What Is It?

The WARP programme (pdf) is a graded programme intended to search for Cold Dark Matter in the form of weakly interacting, massive sub-atomic sized particles known as WIMP's (Weakly Interacting Massive Particles). These particles may produce via weak interactions nuclear recoils in the energy range 10–100 keV. A cryogenic noble liquid detector such as Argon, unlike an ordinary scintillator, permits the simultaneous detection of both ionisation and scintillation.

Already in 1993 the ICARUS collaboration, in the framework of a systematic study of ultra-pure noble liquids, pointed out for the first time that this property could lead to a unique signature for the energy deposited in the medium by the recoiling nucleus. It was then shown experimentally that, due to the high ionisation density of the recoiling nucleus, the final ionisation signal is strongly suppressed because of electron recombination, while scintillation light will still be produced from the excited states. Therefore electron mediated background events due to cosmic rays and the residual radioactivity in the detector elements and in the regions surrounding the detector itself may be easily rejected, since they generally deposit energy in the liquid close to minimum ionisation.

Initial tests were performed with Xenon. We have switched to Argon, which has even more favourable features and which can be produced in large volumes at a low cost, as already proven by the ICARUS experiment.

How it works?

Inner detector Layout Scheme The main concept of the inner double phase argon detector is sketched on the right. When a particle interacts in the liquid region excitation and ionization occur

  • A prompt primary scintillation signal due to disexcitation of argon excited dimers produce by the impinging particle is produced and detected by the photomultipliers matrix positioned in the gaseous phase;
  • If an opportune electric fields are applied a bunch of ionization electrons produced in the interaction and surviving recombination processes is drifted toward liquid-gas interface, extracted to gas. Once extracted they are accelerated in order to produce, through collisions with atoms, the emission of photons. This light signal, proportional to ionization, is called secondary scintillation or proportional light.
The ratio of primary over secondary signal, as experimentally demonstrated through a 2.3 liters prototype, depends on the kind of particles producing excitation and ionization. Also the shape of the primary signal can be used to discriminate the nature of the ionizing particle. A nuclear recoil induced, for example, by an elastic WIMP interaction, produces a relative small amount of ionization electrons surviving recombination and a faster scintillation signal. This is mainly due to the large local ionization density that favors columnar recombination. On the other side a minimum ionizing particle such as gammas or electrons, of the same energy, produces a larger secondary signal since ionization electrons recombination is reduced and a corresponding slower primary signal.

The aim of experiment is to detect nuclear recoils and their spectrum, possibly induced by WIMPs interactions in the detector, in the energy range 10-100 keV: in this range the substantial background induced by gammas and electrons is strongly suppressed by the proposed techniques that are used as discrimination methode.

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