TRINAT

One key Standard Model prediction is that beta decay neutrinos are exclusively left-handed, or only spin counter-clockwise. Thus, TRINAT is searching for the theoretically forbidden right-handed spin neutrinos, the detection of which would point to gaps in the Standard Model. 

The key experimental challenge is that neutrinos are exceptionally difficult to detect. A neutrino can pass through hundreds of light years of lead without interacting with it. As a result, TRINAT scientists are leading the way in using a form of intricate momentum triangulation to tease-out the spin of beta decay neutrinos.  

TRINAT is a tabletop-sized experimental facility located in ISAC-I that uses state-of-the-art laser trapping and cooling technology. Unlike most TRIUMF experiments, TRINAT uses neutral atoms, and thus its name TRIUMF Neutral Atom Trap. 

To detect neutrino momenta, TRINAT uses a variety of advanced laser techniques to create a pen-tip-sized cluster of almost perfectly still beta-decaying atoms suspended in a vacuum chamber. The atoms are all spin polarized, or magnetically oriented in the identical direction.  

The degree of total polarization is critical to being able to precisely measure the positrons’ decay direction with respect to the spin. Through years of experimental fine tuning, to date (2017) TRINAT has achieved an overall spin polarization rate of 99.1 +- 0.1 \%. This means that TRINAT will detect a right-handed electron neutrino even if it occurs in less than one-in-a-thousand decays. We already knew from the TWIST muon decay experiment at TRIUMF that a right-handed muon neutrino occurs in less than one-in-ten-thousand decays, and one future TRINAT goal is to achieve that level of precision for the electron neutrino. 

From the perspective of a single atom of an isotope, the experimental technique is analogous to a stationary ship at sea (the atom), its bow pointing due north (polarized). On the ship, there’s a single passenger (the positron or electron), and an invisible small dog (the neutrino), which simultaneously jump off the ship (the beta decay). By precisely measuring the movement of the ship and the passenger, TRINAT scientists can precisely deduce the movement of the invisible dog, or neutrino.  

TRINAT requires enormously high precision since only a minute fraction of neutrinos might not be left-handed. To detect any right-handed neutrinos requires both measuring a large number of beta decays and the world’s most precise measurements of beta decay particle momenta. 

 The stopping foil

TRINAT’s first step is to rapidly decelerate and neutralize an isotope beam from ISAC-1 (for example, of the isotope 37K), entering TRINAT at about 80-million-ions-a-second. This is done by colliding the ions into a thin zirconium foil at the back of TRINAT’s collection trap. The ions stop just 100 angstroms, or atomic lengths, into the zirconium. Through interacting with the zirconium atoms, the ions are electrically neutralized. However, the neutralized atoms don’t remain embedded. The foil is orange hot, heated to about 900 °C, which causes the embedded atoms to boil off from the metal’s surface into the collection chamber.  

 The Collection Trap

TRINAT has two atom traps, both of which use a combination of laser light and magnetic fields to gather, condense, and cool the atoms into a millimeter-cubed size cluster (this combination of magnets and optical laser light makes it a magneto-optical trap).  Both traps operate at a strong vacuum, just one-trillionth the atmospheric pressure of the surrounding room.  

In the first trap, the collection trap, the atoms are photon-corralled, and slowed, or cooled, into the centre of the trap using a technique called laser cooling. The trap is surrounded by three pairs of lasers pointed inward—one pair aimed from top-and-bottom; one pair from left-and-right; and the final pair at opposing 90-degree angles. Every atom has a characteristic resonant frequency—a frequency at which it will absorb and be energized by photons at a particular wavelength of light. When an atom absorbs a photon, it gets a momentum kick in the direction the photon was travelling. Atoms absorb more photons closer to the resonant frequency. 

