ACME uses a cryogenic buffer gas beam (CBGB) source to achieve high single-quantum- state intensities of the chemically reactive molecular species ThO. The heart of the cold beam apparatus, the buffer gas cell (see Fig.5.1.3 for a schematic), is similar to those described in existing buffer-gas-cooled beam publications [22,107,133,147,148,174]. The ACME Gen. I beam source was developed primarily by Nick Hutzler and was characterized and described in detail in [105, 106].
The cell is a small copper chamber with a cylindrical inner bore (12.7 mm in diameter and
≈9cm long) whose axis points alongxˆ(see Fig.2.1.1). Precursor targets are set into alcoves along one side of the bore, while the other side has windows for admitting the ablation laser that are offset on long tubes or “snorkels” to reduce clouding due to dust accumulation. The cell is mounted in vacuum and held at a temperature of 16 K with a Cryomech PT415 pulse tube cooler and a small ≈1 W resistive heater (the heater is not needed while the ablation laser is firing). The cell temperature and the temperature of all the cryogenic components
are monitored using Lake Shore silicon diode thermometers (Model DT-670A-CU). Neon (Ne) buffer gas, pre-cooled by heat sinks consisting of tube lengths thermally anchored to the warmer stage of the pulse tube and the exterior of the cell, flows into the cell through a fill line at the “upstream” end of the cylindrical volume. At the “downstream” end, an aperture 5 mm in diameter in a thin (0.5 mm) plate is open to the external vacuum, allowing the buffer gas to flow out as a beam.
The cell is surrounded by two nested chambers of metal that are also thermally connected to the pulse tube cooler (see Fig.3.0.1). The inner chamber is a copper box held at 4 K by the PT415 cold stage that acts as a high-speed, large-capacity cryopump for neon, maintaining a high vacuum of ∼ 3 µTorr in the system despite large buffer gas throughputs. “Sorbs,” which are large surface areas of activated coconut charcoal glued to copper plates with a cryo- compatible, thermally conductive epoxy4 and cooled to 4 K, also help to keep the pressure low by adsorbing residual helium and other gases in the chamber.5 The outer chamber is an aluminum box covered in aluminized Mylar “super insulation” and kept at ≈ 60 K by the PT415 warm stage. It serves to shield the inner cryogenic regions from blackbody radiation emitted by the room-temperature vacuum chamber. The 4 K cryopump chamber, 60 K radiation shield, and vacuum chamber all have windows for optical access and apertures (described in Section2.3.2) to transmit and collimate the buffer gas beam.
The source of ThO molecules is a ceramic target of thoria (ThO2) fabricated in house.
We make the targets using established techniques, including a recipe for ThO2 mock nuclear
fuel pellets furnished by Oak Ridge National Labs [19, 116, 190]. The details of our target- making procedure are provided in AppendixB. The general method is to prepare a mixture of
4The preferred epoxy is Stycast 2850FT mixed with 24 LV catalyst in a ratio of 100 parts epoxy to 7.5 parts catalyst and cured for at least a day.
5The utility of sorbs is well established for CBGBs with helium buffer gas, but it has not been proven for neon-based beams. In fact, neon that reaches the sorbs is likely to clog the pores of the charcoal and limit its effectiveness as a pump for residual helium. We nevertheless include sorbs in our system because they are unlikely to harm and may help the vacuum quality. In addition, about two-thirds of our sorb surfaces are “protected,” meaning that gas must bounce multiple times off of other cryogenic surfaces before reaching the charcoal. This limits the pumping speed of the sorb but should also limit the pore-occluding detrimental effects of neon and other species, which stick to the protecting surfaces instead of the charcoal.
325 mesh (≤44µm) ThO2 powder and the “sintering agent” niobia (Nb2O5) by ball milling,
pre-compacting, meshing, and purifying it in a 1000◦C furnace. The prepared powder is then mixed with a sticky binder to inhibit crumbling and cold-pressed in a 1/2” or 3/4” diameter die to a thickness of≈1/8–1/4”. The pressed pellet is then sintered at least twice at a temperature of 1150◦C or above. This process produces sturdy, very hard targets with masses of 5 to 10 g and densities between 65% and 85% of the theoretical maximum ThO2
density of 10.0 g/cm3.
