JRA3: STREGA - TASK M6
Report on room temperature measurements on Al using an
electron particle beam
First year report (04/2004, 04/2005)
Leading group: Frascati, INFN (3)
Introduction
The aim of task M6 is the measurements of the mechanical vibrations caused by the interaction
particle inside different media at different thermodynamic temperature where an enhancement of
effect has been observed in resonant gravitational wave detectors triggered by cosmic ray particles.
In the interaction of energetic particles within a massive detector, energy is released due to standard
nuclear and electromagnetic processes with a consequent temporary local increase of temperature
and pressure inside the detector itself. This pressure pulse generates acoustic vibrations in the
material exciting the modes of oscillation. This behavior is well explained by the so-called ThermoAcoustic Model [1, 2, 3], and some experiments [4, 5] have confirmed its validity, at least at room
temperature. During the runs performed in 1998 and 2000, the gravitational wave detector
NAUTILUS has recorded signals of cosmic rays passage [6] whose amplitude was in strong
disagreement with the prediction of the Thermo-Acoustic Model. The “anomalous” measurements
were taken with the antenna in the superconducting state (T=0.14 K).
It is not clear how to interpret this phenomenon. Nevertheless, there are some aspects of the
Thermo-Acoustic Model that are not already well defined, like the behavior of the Grüneisen
parameter for aluminum below the temperature of transition to the superconducting state or the
possibility of an enhancement of the energy conversion at the lowest temperatures. Other
possibilities to explain these results can come from anomalous composition of the cosmic rays,
such as the presence of exotic matter in the cosmic ray shower (nuclearites, Q-balls).
More recent data, coming from the EXPLORER antenna working at the thermodynamic
temperature of 2.7K, show that the anomaly looks not to be anymore correlated in to the
superconducting state of the detector.
Task M6, known with the acronym of RAP [7, 8] (Rivelazione Acustica di Prticelle), is intended to
investigate these topics, making measurements at different temperatures, from 300 K down to
significantly below 1 K, i.e. both in normal-conducting and in superconducting regime for
aluminum.
This study is important for the evaluation of the noise induced by cosmic rays in the next
generation of gravitational wave detectors, both of interferometer and resonant mass type. In
particular it can drive the choice of an underground site for the future projects of large resonant
mass detectors operated at ultra low temperatures.
RAP collaboration is an international collaboration between INFN (Frascati, Roma1, Roma2, Gran
Sasso Laboratory), La Sapienza and Tor Vergata Universities (Rome, Italy), Leiden University
(The Netherlands). A new collaboration with Australian Perth group is just started in order to
investigate the effect in a Niobe material sample.
The acoustic emission detector and Frascati cryogenic facility
The RAP detector consist of a small cylindrical aluminum bar placed inside a cryostat that is going
to be equipped with a dilution refrigerator system, installed at the DAFNE Beam Test Facility,
where a 510 MeV electron beam is used to generate the Thermo-Acoustic effect inside the massive
detector. The oscillating test mass is constituted by a cylindrical bar (2R=181.7 mm, L=500 mm)
made of AL5056, the same aluminum alloy (5.2 Mg%, 0.1%Mn, 0.1% Cr) used in NAUTILUS.
The resonance frequency of the first longitudinal mode is about 5.096 kHz at T=300 K.
The suspension system is a vertical cascade of seven attenuation stages (mechanical filters). The
aim of the cascade is to provide the requested level of attenuation inside the working frequency
window (-150 dB attenuation). The system also provides thermal link between the bar and the
dilution refrigerator.
The cryogenic and vacuum system is basically composed by a commercial cryostat (height =3200
mm, diameter =1016 mm) and a 3He-4He dilution refrigerator (base temperature =100 mK, cooling
power at 120 mK=1mW). The assembly minimizes the acoustic interference, since there is no
direct contact between cryogenics and detector, except for the weak thermal connections between
the refrigerator and the suspension system.
A mechanical structure encloses the cryostat allowing an easy positioning of the detector on the
beam line and the consequent removal after the expiration of the dedicated periods of data taking.
The RAP experiment installed in the DAFNE Beam Test Facility: schematic layout of the
experiment installed in the BTF area, picture of the cryogenic facility, mechanical structure and the
bar detector
The readout is based on commercial piezoelectric ceramics (PZT) followed by a low-noise custom
JFET pre-amplifier, and a Stanford SR560 amplifier. A noise of 1nV/√Hz. for the first amplifying
stage at 5 kHz in the 25 kHz band has been measured, while the SR560 second stage showed a
noise of ≈ 4 nV/√Hz at high gain (G=50000). The methods used for calibrating the detector are
based on the use of the PZT self-calibration technique and on the use of an accelerometer [10].
The data acquisition system , based on a 200 ksample/s peak sensing 16 bit VME ADC (VMIC
3123) and a VME Pentium III CPU (VMIC 7740) running Linux, has been developed in the
LabVIEW environment (with low-level C calls); it collects data coming from the PZT, the
accelerometer, the environmental sensors and from the beam signals, originated by the upstream
beam monitor detector. The overall data throughput on disk is 0.3 MB/s. An online monitor of the
measured amplitude, also performing real-time fast Fourier transform analysis, has also been
developed.