So, in an experiment with potassium-37, the six lasers are set to shine light at a frequency just below the resonant frequency of potassium-37. The result is that if, for example, an atom is moving upwards, its upward momentum causes it to experience the laser light hitting it from above as slightly Doppler shifted to the blue, or to a higher frequency. Thus, the 37K atom absorbs more of these resonant photons, than photons coming from below, and slows down. In this way, all the potassium-37 atoms moving in any direction are dramatically slowed or cooled. The six titanium: sapphire tunable ring laser beams are driven by the same type of green laser used in many laser pointers, but with ten thousand times the power.  

To tight cluster the slowed atoms, TRINAT’s collection trap uses a finely calibrated magnetic field produced by two magnetic coils, one located above, the other below, the trap. Just as the Earth has poles, every atom has a slight magnetic moment. The collection trap magnets are arranged so that the magnetic field is weakest at a tiny point in the very centre of the trap, a point called a magnetic well. Based on the physics of resonance frequencies, the further an atom is from the well, the more photons it absorbs, and is thus be pushed back towards the magnetic low point. 

In this way, about one-in-a-thousand of the atoms are centrally trapped, leaving the bulk of them lining the wall’s Pyrex walls.  

The beta-decay from these untrapped, rapidly moving atoms would overwhelm TRINAT’s detectors with unwanted signals.  

To avoid signals from these untrapped decaying atoms, TRINAT has a second trap, the detector trap. Every second, the six trap beams are turned down to ten-percent energy for 40-thousandths-a-second. A seventh laser beam is then turned on which pushes the tight cluster of trapped atoms, like corks on a river, through an opening into a second trap.  

The Detector Trap

To calculate accurate neutrino momenta, the isotope cluster must be held at lowest possible temperature and size since this provides the baseline location from which the recoil particle and positron are measured. 

The detector trap uses the same combination of six lasers and a magnetic field to force the atoms into a suspended, still, pin-point cluster of about one-millimeter cubed.  The atoms are cooled to about 0.0003 °K, at which temperature an atom is moving at about half-a-meter-per-second. For comparison, the same atom at room temperature would be moving at about 500 meters-per-second. 

In the detector trap, there’s another key step: the atoms are all made to spin in the same direction, or spin polarized. This is critical in order to confine, or know, the spin of the atoms that beta decay, a key part of determining the angular momentum component of its overall momentum. 

The atoms are spin polarized using a rapid-fire sequence of repeated steps in order to both polarize and continue to contain them. For a series of about 100 cycles, the laser-magnetic trap is turned-off for 1.9 milliseconds during which time another laser shines circularly polarized light on the cluster forcing the atoms to spin in the same direction, and during which the beta decays are measured. Then, the laser-magnetic trap is turned back on for three milliseconds, refocusing the atoms into a tight cluster, but destroying the spin polarization. 

Detectors

37K has a half-life of about one second before decaying into argon-37, a positron and a neutrino, and a TRINAT experimental run measures between 300,000 and a million of these beta decays. The detector measurements are only made during the period in the detector trap when the atoms are spin polarized. TRINAT has two separate detectors, one for the argon (also known as the recoil product) and one for the positron. 

The positron’s location and timing is captured by detectors located along the top and bottom of the detector trap. TRINAT scientists determine the positron momentum from where it hits on the detector and the total energy it deposits.  

When the 37K transforms into argon-37, the beta decay process kicks the atom from the magnetic well. Because an argon-37 atom has a different resonant frequency than 37K, it isn’t pushed back by the laser light. Some argon-37 atoms are produced as ions in the decay, and are captured by a uniform electric field which accelerates them towards a microchannel plate detector which records their position and timing, from which their momentum is deduced. 

Based on Newton’s law of the conservation of momentum, the overall momentum of the system must be conserved. Thus, measured in coincidence, the positron and argon-37 measurements provide TRINAT scientists with the invisible neutrino’s momentum. Similarly, the neutrino’s average spin is deduced from its average direction with respect to the measured spins of the beta particle and 37K .