The apparatus is shown in Fig. 2. It has been described in several publications, for example in Ref. [10]. It has a base pressure of < 1×10−6 mbar. The central source part is described in Ref. [4]. The emitter is a cylindrical (extruded) sample of an industrial iron oxide catalyst doped with K [24, 25], a so called styrene catalyst type Shell S-105 (obsolete type number). This type of catalyst is an efficient hydrogen abstraction and transfer catalyst. The source metal tube can be heated by an AC current through its wall up to 400 K. Deuterium gas (99.8%) is admitted through the tube at a pressure up to 1 × 10−5 mbar in the chamber.

Fig. 2 Horizontal cut through the apparatus Full size image

The metal target is constructed for direct heating with a 50 Hz AC current. The carrier Ni foil with dimension 12 × 15 mm has a thickness of 0.2 mm. It is spot welded to two thinner foils of Ta with thickness 0.1 mm which carry the heating current. On the carrier, short Ir rods with 2 mm diameter are spot-welded. The carrier foil is mounted at 45° to the vertical direction and located approximately 1 cm below the source tip. The heating current through the carrier and its supporting foils is taken from an external ring transformer with a few turns of secondary winding. The temperature of the carrier is measured by a type K thermocouple (TC), spot welded at the upper half of the carrier foil. The cold end of the TC is at the screw support on the arms holding the carrier foil, at a distance of 20 cm from the carrier.

A Nd:YAG laser with an energy of < 125 mJ in 5 ns long pulses at 10 Hz is used at 532 nm. The laser beam is focused on the carrier at the center of the chamber with an f = 400 mm spherical lens. The lens is mounted in a vertical motion translation stage. The intensity in the beam waist of (nominally) 70 µm diameter is relatively low, ≤ 1012 W cm−2 as calculated for a Gaussian beam. A dynode-scintillator-photomultiplier detector which measures the time-of-flight (TOF) spectra of the neutral and ionized flux from the laser initiated processes is shown in Fig. 2. This detector can be rotated around the carrier at the center of the chamber. Fast neutral particles impact on a steel catcher foil in the detector, and ions ejected from there (or in the beam) are drawn towards a Cu–Be dynode held at − 7.0 kV inside the detector. The total effective flight distance for the particles from the laser focus to the catcher foil is 101 mm by direct measurement and internal calibration [5, 10]. The photomultiplier (PMT) is Electron Tubes 9128B (single electron rise time of 2.5 ns and transit time of 30 ns). This PMT is covered by Al foil and black plastic tape giving only a small active cathode area of 2–3 mm2 to avoid saturation. Blue glass filters in front of the PMTs remove any scattered laser light. A fast preamplifier (Ortec VT120A, gain 200, bandwidth 10–350 MHz) is used. The signal from the PMT is sometimes collected on a fast digital oscilloscope (Tektronix TDS 3032, 300 MHz). The average function in the oscilloscope is used. A multi-channel scaler (MCS) with 5 ns dwell time per channel is used (EG&G Ortec Turbo-MCS) for the TOF–MS spectra. Each MCS spectrum consists of the sum of 500 laser shots.

The laser-induced mass spectrometry used here is described in several publications [3, 4, 10,11,12]. Due to the very short bond distances in ultra-dense hydrogen and also in low levels of ordinary hydrogen Rydberg matter H(1) and H(2), the kinetic energy release (KER) given to the cluster fragments by the Coulomb explosions (CE) is quite high. It is also well-defined, due to the easy removal of the orbiting Rydberg electrons by the laser pulse, without any large changes of the structure before the fs long repulsion period between the fragments [26]. The total kinetic energy of the fragments gives directly their initial distance as

$$r = \frac{1}{{4\pi \varepsilon_{0} }}\frac{{e^{2} }}{{E_{kin} }}$$ (4)

where ε 0 is the vacuum permittivity, e the unit charge on the fragment ions and E kin the sum kinetic energy for the fragments (KER) from the CE. The fraction of the KER that is observed on each fragment depends on the mass ratio of the fragments. The kinetic energy is determined by measuring the time-of-flight (TOF) of the fragments and then converting this quantity to kinetic energy. In the case of long H(0) chain clusters, a central fragmentation is often observed, thus giving two identical cluster fragments, each carrying half the total KER. The most common state of H(0) has s = 2 [3]. It has 2.3 pm H–H distance and gives a total KER of 640 eV.

Part of the flux from the laser-induced processes on the target is taken out through a small opening in the chamber wall to a separately pumped and valved chamber with two collectors consisting of a metal wire loop covered with 1–2 layers of 20 µm thick Al foil. The loops have a diameter of 50 mm inside the tube with internal diameter 63 mm. The collectors are at a distance of 66 cm (inner collector) and 163 cm (outer collector) from the target as seen in Fig. 2. The signal to them is directly observed on a fast digital oscilloscope (Tektronix TDS 3032, 300 MHz). The oscilloscope is triggered by a pulse from a photodiode close to the laser. The diode is located such that the trigger delay in the cable to the oscilloscope is close to the time for the pulse to move to the target and through the cable to the oscilloscope. The error is estimated to be ± 1–2 ns [27]. The typical behavior of the signals is shown in several publications [1, 23]. The decay times of these signals are characteristic for the decaying mesons. The signal is normally due to fast (relativistic) muons arriving to the collectors from the relatively slow decaying mesons (kaons and pions).

In some tests the inner collector is replaced by a wire coil wound on a ferrite toroid. This coil works as a current transformer for the particle beam current. The signal is due to charged fast particles, and photons and other neutral particles like neutral kaons cannot be observed by this coil. A direct comparison of the coil signal and the outer collector signal shows that the same particles are observed, thus proving that the outer collector signal is due to fast charged particles and not to photons or neutral particles. The measuring coil is wound on a N30 MnZn soft ferrite toroid with epoxy cover (EPCOS) with inner diameter 25 mm. There are 19 turns of enameled copper wire wound on it in the negative direction. The toroid hangs freely in the conductor wires in the particle beam. One end of the coil is in turn connected to the 50 Ω oscilloscope input, while the other end is connected to a 50 Ω BNC termination to the mounting flange. The sign of the beam current is determined by calibration using a current pulse in a single wire through the opening of the coil, to avoid possible sign errors.