Radon sensor networks for large buildings: balancing the trade-off between energy efficiency and health

This is a copyedi ed e sion o a jou nal a icle published in m - Technisches Messen, 2025, 10.1515/ eme-2025-0036. The p esen e sion is also a ailable o open access, DOI: 10.7795/120.20250711 Radon senso ne wo ks o la ge buildings: balancing he ade-o be ween ene gy e iciency and heal h Rö ge , S e an1 0000-0002-4750-9563; Nö zel, Ral 2; Honig, Anja1 0009-0007-9744-9822; Sabo , Benoi 3 0000-0003-3043-8006; Weinbe g, Ke s in2 0000-0002-2213-8401 1 Physikalisch-Technische Bundesans al , B aunschweig, Ge many. 2 Leh s uhl ü Fes kö pe mechanik, Depa men Maschinenbau, Uni e si ä Siegen, Siegen, Ge many. 3 Labo a oi e Na ional Hen i Becque el (LNE-LNHB), Uni e si é Pa is-Saclay, CEA, Lis , Palaiseau, F ance. DOI: h ps://doi.o g/10.1515/ eme-2025-0036 Yea o publica ion: 2025 Acknowledgemen : Resea ch unding: The p ojec 23IND07 RadonNET has ecei ed unding om he Eu opean Pa ne ship on Me ology, co- inanced om he Eu opean Union’s Ho izon Eu ope Resea ch and Inno a ion P og amme and by he Pa icipa ing S a es. Copy igh in o ma ion: BY 4.0; This wo k is licensed unde he C ea i e Commons A ibu ion 4.0 In e na ional License. © 2025 he au ho (s), published by De G uy e , Be lin/Bos on Ci a ion o m: Rö ge , S e an, Nö zel, Ral , Honig, Anja, Sabo , Benoi and Weinbe g, Ke s in. "Radon senso ne wo ks o la ge buildings: balancing he ade- o be ween ene gy e iciency and heal h" m - Technisches Messen, 2025. h ps://doi.o g/10.1515/ eme-2025-0036 m–Technisches Messen 2025; aop Re iew A icle S e an Rö ge *, Ral Nö zel, Anja Honig, Benoi Sabo and Ke s in Weinbe g Radon senso ne wo ks o la ge buildings: balancing he ade-o be ween ene gy e iciency and heal h Radon-Senso ne zwe ke ü g oße Gebäude: Abwägung zwischen Ene giee izienz und Gesundhei h ps://doi.o g/10.1515/ eme-2025-0036 Recei ed Ma ch 27, 2025; accep ed June 21, 2025; published online July 11, 2025 Abs ac :Radon (222Rn) is a adioac i e noble gas ha is o med in he decay o he p imo dial 238U decay chain, wi h an a e age abundance o 3 pa s pe million (3 g ⋅ −1)in he ea h’sc us .Since adonis heonlynoblegasin hedecay chain, i has wi h i s hal li e T1 2 =3.8232(8) d, he abili y o emana e om he g ound o building ma e ials and o in e e e wi h he biosphe e. Taking in o accoun Di ec i e 2013/59/Eu a om, he EURAMET EPM p ojec RadonNET was ounded o find a ade-o be ween ene gy e iciency and heal h e ec s by means o measu emen s wi h small unce - ain ies. Wi h his in mind, an pulse ioniza ion chambe o he ime- esol ed measu emen o adon ac i i y a low con- cen a ions is desc ibed he e. The solu ion shown is cha ac- e ized by low sensi i i y o oom and impac noise. Expe - imen al in es iga ions show he p ope ies o he pulse ion- iza ion chambe unde a ious condi ions, pa icula ly wi h ega d o i s use in senso ne wo ks in la ge buildings o u u e ci ies. Keywo ds: adon; low-le el ac i i y concen a ion; Radon- NET; pulse ioniza ion chambe Zusammen assung:Radon (222Rn) is ein adioak i es Edel- gas, das beim Ze all de p imo dialen 238U-Ze alls eihe en s eh , das im E dman el du chschni lich mi 3 ppm *Co esponding au ho : S e an Rö ge , Physikalisch-Technische Bun- desans al , B aunschweig, Ge many, E-mail: [email p o ec ed] Ral Nö zel and Ke s in Weinbe g, Leh s uhl ü Fes kö pe mechanik, Depa men Maschinenbau, Uni e si ä Siegen, 57068 Siegen, Ge many, E-mail: Ral .Noe [email protected] (R. Nö zel), Ke s in.Weinbe [email protected] (K. Weinbe g) Anja Honig, Physikalisch-Technische Bundesans al , B aunschweig, Ge many, E-mail: [email p o