ThO molecules are introduced into the cell via laser ablation: A Litron Nano TRL 80- 200 pulsed Nd:YAG laser is fired at the ThO2 target, creating an initially hot plume of
gas-phase ThO molecules, plus other detritus. The ablation pulse energy is typically set to 60–100 mJ, and the repetition rate is 50 Hz. The pulse length is≈5ns. Each target lasts for approximately 1 month of hard (10 h/day) running, or ∼ 50 million ablation shots, before becoming depleted (i.e. producing 1/3 to 1/2 of their original yields). Upon removal, the targets have typically lost 30% to 50% of their original mass and appear darkened, pitted, and spiky all over from ablation damage. Under typical running conditions, the in-cell production rate of ThO is ∼ 1014 molecules/s [105, 106]. For a month of running time at
10 h/day, this makes a total of∼1020 molecules produced per target. At a mass of 248 amu
per molecule, this implies that ∼ 40 mg or ∼ 1% of the ablation product from a target is converted into gas-phase ThO molecules.
On a timescale rapid compared to the emptying time of the cell into the beam region, the hot ThO molecules in the ablation plume thermalize with the 16 K buffer gas in the cell. Continuous Ne flow at ≈ 40 SCCM (standard cubic centimeters per minute; for reference, 1 SCCM = 4.48× 1017 atoms/s) maintains a buffer gas density of n
0 ≈ 1015–1016 cm−3
(≈10−3–10−2 Torr, where the subscript “0” indicates the steady-state value of the quantity
in the cell). This is sufficient for rapid translational and rotational thermalization of the molecules and for producing hydrodynamic flow out of the cell aperture that sweeps along and extracts a significant fraction of the molecules before they can diffuse to the cell walls
and stick. The measured extraction fraction is ≈ 10% [106]. The result is a ≈ 2 ms long pulsed beam of cold ThO molecules entrained in a continuous flow of buffer gas.
Just outside the cell exit, the buffer gas density is still high enough for ThO–Ne collisions to play a significant role in the beam dynamics. The average thermal velocity of the buffer gas atoms is higher than that of the molecules by a factor of√mmol/mb, where the subscripts
“b” and “mol” indicate buffer gas and molecule quantities, respectively. Consequently, the ThO molecules (mmol = 248amu) experience collisions primarily from behind, with the fast
Ne atoms (mb = 20 amu) pushing the slower ThO molecules ahead of them as they exit the
cell. This accelerates the molecules to an average forward velocityvf that is larger than the
thermal velocity of ThO. As the buffer gas pressure in the cell is increased, vf approaches
v0,b, the thermal velocity of the buffer gas.
The angular distribution of the molecule beam has a characteristic apex angle θ given by tan(θ/2) ≡∆v⊥/2vf, where ∆v⊥ is the transverse velocity spread of the beam. For the
ACME beam, the apex angle isθ≈39◦, and the characteristic solid angle isΩ≈0.35sr. The beam velocity is measured to be ∼180 m/s. As the gas cloud expands nearly isentropically out of the cell into the vacuum, it must also cool. The measured final longitudinal and rotational temperature of the beam is ∼ 4 K, yielding a forward velocity distribution ∆v∥
of ∼ 40 m/s FWHM (full width at half maximum) and efficiently populating low-lying rotational levels in the ground electronic state (e.g. ∼ 30% in J = 1). The total number of molecules per pulse in one of the most populated quantum states is measured to be ∼1011.
This slow, cold, high-intensity molecular beam provides ACME with a long interaction time
τ over a short distance, low phase-decoherence due to the narrow velocity spread, and a high count rateN˙.
In practice, we typically run the ACME ablation beam source for no more than about 10–12 hours at a time because of the necessity of regularly warming up to desorb the neon and regenerate the cryopumping surfaces. Our usual running routine is to start the beam source and tune up the experiment in the morning, run through the day, and “de-ice” the
beam box in the evening. After the de-ice procedure, which takes about 1 hour and involves heating the 4 K stage to ∼ 60 K, the pulse tube cooler is switched back on, and the beam source is allowed to cool back down overnight. By the next morning, ≈10 hours later, the beam source is cold and ready to run again.