yield is observed near T = 29 K for CO and near T = 85 K for CO2, when fast sublimation occurs and ice thickness vanishes. Close to sublimation temperature, the decay constant of the (CO)2Cn+ series increases due to a decreasing formation probability of large Cn clusters.

Secondary ion statistics and determination of nanocluster ( m > 10 7 amu) ion registration efficiency

The m/ q spectra of gold nanocluster ions (NCI) (〈 D〉 = 18.1 nm and 〈 m〉 = 4.6 × 10 7 amu) desorbed from nanodispersed targets with 252Cf fission fragments were measured using a tandem time-of-flight mass spectrometer at acceleration of 38 kV. In parallel, the statistics of secondary ions (SI) emitted with NCI from various converters (Au and CsI) were measured and the registration efficiency (RE) of NCI was determined. It was shown that distribution of SI number is most adequately described by the geometric distribution. The resulting values of RE are less by a factor of 1.5–2 than those predicted by the Poisson distribution for the m/ q range from 3.1 × 10 6 to 8.2 × 10 6 amu/e.

Ion cluster desorption from frozen NH 3 induced by impact of fast multi-charged ions

Ion cluster desorption yields due to the impact of 252Cf fission fragments on condensed ammonia are determined. Identification and abundance of the ion desorbed species were provided by a time-of-flight mass spectrometer. Ammonia ice thin layers deposited on Au film were heated from 25 K to complete sublimation. The typical pressure during heating process was 10 −8 mbar. Measurements in the 25–150 K temperature range show emission of (NH 3) n NH m ± ion clusters, where n varies from 0 to ∼30 and m varies from 0 to 3 (for negative ions) or to 5 (for positive ions). The distribution is described by the sum of two decaying exponentials when n increases, which is interpreted by the simultaneous occurrence of two cluster formation regimes: (i) recondensation of sublimated molecules from the projectile track and (ii) emission of pre-formed bulk fragments from the periphery of the impact site. Moreover, the cluster yields of positive and negative ion series are approximately proportional to each other, suggesting that the ion cluster distribution shape is dictated by the neutral molecules but absolute yield is controlled by the emission of the ion species. In the 125–140 K range, sublimation rate increases, the ice thickness decreases rapidly and all the mentioned cluster series tend to disappear. The series (HCN) n CN − then appears, originating in the clustering of residual hydrocarbon contaminants on the metal substrate surface. The residual gases are analyzed as a function of the NH 3 ice target temperature and their partial pressure compared to the ion desorption yield.

Characterization of [formula omitted] clusters produced by electronic sputtering

The structures and abundances of C n = 2 – 16 + clusters produced by electronic sputtering from the impact of 252Cf fission fragments on CO ice and from the UV laser desorption of amorphous carbon and graphite targets are analyzed. A systematic search for the more stable conformers at the DFT/B3LYP level of calculation and their contribution to the mass spectra is reported. A new methodology for a proper taxonomic description of the cluster isomers is proposed. Besides the known linear and monocyclic clusters series, new members of these series and two new series are described: the bi-cyclic and/or tri-cyclic and the rhombic core series.

A source of liquid drop metal nanocluster beams: Construction and application

The principle of operation and design of the source of liquid drop metal nanocluster ions (NCI) based on the desorption effect of islet-grains from nanodispersed targets under bombardment by multi-charged ions– 252Cf fission fragments are described. The NCI source allows for obtaining nanocluster ion beams with the energy 50 kV ⁎ q ( q = 1–20e) with the intensity up to ∼10 5 s − 1 . The results of the first experiments on desorption of biomolecules over the range up to 7 ⁎ 10 3 Da under the action of accelerated NCI of gold as well as studies of the changes of the NCI morphology and the surface itself under bombardment thereof by gold NCI are given.

Electronic Sputtering Analysis of Astrophysical Ices

Experimental results on fast ion collision with icy surfaces having astrophysical interest are presented. 252Cf fission fragments projectiles were used to induce ejection of ionized material from H2O, CO2, CO, NH3, N2, O2 and Ar ices; the secondary ions were identified by time-of-flight mass spectrometry. It is observed that all the bombarded frozen gas targets emit cluster ions which have the structure XnR±, where X is the neutral ice molecule and R± is either an atomic or a molecular ion. The shape of the positive or negative ion mass spectra is characterized by a decreasing yield as the emitted ion mass increases and is generally described by the sum of two exponential functions. The positive ion water ice spectrum is dominated by the series (H2O)nH3O+ and the negative ion spectrum by the series (H2O)nOH− and (H2O)nO−. The positive ion CO2 ice spectrum is characterized by R+ = C+, O+, CO+, O2+ or CO2+ and the negative one by R− = CO3−. The dominant series for ammonia ice correspond to R+ = NH4+ and to R− = NH2−. The oxygen series are better described by (O3)nOm+ secondary ions where m = 1, 2 or 3. Two positive ion series exist for N2 ice: (N2)nN2+ and (N2)nN+. For argon positive secondary ions, only the (Ar)nAr+ series was observed. Most of the detected molecular ions were formed by one-step reactions. Ice temperature was varied from ∼20 K to complete sublimation.

Secondary ion emission from CO 2–H 2O ice irradiated by energetic heavy ions : Part II: Analysis–search for organic ions

Secondary ion mass spectrometry is used to investigate ion emission from a frozen-gas mixture of CO 2 and H 2O ( T = 80–90 K) bombarded by MeV nitrogen ions and by 252Cf fission fragments. The aim of the experiment is to detect organic molecules, produced in the highly excited material around the nuclear track, which appear as ions in the flux of sputtered particles. Part I of the present work [L.S. Farenzena, V.M. Collado, C.R Ponciano, E.F. da Silveira, K. Wien. Int. J. Mass Spectrom. 243 (2005) 85–93] described the method and presented the time-of-flight mass spectra; a list of the CO 2 specific and H 2O specific reaction products was provided. In Part II, the peaks of the TOF mass spectra are analyzed. Projectile-ice direct coulomb interaction leads to the production in the track of the H +, C +, O +, O 2 +, CO + and CO 2 + primarily ions, which react in the highly energized nuclear track plasma mainly with CO 2 and H 2O or H 2CO 3. The positive molecular hybrid ions formed are identified as organic species like COH +, COOH +, CH n = 1–3 +, H n = 1,2 COOH + and cluster ions. Similarly, the negative primarily ions O − and OH − formed by electron capture produce negative hybrid ions such as (CO 2) n OH −, the four ions (CO 4H m = 0–3 ) − and the corresponding cluster ions. By far, most of the molecular ions have been formed by one-step reactions; exceptions are eventually the CO 4H m − ions created by a reaction between CO 3 − and water molecules. An intense mass line corresponding to HCO 3 + has been observed, but not the one due to formaldehyde ion. Weak signals of ionic ketene, hydrogen peroxide and carbonic acid were seen.