ec ed] Benoi Sabo , Labo a oi e Na ional Hen i Becque el (LNE-LNHB), Uni e si é Pa is-Saclay, CEA, Lis , F-91120, Palaiseau, F ance, E-mail: [email p o ec ed] (3 g ⋅ −1) en hal en is . Da Radon das einzige Gas (so- ga Edelgas) in diese Ze allske e is , ha es mi sei- ne Halbwe szei T1∕2=3,8232(8) d die Möglichkei , aus dem E d eich ode Bauma e ialien zu emanie en und mi de Biosphä e zu eagie en. Un e Be ücksich- igung de Rich linie 2013/59/Eu a om wu de das EU- RAMET EPM P ojek RadonNET mi dem Ziel ge ö - de , den Komp omiss zwischen Ene giee izienz (besse- e Gebäudehüllenabdich ung) und Gesundhei (s ä ke e Du chlü ung) du ch Messungen mi ge inge Unsiche hei zu finden. In diesem Sinne wi d hie eine Impulsionisa ions- kamme ü die zei au gelös e Messung de Radonak i i ä bei nied igen Ak i i ä skonzen a ionen besch ieben. Die gezeig e Lösung zeichne sich du ch ge inge Empfindlich- kei gegenübe Raum- und T i schall, sowie ge inge Kos en aus. Expe imen elle Un e suchungen zeigen die Eigenscha - en de Impulsionisa ionskamme un e e schiedenen Be- dingungen, insbesonde e im Hinblick au ih en Einsa z in Senso ne zwe ken in g öße en Gebäudekomplexen. Schlagwö e : Radon; niede ige Ak i i ä skonzen a ionen; RadonNET; Impulsionisa ionskamme 1 In oduc ion Radon gases and hei decay p oduc s pe o m p edomi- nan ly 𝛼-decay and occu in many a eas o human li e and wo k. O na u al o igin a e mainly 222Rn ( adon) o he 238U decay chain and 220Rn ( ho on) o he 232Th decay chain. Due o he na u al dis ibu ion o u anium and ho ium in he Ea h’s c us , he adon gases can escape o he su ace. Inc eased ac i i y concen a ion can be ound in he a ea o o e deposi s, e.g., in he O e Moun ains o he Black Fo es . The adon is quickly dispe sed ou doo s, and he measu - able ac i i y concen a ions a e usually only a ew Bq m−3. Howe e , he e can be a subs an ial accumula ion o se e al Open Access. ©2025 he au ho (s), published by De G uy e . This wo k is licensed unde he C ea i e Commons A ibu ion 4.0 In e na ional License. 2—S. Rö ge e al.: Radon senso ne wo ks 100 kBq m−3in buildings, cella s, na u al ock c e ices, o mining unnels. I a human inhales ai con aining adon, a ce ain p opo ion decays in o 𝛼-emi ing decay p oduc s, and some o hem emain in he lungs. Depending on he ac i i y concen a ion o he ai and he exposu e ime, his leads o an inc eased isk o lung cance . Al hough hese ac s ha e been known o some ime, legal egula ions o p o ec ion agains adon ha e only been in place since he end o 2020 [1]. The e o e, in compliance wi h he Di ec i e 2013/59/Eu a om, membe s a es a e equi ed o mi iga e 222Rn ac i i y concen a ion in public and p i a e buildings, whe e le els egula ly exceed he e e ence h eshold o 300 Bq m−3. Au ho i ies a e manda ed o implemen mi iga ion s a egies ha adhe e o basic sa e y s anda ds, sa egua ding ci izens om ionising adia ion exposu e due o 222Rn p ogeny. The EURAMET EPM P ojec RadonNET (23IND07) [2] add esses he challenge o quan i ying 222Rn ac i i y concen a ions indoo s, pa icula ly in la ge buildings o u u e ci ies wi h a ocus on connec ed, low-ene gy consump ion buildings. The de elopmen o me hods and senso s o de ec ing adon ac i i y con- cen a ion as well as he c ea ion o quali y assu ed senso ne wo k will enhance on-si e 222Rn me ology and p o ide suppo o he Eu opean adia ion p o ec ion indus y. 1.1 Objec i es He e, he cha ac e iza ion o an adap ed pulse ioniza ion chambe o he de ec ion o 222Rn and he 𝛼-emi ing decay p oduc s 218Po, 214Po and 210Po is p esen ed. The echnical cons uc ion and he elec onical ea u es o he de ec o a e explained and a aceable