Theoretical x-ray transition probabilities for high-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Z</mml:mi></mml:math>superheavy elements

Anholt, R.; Rasmussen, J. O.

doi:10.1103/physreva.9.585

Cited by 47 publications

(9 citation statements)

References 16 publications

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“…Considering the above, we will use an estimate of P~-0.7%; to which we estimate an uncertainty of a factor of two. On the other hand, there is a rather sxnall uncertainty in the the lifetime of Yb Kvacancies: r = 20x10 is s [12], a value corroborated by the intrinsic line widths [13]. In the present experiinent, one would expect this time to increase an insignificant amount due to multiple ionization.…”

supporting

confidence: 48%

Estimation of the time scale of last chance alpha emission using an ‘‘atomic clock’’

et al. 1994

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The probability of Slling a K-vacancy, created on the incoming part of the coQision, before nparticle emission is used to time the decay of Yb compound nuclei. These nuclei were produced in the fusion reaction 250 MeV Ni + Mo. In general the nuclear decays are too fast to be timed by this clock, however, n-particle emitting compound nucleus states have lifetimes sufBciently long for this technique to work when both the a-particle has low energy and the compound nucleus spin is large. This supports the existence of last, or near last, chance o;-particle emission in the deexcitation of high spin compound nuclei.PACS number(s): 24.60. Dr, 25.70.Gh Thirty years ago Gugelot [1] pointed out that one can calibrate the decay width (I', ) of a compound nucleus in terms of the K x-ray width (I' ) for atomic vacancies. All one has to do is prepare an atoxn with an excited nucleus and a K vacancy and then measure the branching ratio I' /I', . In practice, there are several problems in applying this idea. The K-vacancy production probabilities P~are low (of the order of 1%) for typical low energy fusion reactions and, in contrast to atomic collisions below the Coulomb barrier, the x rays of interest must be detected in a large background resulting &om internal conversion x rays and p rays emitted by excited nuclear reaction products. Another important issue is photon production from the filling of vacancies in a charged particle exit channel during the time after exnission but before the emitted particle is well outside the K-shell Bohr radius (molecular orbital emission). Due to these problems, this old idea has met with success only recently [2][3][4].This technique is best suited to the study of the lifetimes of last or near last chance decays of compound systems in the bottom portion (high charge) of the periodic table. Only under these conditions does the K-vacancy lifetime approach the compound nuclear lifetime. Two interesting topics for which this technique is quite promising are: (1) the study of slow fission components and (2) slow "yrast" a-particle emission. The former involves actinide nuclei for which the K-vacancy lifetime is less than 10~s, and the latter, heavy rare earth nuclei with Kvacancy lifetixnes between 10 and 10 s. The recent paper by Molitoris et al. [4] addresses the first of these topics and this work deals with the second.Statistical model codes predict that the emission of a particles competes with p-ray emission at very low excitation energies when the nuclei are trapped near the yrast line at high to moderate spins [5,6]. Grover and Gilat first discussed this process in work published 25 years ago [5]. These early calculations, as well as more recent ones [7], predict that these "stretched" and "last chance" o. particles would be emitted &om states with lifetimes similar to the K-vacancy lifetime and therefore present an ideal application of the atoxnic clock xnethod. The predicted a-particle energies are low for these "last chance" particle emissions which, despite the angular moxnentum re...

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confidence: 48%

Estimation of the time scale of last chance alpha emission using an ‘‘atomic clock’’

et al. 1994

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“…The number of created K-vacancies per collision with an impact parameter b ~< 20 fm is typically of the order of 10~o [21,24,25]. The transition time (v-decay) of an electron from a higher shell to the K-shell is about 10-17s [26]. This has to be compared with nuclear transition times of about 10-13s, after which conversion takes place.…”

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confidence: 98%

Internal electron-positron pair production from electric monopole transitions

Soff¹,

Schlüter

Greiner

1981

Z Physik A

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Nuclear E0-transitions above the pair production threshold of co--2rn c2,-, 1 MeV are considered. The ration r/of the electron-positron pair creation probability to the K-shell conversion probability is calculated. For 92U and co= 1,450keV a typical value of t/ =2,10 .3 is obtained. Good agreement with experimental data is found. Our numerical results for finite size nuclei are compared with analytical expressions utilizing relativistic electron wavefunctions for point nuclei. The energy distributions of emitted positrons are presented for various nuclear charge values and transition energies. The possibility of monoenergetic positron emission is discussed. Of particular interest is the relation of the theoretical results to the recently observed structure in the positron spectra, which have been measured in high energy collisions of very heavy ions.

