Compact radiation shield for a superconducting array molecule detector

Compact radiation shield for a superconducting array molecule detector Shigetomo Shiki1,2, Masahiro Ukibe1, Yutaka Shimizugawa1 Ryutaro Maeda1, and Masataka Okubo1 1

National Institute of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan

Recent development on large-scale superconducting array detectors requires a large through hole along the molecular flight path in a cryostat. The through hole causes degradation of detector performance due to the 300K radiation and a short holding time at the cryostat base temperature. To realize a compact detector system, we designed and fabricated infrared radiation shields with a fine-honeycomb collimator and a micro-structured metal-mesh. The infrared flux through the honeycomb collimator located at 50K was 300—500µW. A test run in a cryostat showed a holding time of 8 hours. We fabricated a metal-mesh consisting of a self standing Cr/Cu film with an array of 2.0µm holes having a pitch of 3.5µm. The metal-mesh was supported by a Si reinforce structure. Spectral measurement indicates that the transmission of the 300K radiation is less than 1%. PACS numbers: 07.20 Mc, 42.79 Ci, 82.80 Ms 1. INTRODUCTION Superconducting detectors are promising for mass spectrometry (MS), because of the capability of detecting huge molecules (>1MDa) and analyzing fragments including neutrals1,2, which cannot be achieved using conventional ion detectors such as micro channel plates. Recent developments on large scale superconducting array detectors3 require a large through hole along the molecular flight path in cryostats. The through hole causes degradation of detector performance due to the 300K radiation and a short holding time of cryostats. Previously, these problems were solved using a long radiation shield in mass spectroscopy4, but the size of the shield was inconvenient for practical applications. To realize a compact detector system, we designed and tested a compact radiation shield with a fine-honeycomb collimator and a metalmesh infrared (IR) filter. Previous study showed that a metal-mesh can

Shiki et. al work as an IR filter, however the size was not enough for our array detectors of 10mm5. In this paper, the performance of a fine-honeycomb collimator and a metal-mesh IR filter is reported.

2. EXPERIMENTS 2.1. DESIGN OF RADIATION SHIELD We designed the radiation shield system (Fig.1) according to the following requirements: (a) molecules should pass through radiation shields, (b) the ratio of transmission of particle and radiation (TION/T300KBB) is larger than 1, (c) the heat load of the cold stage is less than 10µW, (d) the shield should cover a large-scale array detector of 8mm x 1mm, (e) length of the shield is less than 10mm.

radiation shield 25-77K 4K metal mesh

300K detector

molecule

300K blackbody 450µW mm-2 0.3K stage cooling power 100µW fine honeycomb collimator 0.9-1.25mm interval L > 10mm t=0.015-0.1mm

Fig1. A schematic diagram of radiation shield. system.

Compact Radiation Shield with fine honeycomb and metal mesh The honeycomb structure works as a first window to limit the solid angle seen from the 0.3K detector stage. The shield is connected to a heat bath of 25K, so that the shield absorbs most of the infrared radiation from room temperature. The second window after the honeycomb collimator is a metal-mesh, which reflects the 300K IR radiation and enables molecules to pass through. The pitch of the hole array is designed to be much shorter than a wavelength of the 300K blackbody radiation. 2.2. FINE HONEYCOMB COLLIMATOR The honeycomb collimators can be constructed from aluminum foil or ceramics (cordierite), have a length of 10mm, a diameter of 10mm, and a square or hexagonal cells of 0.7-1mm in size. The infrared flux through the honeycomb collimators located at 50K was measured with a custom-made bolometer. The material, dimension, and measured transmission flux values are listed in Table 1. The observed transmission was two times larger than the nominal value expected from the geometry. The difference is due to reflection and scattering of IR radiation at the inner surfaces of the collimator cells, or transmission through the thin wall of the cells. The shielding performance of the sample (a) was tested in a 3He cryostat. The honeycomb collimator was located at a 25K shield. The temperature of the 0.3K cold stage rose to 0.54K from 0.29K, and the holdtime of the base temperature was shortened from 170h to 8h (Fig.2). Nevertheless, the 8h holding time is much better than a few minutes without the collimator. The leakage current of a Nb-Al/AlOx/Al-Nb STJ detector with the collimator was still 25 times larger than that at 0.29K. Table 1. Properties of fine-honeycomb-structured collimators. Transmission Material Thickness Pitch Surface [mm] [mm] [µW] (a) Cordierite 0.27 1.25 No coat 390 (b) Cordierite 0.27 1.25 Carbon black 260 (c) Cordierite 0.05 0.73 No coat 500 (d) Cordierite 0.05 0.73 Carbon black 220 (e) Aluminum 0.015 0.9 No coat 11000 (f) Aluminum 0.015 0.9 MnO2 290

Shiki et. al 0.8 0.75

temperature [K]

0.7 0.65 0.6 0.55 0.5 0.45 0.4 0

2

4

6 time t [hour]

8

10

12

Fig. 2 Temperature of the cold stage head as a function of time in a cryostat with the collimator only. A shutter at 25K was opened at t=1.2 hours, and the base temperature was kept for 8 hours.

2.3. METAL-MESH INFRARED FILTER In order to avoid the effects of the 300K radiation, we employed a self-standing metal-mesh IR filter, which was constructed from a Cu film. The metal-mesh was fabricated as follows. The Cu film with a thickness of 500nm was deposited on a Si wafer covered with a Cr adhesive layer by a DC magnetron sputtering apparatus. A mesh having a hole size of 2.0µm and a pitch of 3.5µm was patterned with g-line photolithography and Ar milling etching. A Si reinforcement structure with a frame size of 12mm x 5mm was fabricated with dry deep etching to support the self-standing Cu/Cr film. The details of these processes will be described elsewhere. Room temperature measurements showed that the performance of the metal-mesh was suitable for MS applications. An SEM image showed the hole size of 1.8µm and the pitch of 3.6µm (Fig.3). Particle transmission of 20% was actually measured with 3keV-Ar+ ions, and the measured transmission is compatible with the geometrical open area of the metal-mesh.

Compact Radiation Shield with fine honeycomb and metal mesh

Fig.3 SEM image of a metal-mesh.

Fig. 4 Spectral transmission coefficient of the metal-mesh. The solid line shows transmission of metal-mesh. The dotted line shows the intensity of the 300K radiation. It is clearly seen that the metal mesh cut off the room temperature blackbody radiation.

Shiki et. al IR transmission of a metal-mesh was measured with Fourier transform IR spectroscopy (Fig.4). The total transmission of the 300K blackbody radiation through a metal-mesh filter was 1%, which was evaluated by integrating transmission flux over entire wavelengths. It is expected that the hold-time of 3He cryostat will increase dramatically using the metal mesh filter. The metal-mesh naturally works as an IR filter even though it was placed at the 4.2K shield. I-V curves were measured using the metal-mesh and the collimator. No change was observed when the 4.2K shutter was open. 3. CONCLUDING REMARKS We have designed a new compact radiation shield for molecule detection. The shield consists of a collimator with the fine-honeycomb structure and a metal-mesh IR filter. It has been demonstrated that the new compact radiation shield is effective for large-scale superconducting array detectors. ACKNOWLEDGEMENT The authors thank to Dr. Takayuki TAKANO and Dr. Toshihiko NOGUCHI for their kind support in the fabrication of the metal-mesh filters. The study was supported by the JST-SENTAN project. REFERENCES 1. 2. 3. 4. 5.

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