calib a ion due o na ional s anda ds is pe o med which e eal he pe o - mance o his de ec o wi h ega d o he Eu opean Coun- cil Di ec i e 2013/59/Eu a om and he RadonNET p ojec goals. Two main objec i es o RadonNET a e o: 1. de elop new concep s and me hods o senso s de ec - ing adon ac i i y concen a ion wi h lowe ed esponse ime, inc eased sensi i i y and educed unce ain y compa ed o exis ing solu ions. To build he senso s in a cos -e ec i e and ma e ial-sa ing way h ough ad anced manu ac u ing echniques using indus ial p oduc ion by Small and Medium En e p ises (SME), 2. de elop aceable, in-si u ope ando calib a ion p ocedu es o hese senso s wi h less han 10 % unce ain y a an ac i i y concen a ion le el down o Bq m−3, allowing o esponse ime and dynamic linea i y es ing. These goals can be eached wi h an app op ia e ans e s anda d, as which he he e p esen ed de ec o migh se e. 1.2 De ec ion p inciples Se e al measu emen me hods ha e been es ablished o measu ing he 𝛼-ac i i y in he ai : – PIN diodes and collec ion o he decay p oduc s om 222Rn by means o high ol age on he su ace [3], – Lucas cells wi h pho o mul iplie ubes and special sen- so echnology om e.g. ANSTO [4]–[6], – Pulse Ioniza ion Chambe s wi h ai filling o ai flow [7]–[12]. Since he e ec i e su ace a ea o PIN diodes is only a ew squa e cen ime es, he 𝛼-emi ing decay p oduc s (218Po, 214Po and 210Po) a e collec ed on he su ace by means o an elec ic field. So-called Lucas cells a e combina ions o pho omul iplie ubes (PMT) and s uc u es coa ed wi h zinc sulphide. Each 𝛼-decay p oduces a flash o ligh , which is coun ed wi h he PMT; due o he sys em, spec oscopy is no possible. In con as , pulse ioniza ion chambe s wi h ai filling allow di ec measu emen o 222Rn and i s decay p od- uc s as well as 𝛼-spec ome y. Such a sys em is desc ibed below. We no e ha ega dless o he measu ing p inciple, he la ges possible ac i e de ec ion olume should be used o he measu emen o achie e a low s a is ical unce ain y wi hin he sho es possible measu ing ime. In he ollowing we p esen a modified pulse ioniza ion chambe o de ec ing 222Rn and he 𝛼-emi ing decay p od- uc s 218Po, 214Po, and 210Po. The basic s uc u e o such pulse ioniza ion chambe is always simila . Two elec odes o m a capaci o wi h ai as he dielec ic. A high DC ol age in he ange o 500 V–2500 V is applied o his capaci o . When an 𝛼-pa icle passes h ough he ai -filled a ea be ween he elec odes, he 𝛼-pa icle is slowed down by collisions wi h oxygen and ni ogen molecules, and he ai is ionized. This p ocess p oduces ee cha ge ca ie s in he o m o posi i ely and nega i ely cha ged ions and elec ons. F ee ions a e hen collec ed on he elec odes and his cha ge can be de ec ed wi h a sensi i e amplifie . As he e is a p opo ionali y be ween he amoun o cha ge and he ene gy o he 𝛼-pa icles, a pulse ioniza ion chambe enables an 𝛼-spec ome ic e alua ion o he 𝛼-ene gies. 2 A p o o ype mul iwi e pulse ioniza ion chambe Because he ai does no conduc ee elec ons, hese a e cap u ed by elec onega i e oxygen molecules wi hin S. Rö ge e al.: Radon senso ne wo ks —3 a ound 3 ⋅10−7s. Tha means ha only posi i e and nega i e ions emain a ailable as cha ge ca ie s. Unde he o ce o he elec ic field, he ee ions d i owa ds he capaci o elec odes and can be de ec ed as a cha ge pulse. The d i speed o hese ions is ela i ely low and mainly dependen on he s eng h o he elec ic field. Ex ensi e s udies we e conduc ed in e . [13] and could be confi med selec i ely by measu emen s. A a field s eng h o a ound 200 V m−1, he ions equi e a ound 2 ms–3 ms o a dis ance o 1 cm. This pa icula