{"id":7297,"date":"2020-07-09T22:13:45","date_gmt":"2020-07-09T22:13:45","guid":{"rendered":"https:\/\/www.particlebites.com\/?p=7297"},"modified":"2020-07-09T22:13:46","modified_gmt":"2020-07-09T22:13:46","slug":"crystals-are-dark-matters-best-friends","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=7297","title":{"rendered":"Crystals are dark matter\u2019s best friends"},"content":{"rendered":"

Article title:<\/strong> “Development of ultra-pure NaI(Tl) detector for COSINE-200 experiment”
\n<\/strong><\/p>\n

Authors:<\/strong> B.J. Park et el.<\/p>\n

Reference:<\/strong> arxiv:2004.06287<\/a><\/p>\n

The landscape of direct detection of dark matter is a perplexing one; all experiments have so far come up with deafening silence, except for a single one which promises a symphony. This is the DAMA\/LIBRA experiment in Gran Sasso, Italy, which has been seeing an annual modulation in its signal for two decades now.<\/p>\n

Such an annual modulation is as dark-matter-like as it gets. First proposed by Katherine Freese in 1987, it would be the result of earth\u2019s motion inside the galactic halo of dark matter in the same direction as the sun for half of the year and in the opposite direction during the other half. However, DAMA\/LIBRA\u2019s results are in conflict with other experiments \u2013 but with the catch that none of those used the same setup. The way to settle this is obviously to build more experiments with the DAMA\/LIBRA setup. This is an ongoing effort which ultimately focuses on the crystals at its heart.<\/p>\n

\"\"<\/a>
Cylindrical crystals wrapped in reflector, bounded by photomultipliers (PMTs) and surrounded by scintillators. (COSINE-100<\/a>)<\/figcaption><\/figure>\n

The specific crystals are made of the scintillating material thallium-doped sodium iodide, NaI(Tl). Dark matter particles, and particularly WIMPs, would collide elastically with atomic nuclei and the recoil would give off photons, which would eventually be captured by photomultiplier tubes at the ends of each crystal.<\/p>\n

Right now a number of NaI(Tl)-based experiments are at various stages of preparation around the world, with COSINE-100 at the Yangyang mountain, S.Korea, already producing negative results. However, these are still not on equal footing with DAMA\/LIBRA\u2019s because of higher backgrounds at COSINE-100. What is the collaboration to do, then? The answer is focus even more on the crystals and how they are prepared.<\/p>\n

\"\"<\/a>
Setup of the COSINE-100 experiment. (COSINE-100<\/a>)<\/figcaption><\/figure>\n

Over the last couple of years some serious R&D went into growing better crystals for COSINE-200, the planned upgrade of COSINE-100. Yes, <\/span>a <\/span>crystal <\/span>is something that<\/span> can and <\/span>does <\/span>grow. A seed placed inside the raw material, in this case <\/span>NaI(Tl) <\/span>powder, leads it to organiz<\/span>e<\/span> itself around the seed\u2019s structure over the next <\/span>hours or<\/span> days. <\/span><\/p>\n

In COSINE-100 the most annoying backgrounds came from within the crystals themselves because of the production process, because of natural radioactivity, and because of cosmogenically induced isotopes. Let\u2019s see how each of these was tackled during the experiment\u2019s mission towards a radiopure upgrade.<\/p>\n

Improved techniques of growing and preparing the crystals reduced contamination <\/span>from <\/span>the materials of the grower device <\/span>and from <\/span>the <\/span>ambient environment. At the same time different raw materials were tried out to put the inherent contamination under control. <\/span><\/p>\n

Among a <\/span>handful of naturally present radioactive isotopes<\/span> particular care was given to 40<\/sup>K. <\/span>40<\/sup>K <\/span>can <\/span>decay<\/span> characteristically<\/span> to an X-ray <\/span>of 3.2keV<\/span> and a <\/span>\u03b3<\/span>-ray <\/span>of 1,460keV<\/span>, <\/span>a combination<\/span> convenient for tagging it to a large extent. <\/span>The tagging<\/span> is done with the help of <\/span>2,000 <\/span>liters<\/span> of liquid scintillator <\/span>surrounding the crystals. However, <\/span>if the <\/span>\u03b3<\/span>-ray escapes <\/span>the crystal <\/span>then the left-behind X-ray will mimic the expected signal from WIMPs\u2026 <\/span>Eventually the dangerous <\/span>40<\/sup>K <\/span>was brought down to<\/span> levels comparable to those in <\/span>DAMA\/LIBRA <\/span>through the investigation <\/span>of various techniques and first materials. <\/span><\/p>\n

B<\/span>ut <\/span>the main source of radioactive background <\/span>in COSINE-100<\/span> was <\/span>isotopes such as 3<\/sup>H or 22<\/sup>Na <\/span>created<\/span> inside the crystals<\/span> by cosmic ray <\/span>muons,<\/span> after their <\/span>production<\/span>. <\/span>Now,<\/span> t<\/span>heir abundance was reduced significantly by <\/span>two simple moves: <\/span>the crystals <\/span>we<\/span>re grown locally <\/span>at a very low altitude and installed <\/span>underground <\/span>within a few weeks (<\/span>instead of <\/span>being <\/span>transported from a lab at 1,<\/span>400<\/span> meters <\/span>above sea in <\/span>Colorado). Moreover, most of th<\/span>e remaining cosmogenic<\/span> background <\/span>is to <\/span>decay away within a couple of years. <\/span><\/p>\n

\"\"<\/a>
Components of the background, and temporal evolution of the cosmogenic radioactivity. (Source<\/a>)<\/figcaption><\/figure>\n

Where are these efforts standing? <\/span>The energy range of interest for testing the DAMA\/LIBRA signal is 1-6keV. This corresponds to a background target of 1 count\/kg\/day\/keV. <\/span>After the crystals R&D, t<\/span>he achieved contamination was less than about 0.34 counts. <\/span>In short, e<\/span>verything is ready for <\/span>COSINE-<\/span>1<\/span>00 <\/span>to upgrade to <\/span>COSINE-<\/span>2<\/span>00 <\/span>and test the annual <\/span>modulation without the <\/span>previous <\/span>ambiguities <\/span>that stood in the way. <\/span><\/p>\n

Learn more: <\/strong><\/p>\n

More<\/a> on DAMA\/LIBRA in ParticleBites.<\/p>\n

Cross-checking<\/a> the modulation.<\/p>\n

The COSINE-100<\/a> experiment.<\/p>\n

First COSINE-100 results<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

Or how to cross-check dark matter hopes.<\/p>\n","protected":false},"author":31,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[38,57],"tags":[10,21],"class_list":["post-7297","post","type-post","status-publish","format-standard","hentry","category-dark-matter","category-experimental-techniques","tag-dark-matter","tag-experiment"],"_links":{"self":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts\/7297","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/users\/31"}],"replies":[{"embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=7297"}],"version-history":[{"count":12,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts\/7297\/revisions"}],"predecessor-version":[{"id":7315,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=\/wp\/v2\/posts\/7297\/revisions\/7315"}],"wp:attachment":[{"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7297"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7297"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.particlebites.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7297"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}