Room Temperature measurements, first result at low temperature
During the year 2003, the commissioning and the first runs at room temperature on the beam have
concluded the first phase of the experiment, dedicated to testing the read-out, electronics, data
acquisition system, suspension and mechanics. The results obtained in this first runs are in
agreement with data obtained by previous experiments [4, 5]. Major uncertainty came from the
beam flux monitor, needed in to evaluate the average energy of the beam impinging on the bar.
Anyhow the measurement performed lowers the errors with respect to previous experiments based
on similar techniques.
In 2004, the cryogenic tests and the runs at liquid He temperature (4.2 K) were planned.
Unfortunately, in February 2004 a severe cryostat failure (a big crack of a welding in the nitrogen
dewar) occurred, and the cryogenic test was stopped for the repair [9].
Then, in May 2004, after the fixing, the cryostat was directly placed in the DAFNE Beam Test
Facility for the cryogenic runs with the electron beam.
The cryogenic operation started with the filling of the cryostat with liquid Nitrogen, both in the
Nitrogen and in the Helium dewars, followed by the final cool-down to 4.2 K with liquid Helium.
A very slow cool-down was made between 300 and 77 K to avoid as much as possible fast
contraction of the cryostat material. During the cool-down, the space surrounding the bar was filled
with about 1 mbar of gaseous Helium, in order to allows the detector cooling. This was indeed the
only thermal link between the bar and the liquid Helium, the bar being mechanically disconnected
from the cryostat, with the exception of three thin stainless steel wires.
During the period May 26 - June 17 a number of shots, with the bar at different temperatures, was
recorded by the data acquisition system. The signal of the piezoelectric ceramics, enhanced by a
factor 103 by the SR560 amplifier, has been sent both to the ADC of the DAQ system for the data
storage and to a spectrum analyzer for a visual analysis.
The data analysis is still in progress. The relationship between the measured signal and the
amplitude of the first longitudinal mechanical oscillation B0 of the bar per unit of released energy
W is
B0
V

W N E  (T)
[m J -1 ]
where V is the amplified piezoelectric signal, N is the number of electrons of the beam, E is the
mean energy released from every electron to the bar (Monte Carlo simulation gives an estimate of
195 ± 7 MeV/electron for the 510 MeV electrons coming from the DAFNE LINAC) and α(T) is the
electro-mechanic conversion factor.
Plot of the first longitudinal mode amplitude per unit of released energy versus T
Figure shows the results of the visual analysis performed during the 2004 data taking together with
the results obtained in 2003, in comparison with the results obtained by a past experiment [5]
adapted to the RAP geometrical parameters. The lines show the theoretical prediction (both 1-D
calculation and 3-D simulations) of the Thermo-Acoustic model. 1-D line is the result of an
analytical calculation [4], whose expression is
B 0 2 L

W  Cv M
[m J -1 ]
where λ is the linear thermal expansion, Cv is the specific heat, L and M the length and the mass of
the bar, while 3-D line is obtained from a finite element simulation.These first results show a quite
good agreement with the theory [11]. Nevertheless, a more accurate offline analysis is now in
progress. This will be the last step before the data taking with the bar in superconducting state,
which is the main purpose of this experiment. First measurements are planned during the year
2005, after the dilution refrigerator characterization.
Conclusions
The Task M6 has achieved the tree milestone scheduled for the first year of the project: the acoustic
detector has been designed and implemented. At the same time the cryogenic facility has been
modified in order to host the detector. The last upgrade, with the introduction of the dilution
refrigerator is in progress. Two runs at room temperature have been performed and the results are
in good agreement with previous experiments. The measurements are characterized by a superior
sensitivity and we are working on data analysis and diagnostic for beam characteristics in order to
reduce calibration errors and discriminate between different models.
The collaboration anticipates the schedule by performing the first world measurement at low
temperature for aluminum. Data are still confirming the Thermo-Acoustic Model predictions. New
runs will be performed in 2005 below aluminum critical temperature A niobium bar detector,
provided by NIOBE collaboration, is under implementation in order to have first results with a
different medium.
Bibliography
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3. G. Liu, B. Barish, Phys. Rev. Lett. 61, 271 (1988) and references therein.
4. A.M. Grassi Strini, G. Strini, G. Tagliaferri, J. Appl. Phys.51(2), 948 (1980).
5. G.D. van Albada et al., Rev. Sci. Instr. 71, 1345 (2000).
6. P. Astone et al., Phys. Lett. B540, 167 (2002).
7. S. Bertolucci et al., INFN-LNF Note, LNF-01/027 (2001)1.
8. G. Mazzitelli et al., Class. Quantum Grav., 21, S1197 (2004).
9. G. Delle Monache, C. Ligi, RAP TECHNICAL NOTE 005 (2005)
10. S. Panella, RAP TECHNICAL NOTE 004 (2005)
11. C. Ligi, International Journal of Modern Physics A, Proceeding of 19th
European Cosmic Rays Symposium (2004)
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LNF and RAP technical notes are available at http://www.lnf.infn.it/esperimenti/rap/docs/