ea u e mus be conside ed in he mechanical cons uc ion and design o he elec onic amplifie . The ene gy o an 𝛼-pa icle ionizing he ai is p opo - ional o he numbe o ions p oduced and he e o e also p opo ional o he a ea o a cha ge pulse collec ed om an ioniza ion chambe . Due o he andom o ien a ion o an 𝛼-pa icle ajec o y in ela ion o he geome y o he pulse ioniza ion chambe , he maximum cha ge is a iable o he same 𝛼-ene gy. Ea lie in es iga ions we e domina ed by designs ha gene a ed a field s eng h ha was as homogeneous as possi- ble wi hin he pulse ioniza ion chambe . In o de o achie e his, ela i ely hick wi es o 0.5 mm up o 2 mm in diame e we e used as elec odes and small dis ances o 1 cm up o 3 cm we e chosen be ween he elec odes [12],[14]. The aim o hese measu es we e o keep he cha ge ca ie collec ion imes o he ions gene a ed by he 𝛼-decays as cons an and as sho as possible. Wi h he same cha ge ca ie collec ion ime, he pulse heigh o he signals is only dependen on he ene gy o he 𝛼-pa icles. Me hods o iginally de eloped o 𝛾-spec ome y we e used o e alua ion o enable eal- ime analysis [15].In e . [16], di e en a ian s we e in es iga ed, and 𝛼-ene gy esolu ion be ween 5 % and 10 % we e achie ed. An analysis o he possibili y o imp o ing hese esul s e eals wo dominan aspec s: Sho ened pa hs o he 𝛼-pa icles and low- equency, mechanical ib a ions caused by oom and impac noise du ing he measu emen [16]. Due o he la ge numbe and ela i ely la ge wi e diame e 𝛼-pa icles can collide wi h he wi es be o e he 𝛼-ene gy can be ully con e ed in o ionized cha ge ca ie s. This esul s in many pulses wi h lowe ene gy in he spec um. Unde he e ec o sound wa es, he dis ance be ween he pla es o wi es o he cha ged capaci o o he pulse ioniza ion chambe changes. This leads o changes in capaci ance and ol age pulses as in e e ence. Un o una ely, he equency spec um o he elec ical pulses, he mechanical na u al equencies o he pulse ioniza ion chambe , and he elec ical in e e ence om he 50 Hz powe g id a e in hesame ange. The pulse ioniza ion chambe used he e akes a di - e en app oach, ollowing he ideas o e s. [7],[8]. In e nal cons uc ion ma e ials wi hin he ajec o ies a e minimized as a as possible, and he dis ances be ween he elec odes we e chosen o c ea e an undis u bed a- jec o y o mos 𝛼-decays. Depending on he ene gy o he 𝛼-pa icles, his is be ween 4 cm and 8 cm in ai [17].The e- o e, he 𝛼-pa icles’ ene gy is p opo ional o he a ea o he pulses gene a ed. Due o he unde e mined o ien a ion o his ajec o y in ela ion o he geome y o he pulse ioniza ion chambe , he maximum alue is a iable o he same 𝛼-ene gy. To achie e an op imum measu emen esul , measu es mus be aken o minimize mechanical and elec- ical in e e ence and o accoun o he special ea u es o signal echnology. In e s. [8],[18] a solu ion is p oposed which mini- mizes he influence o mechanical ib a ions. The design o he pulse ioniza ion chambe is coaxial. Two s ainless s eel ubes wi h diame e s D1=10 mm and D3=168 mm con- nec ed elec ically ep esen one elec ode o he capaci o , D2=64 mm as a cylind ical cons uc ion made o ensioned s ainless s eel wi es (diame e 0.05 mm) ep esen s he sec- ond elec ode. The o al capaci ance C o al is made up o he calcula ed a eas esul ing om he adii a ios D2∕D1and D3∕D2 C o al =C(D2 D1)+C(D3 D2) =2𝜋𝜀0𝜀 l(1 ln(D2∕D1)+1 ln(D3∕D2))(1) He e 𝜀0and 𝜀 deno e he absolu e and ela i e pe mi - i i y, and l=370mmis hewo kingleng ho hesenso . I he ensioned wi es o D2mo e, he dis ances and he e o e he pa ial capaci ances change in opposi e di ec- ions. One pa ial capaci ance dec eases, while he second pa ial capaci ance inc eases. Se ing up he equilib ium condi ion C(D2 D1)=C(D3 D2)(2) esul s in a comple e compensa ion o he capaci ance changes. This measu e achie es e y good damping o he pulse ioniza ion chambe agains mechanical ib a ions. The equilib ium condi ion leads o D2=√D3⋅D1(3) Fo he design o ou pulse ioniza ion chambe he ol- lowing dimensions we e selec ed: D1=10 mm, D2=64 mm and D3=168 mm. Wi h a leng h o l=370 mm, he e ec i e chambe olume esul s o 7.7 L. Figu e 1 shows he ealized pulse ioniza ion chambe as a CAD cu . 