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“…For EL or ML transitions the bound is x>0.1 approximately and x>0.01 for E0 transitions. It is highly questionable whether this is realistic since a K-hole in an Uranium atom has a lifetime due to radiative transitions [31] of about 10 -17 s, if there are any electrons in higher bound states present while a typical nuclear transition time is 10 -12 s. This ratio should suppress the probability of monoenergetic positron formation by five orders of magnitude. Even for a totally empty K-shell (x=2) we have to assume a rather big fraction of excited nuclei, similar to those of case (a), for which we have no obvious explanation.…”

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confidence: 94%

Can the observed structures in positron spectra be caused by internal conversion?

et al. 1983

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The energy distribution of positrons emitted in quasimolecular collisions of Uranium on Uranium and Uranium on Curium has been measured by several groups. Peak structures in the positron spectra were observed. We discuss the possibility that these structures originate from internal conversion processes following nuclear Coulomb excitation or transfer reactions. Consequences for the nuclear photon spectra and the 3-ray distribution are pointed out and experimental procedures for an unambiguous determination of the significance of conversion processes are discussed.The spectra of emitted positrons produced in quasimolecular collisions of very heavy ions were subject of widespread experimental investigations [1][2][3][4][5][6][7][8][9][10][11][12] during the last years. Of particular interest are encounters with combined nuclear charges of Z 1 +Z2>173 and bombarding energies close to the nuclear Coulomb barrier (El, b-~ 5.9 MeV/u). In these colliding systems supercritical electric fields are created for a short period of ~10 -21 s, which may lead to spontaneous positron production provided a partial depletion of the K-shell has occured on the incoming path of the trajectory. A more profound discussion of this peculiar process is presented in [13] and in the references therein. The energy distribution of positrons measured [2][3][4][5][6][7][8] in 92U-92 U and 92U-96Cm collisions for particular kinematical conditions displays a distinct peak structure at a kinetic positron energy of E~in-~0.63-~320keV (natural units h=c=m=l are used) with a width of about 0.14_~70keV. Depending on the ion scattering angle and on the projectile energy further, but less pronounced structures appear at higher positron energies. As an example we relate our following inferences to the positron distribution detected [2] in U+U collisions at Elau~--5.73 MeV/u and for ion scattering angles between 38 ~ and 52 ~ . Two different explanations for the physical origin of these structures were discussed. According to the suggestion of J. Reinhardt et al. [13][14][15][16] a small fraction of about 5.10 .3 of the scattered ions within the given experimental angular window originate not from Rutherford collisions but from "sticking" reactions. In these reactions a giant nuclear system with Z= 184 is assumed to be formed with a lifetime that is slightly smaller than the characteristic time scale for spontaneous positron creation of T~10 -19 s. As the calculations in [14] have shown this causes a striking "spontaneous peak" in the positron spectra in fair agreement with the measured data [1][2][3][4][5][6][7][8]. The experimental verification of these ideas, indeed, would prove for the first time the spontaneous decay of the vacuum and simultaneously the existence of superheavy nuclear quasimolecules and thus would have far-reaching consequences. Alternatively, it was argued that internal pair creation following nuclear Coulomb excitation or transfer reactions might be responsible for peaks in the energy distribution of created positrons (cf. also [17,

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Theoretical x-ray transition probabilities for high-Zsuperheavy elements

Cited by 47 publications

References 16 publications

Estimation of the time scale of last chance alpha emission using an ‘‘atomic clock’’

Estimation of the time scale of last chance alpha emission using an ‘‘atomic clock’’

Internal electron-positron pair production from electric monopole transitions

Can the observed structures in positron spectra be caused by internal conversion?

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