Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

First dedicated in-beam X-ray measurement at GARIS
C. Berner,∗1,∗3 H. Baba,∗3 R. Gernhäuser,∗1 S. Hellgartner,∗1 W. Henning,∗2,∗3 D. Kaji,∗3 R. Lutter,∗4
L. Maier,∗1 K. Morimoto,∗3 K. Morita,∗3 D. Mücher,∗1 and Y. Wakabayashi∗3

∗1
∗2
∗3
∗4a)

(@Ex = 74 keV) at a trigger rate of 133 kcps. Additionally, neutron damage was measured to be negligible
due to the thin and planar structure of the detector:Analysis of the neutron edges in the spectra showeda minimum detector lifetime of more than 40 days at
10full beam
intensity before any visible neutron damage
4,5would
influence the measurement.
10A
11Henning,
et al.
4,532( Si, S)
Henning,et al.
Chmielewska,
et al.
2832( Si,(12S)C, α)
Chmielewska,
et al.
Karwowski,et al.
12) 7Li)( C, (α6Li,Karwowski,et al. et al.
Deconninck,67α)( Li, (Li)
Henning,etetal.al.
Deconninck,(α) (48Ca)
Henning,al.
MLLet- expected48( Ca)MLL - 132Ce133
MLLMLL- expected- Ce132
MLL (-16O)Ce133
MLLA-4,5- Cedependency
16( O)187-xAu* (48Ti)
4,5A - dependency
250 * 48 300
350
187-x
Au ( Ti)

A 28

0

100

<MK>

10<MK>We report on an experiment aiming at in-beam Xray spectroscopy of heavy and superheavy elements(SHE). The goal is to establish K-X-ray spectroscopy
as a sensitive tool to identify SHE produced in fusion
reactions. SHE are usually identified via the alphadecay products, which have to be connected to wellknown elements. However, various theories predict
spontaneous fission as the dominant decay mode forthe daughter nuclides. Additionally, half-lives of theseelements are expected to increase to values1,2) impeding the identification of SHE solely by their decay.
The in-beam identification of the characteristic X-rays
would independently allow to identify the charge number of the produced SHE.
We performed dedicated experiments for in-beam Xray recoil-decay-tagging spectroscopy at GARIS in order to study the dependence of the mean K-X-ray multiplicity MK  on the mass-number of the producedevaporation residue. MK  is predicted3) to increase
to values well above one when approaching the SHEregion (see Fig. 1).
The fact that a single compound nucleus can emit
more than one X-ray after formation is a consequenceof the filling times of an empty inner atomic orbit (typically 10−13 . . . 10−14 s4) ) being significantly
shorter than the typical lifetime of nuclear levels
decaying by electron conversion (typically the picosecond range).Therefore many subsequent conversions
are possible in the decay cascade of a compound nucleus.
Experiments were performed at the RIKEN Nishina
Centre for Accelerator based Science by using the gasfilled magnet separator GARIS for superheavy elementdetection. A high-purity, low-energy planar germanium LEGe-detectora) was adapted to the GARIS system at the target place for the first time in order
to measure the element-characteristic, prompt X-rayemission.
In September and October 2014, first tests concerning the rate-acceptance and resolution-deterioration of
the LEGe-detector as well as background studies dueto different targets in heavy-ion fusion reactions havebeen carried out. By measuring the γ-ray background
during the reaction 248 Cm(48 Ca, xn)296−x Lv∗ with I =650 pnA average beam-intensity at a distance of 76 cm
to the target the detector performance was excellent:Superior energy-resolution of ∆EF W HM = 800 eV

139La(48Ti,xn)187-x Au*: σ ≈ 18 mb

139La(48Ti,xn)187-x Au*: σ ≈ 18 mb

-1

10-1

10-2

10-2

10

00

50
50

100
100

150

200
ACN
200

150

250

300

350

ACNFig. 1. Experimental values for the mean K-X-ray multiplicity MK  as a function of the mass number ACN .

Dedicated X-ray spectroscopy was performed inJune, 2015: The reaction 139 La(48 Ti, xn)187−x Au waschosen to show the general feasibility of this new detection method. Due to space limitations, a fixed target of 139 La with 300 µm/cm2 on 3 µm Ti-backing
had to be used. Therefore, intensity of the 48 Ti beam
(E = 242 MeV ) was limited to 45 pnA. The production cross-section of the compound nucleus 187−x Au
was measured to be σ = 18.5 ± 4.6 mb. As can beseen in Fig. 1, the measured value for the multiplicityvalue reveals an excellent agreement with the semiempirical prediction of MK  = 1.23 ± 0.29. Encountering the absolute detection efficiencies as well as thetotal dead-time of the electronics a detection-limit incross-section can be estimated for in-beam X-ray spectroscopy using a whole array of LEGe-detectors to be
220 pb. This value - being nearly two orders of magnitude lower than the current limit for in-beam γ-ray
spectroscopy - encourages for further studies.
References

Department of Physics, Technische Universität MünchenANL, Argonne National LaboratoryRIKEN Nishina Center
Faculty of Physics, Ludwig Maximilians UniversitätCanberra Type LEGe1010

1) A. Staszczak et al., Phys. Rev. C 87, 024320 (2013).
2) Y.Oganessian, Rep. Prog. Phys. 78, (2015).
3) W. Henning, Nucl. Phys. A 400, (1983).
4) H.J. Karwowski et al., Phys. Rev. C 25, (1982).

- 55 -