4—S. Rö ge e al.: Radon senso ne wo ks Figu e 1: CAD d awing, showing a cu h ough he pulse ioniza ion chambe . The leng h o he elec odes is 370 mm and he inne diame e o he ou e elec ode is 168 mm. Figu e 2: This pic u e shows he dedica ed calib a ion se up o he pulse ioniza ion chambe . The di e en componen s shown a e gi en in he ex . As a u he measu e, he pulse ioniza ion chambe was ins alled in a s able wooden box (Figu e 2). Two ubes o ai inle and ou le we e connec ed o he pulse ioniza ion chambe and guided h ough he wooden box. The box is glued o he inside wi h a laye o 18 mm hick, high-densi y plas e boa d and placed on coo dina ed elas ome bu e s made o polyme ic oam o he ype Regu oam ib a ion 150 [19]. All in all, hese measu es lead o an ex ao dina ily good supp ession o oom and impac sound. The esul ing elec ical signals a e e y small and lie in he ange o a ound 100 μV. To supp ess any elec omag- ne ic influences, he sys em is su ounded by an elec ically conduc i e shea h. This is ea hed wi h a low esis ance. I is ecommended o ensu e ha no elec ical equalizing cu en s flow h ough his ea h connec ion. The cha ge amplifie la gely co esponds o he design acco ding o e . [14]. A low-noise field-e ec ansis o o ype BF862 amplifies he esul ing pulse. The dimensioning mus be adap ed as op imally as possible o he exis ing condi ions o he pulse ioniza ion chambe by means o op imisa ion. This means ha he discha ge esis ance mus be adap ed o he s a is ical mean alue o he cha ge ca ie collec ion ime. Thus he ollowing condi ion o he cha ge ca ie collec ion ime 𝜏mus be ulfilled: 𝜏cha ge =𝜏discha ge.(4) In ou case, he magni ude o 𝜏=3.5 ms co esponds o a discha ge esis ance o R=1GΩ. The eedback capaci o o he cha ge amplifie has a alue o 10 pF. This alue was also de e mined empi ically. A e op imiza ion, a ol age signal- o-noise a io o a ound 50:1 was achie ed wi h he cha ge amplifie . Wi h a subsequen ins umen a ion amplifie (INA111), he e e ence g ound is decoupled om he cha ge ampli- fie and a low-impedance measu emen signal is gene a ed. The o al ol age amplifica ion is 2000 and his signal is ead in o he pe sonal compu e using a USB sound ca d. Fu he p ocessing o he signals is ca ied ou wi h Ma lab [20]. 3 Expe imen al se -up and calib a ion To de e mine he esponse cha ac e is ics o he pulse ion- iza ion chambe , a dedica ed calib a ion olume has been se up a he Physikalisch-Technische Bundesans al (PTB) wi hin a clima e con olled labo a o y a ea. A pic u e o his se up is shown in Figu e 2. The di e en componen s shown in Figu e 2 a e: 1. Pulse ioniza ion chambe discussed he e (CIPIC), 2. Mul i-Wi e-Pulse-Ioniza ion-Chambe o PTB as com- pa ison (VIIK, see Figu e 3), 3. Re e ence olume o low le el adon e e ence a mo- sphe es (LLRRC), 4. Radon emana ion sou ce e e ed o as In eg a ed Radon Sou ce De ec o (IRSD) [21],wi habsolu e226Ra ac i i ies used be ween 50 Bq and 200 Bq, 5. Addi ional delay and silence olume o he VIIK. The whole calib a ion sys em is he me ically closed o adon. Volumes and connec ing ubes a e, whe e e e