Ever since the groundbreaking discovery that the universe is comprised of only 4.6% “normal” matter that we see and interact with everyday, scientists have been searching for an explanation as to where the other 94.6% of the matter is. This search for the missing matter in the universe has led to the theory of dark matter, a substance that does not interact directly with light or ordinary matter, making it extremely difficult to observe. In the Cold Dark Matter (CDM) theory, a small fraction of dark matter is said to consist of Weakly Interacting Massive Particles (WIMPs). These WIMPs are extremely massive (about 1000 times the mass of a proton) and have low energy. These particles consist of baryonic matter, which is composed of particles that can be detected and do interact with matter, although weakly. Yet how can such a particle be detected? How can a substance that has eluded many attempts of analysis be once and for all revealed?
This method used Superheated Drop Detectors (SDDs), which are containers filled with a medium (usually a polymer or gel) in which many droplets of low-boiling point liquid are pressurized and injected.
Several experiments that attempt to detect these WIMPs are currently underway. The objective of these experiments is to detect the nuclear recoils caused by interaction of WIMPs with the atoms in the detector material. During an internship at the SAHA Institute of Nuclear Physics in Kolkata, India, I used one method of detecting these interactions. This method used Superheated Drop Detectors (SDDs), which are containers filled with a medium (usually a polymer or gel) in which many droplets of low-boiling point liquid are pressurized and injected. As a result, these liquid drops are forced to remain liquid and become superheated. In this meta-stable state, the drops require only a very small amount of energy in order to be vaporized. When particles such as neutrons or high-energy photons come into contact with these droplets, they create bubbles of gas, indicating that a weakly-interacting particle has passed. Yet what is truly marvelous about this detector is how it analyzes these bubbles of gas. Rather than counting or measuring the gas content of the bubbles, the SDD actually “hears” the creation of the gas bubble.
The creation of these gas bubbles is an intricate process, and is key in the SDD’s method of detecting dark matter. When a particle (such as a neutron) passes through a droplet, it makes an elastic collision with the nucleus of an atom in the droplet. The kinetic energy of the particle is transferred to the nucleus; the nucleus then dissipates the energy in the droplet. A photon, however, interacts similarly with an electron rather than the nucleus of the atom. If the dissipated energy of a collision is above a certain threshold value, some molecules of the liquid transition to the vapor phase, forming a micro bubble inside the droplet. The energy must be high enough to allow the micro bubble to reach a critical radius, without which the bubble could not expand further and would eventually recollapse into the liquid state due to surface tension.
If the bubble reaches this critical radius, it rapidly expands, evaporating the nearby liquid until it converts the entire droplet into vapor. This entire process takes about 10 nanoseconds, and this rapid expansion of gas creates an acoustic signal – a sound wave – that can be detected by a condenser microphone. In fact, the frequency range of most of these signals is within the audible range of the human ear, and can be heard as a slight popping sound if the droplets are very close to the ear. The condenser microphones attached to the detector capture these sound waves and convert them to electrical signals.
For my lab, the experimental setup consisted of a sample of the detector being tested to determine the sensitivity towards neutron detection from a Californium source (252Cf). The sample was placed in a beaker of water which was wrapped in heat-generating coils in order to maintain a relatively constant temperature. The sample was then capped with a condenser microphone, which sensed the acoustic signals give off by the detector and converted it into electrical signals. These signals recorded by the oscilloscope, which then recorded their amplitude and frequency by measuring voltage and time.
Finally, all this information was used to plot a Pvar value, which is helpful for determining the characteristics of the particle that created a particular signal. The Pvar value is calculated by first finding the sum of the squares of the amplitudes; this sum is dubbed P. Pvar is then found by taking the log10P and multiplying the result by the time interval of the pulse. This final Pvar value is highly useful in categorizing signals. By taking measurements of controlled sources of radiation – such as neutrons, gamma rays, and alpha particles – and plotting their Pvar values, signals can be quickly matched to the particles which produced them. For example, signals caused by gamma ray emission have been shown to exhibit low Pvar values, while high Pvar values are characteristic of signals caused by neutrons. The data collected can then be analyzed accordingly to determine if the signal was produced by known particles such as neutrons, gamma rays etc, or by some unknown source, possibly dark matter.
Like other methods, this technique is not flawless and is continuously being changed and improved. Currently, new research and development is under way to perfect this method of detection by reducing background radiation emanating from the medium of the detector. This will help differentiate the signals given by neutrons, alpha particles, gamma rays, and other background, thereby increasing the likelihood of dark matter detection. The current model of the detector uses a certain polymer as the medium for containing the droplets used in the detector. New types of mediums are being tested to decrease detector sensitivity toward certain particles in an effort to further pinpoint and filter out various background radiations, thereby clearing the way for dark matter detection.
One such experiment is using a new glycerin based polymer, as opposed to the glycol based polymer currently being used in SNOLab (Sudbury Neutrino Observatory) in Canada. The new polymer is currently being tested and researched, and will hopefully provide a more efficient and precise method of sorting through the background radiation that the detectors receive. Tests have shown that the radioactive count of the new polymer is significantly lower than the current one, meaning that it minimizes signals from within the detector and is therefore much more effective at detecting dark matter.
Yet another new development is a method to increase the lifespan of these detectors by reforming the droplets after they have become bubbles. Every time a signal is detected, one droplet is converted into a bubble and cannot be used again. Therefore, the number of signals a detector can sense is limited to the number of droplets placed within the detector. After the majority of droplets have formed bubbles, the detector must be substituted with a new one replenished with superheated drops. Research aims to reduce this need by re-pressurizing the system to compress the bubbles into the liquid droplets once more, effectively doubling the lifetime of the detector.
An important thing to keep in mind is that the concept of WIMPs is still only a theory, and despite hard efforts, SDD’s may never detect dark matter. In fact, dark matter may consist of a completely different variety of particles, or may not even consist of particles at all! However, until more information is uncovered, scientists will continue to try and detect the elusive constituent of the universe, with the aid of new and innovative methods such as the SDD.