possible, made om s ainless s eel wi h acuum igh con- nec ions. All olumes ha e been measu ed wi h a aceable me e and he assigned unce ain ies ha e been de e mined acco ding o GUM [22]. The calib a ion olume includes he olumes z, all he ex e nal de ec o olumes x,y, he addi ional olumes |, he sou ce con aining olume {and all connec ing ubing. All he ai displacing ol- umes o he de ices placed inside we e sub ac ed om his calib a ion olume. (The ai displacing olumes we e S. Rö ge e al.: Radon senso ne wo ks —5 Figu e 3: Mul i-wi e-puls-ioniza ion-chambe o la ge olume adon de ec o s, designed by PTB [12]. The sensi i e olume o he VIIK ma ked wi h (2) in Figu e 2 is displayed he e. The gold pla ed wi es a e connec ed o di e en elec ical po en ial, o sepa a e and collec he ions o med by 𝛼-pa icles passing h ough he ai . The elec odes a e dis ibu ed o e a diame e o abou 250 mm and ha e a leng h o 300 mm. de e mined mainly by aceable weighing and he densi y o he espec i e ma e ials.) The ull unce ain y budge o he olume de e mina ion esul ed in a calib a ion olume V o al =0.6744(15) m3. The main ela i e unce ain y con i- bu ion o 99.8 % being assigned o he olume de e mina- ion o he LLRRC z. The 222Rn ac i i y concen a ion wi hin he calib a ion olume was homogenised using a memb ane pump wi h a flow a e o abou 2 L min−1. The ex e nal de ec o s (CIPIC, VIIK) whe e supplied wi h calib a ion ai h ough ano he small memb ane pump wi h a flow a e o abou 1 L min−1. The backg ound and leakage o he whole sys em was es ed using syn he ic, aged ai om a 50 L, 200 103hPa canis e a a flow a e o abou 5 L min−1con olled by a B onkho s mass flow con olle . The backg ound coun a e o he CIPIC p o ed o be in a e age 5.7(45) coun s pe 1800 s. The calib a ion sys- em p o ed 222Rn igh . Back di usion o emana ion om 226Ra con amina ed cons uc ion ma e ials a e unde u - he in es iga ion e.g., applying low le el high esolu ion 𝛾-spec ome y. 4 Calib a ion measu emen s Fo he pu pose o aceable calib a ion o he CIPIC and compa ison wi h o he adon ac i i y concen a ion de ec- o s, he signals om he CIPIC we e eco ded and a sepa a e his og am o coun s e sus channel, espec i ely Figu e 4: 𝛼-spec um o he CIPIC as his og am o coun s o e he 𝛼-pa icle-ene gy. The colou ed egions a e he i ed egions o 222Rn (blue), 218Po (o ange) and 214Po (g een). The i unc ion used is he exponen ially modi ied Gaussian. The lowe g aph shows he ela i e esidual be ween he measu emen s and he i . Figu e 5: Ene gy selec ion o he 𝛼-spec um o he CIPIC as his og am o coun s o e he 𝛼-pa icle-ene gy, ha con ain he peaks ound o he 𝛼-pa icle ene gy o 222Rn (blue) and i s p ogeny 218Po (o ange) and 214Po (g een). As in Figu e 4 an exponen ially modi ied Gaussian wi h high and low ene gy ail o e a con inuous s ep unc ion is used o de e mine he a ea unde he peak and he wid h o he peak and he e o e he esolu ion o he de ec o . ene gy was cons uc ed e e y 1,800 s (spec um). To de e - mine he ene gy esolu ion o he de ec o and he shape o he peaks, hese spec a whe e summed up o he du a ion o he calib a ion exe cise and a e shown in Figu es 4 and 5.Figu e 4 shows he o e iew o he ull ene gy ange o he de ec o om below 1 MeV up o abo e 8.5 MeV. The peaks o 222Rn (blue), 218Po (o ange) and 214Po (g een) a e ma ked. Figu e 4 is a cu o he ene gy ange o he iso opes o in e es . 222Rn (blue), 218Po (o ange) and 214Po (g een) a e fi ed wi h exponen ially modified gaussian peak shapes on op o a smoo h s ep unc ion (sigmoid) as a backg ound. The esolu ion o he peaks a e: 222Rn: 𝜎=52.07(4) keV a 5489.48(30) keV, FWHM =122.6(1) keV, (2.2 %) 218Po: 𝜎=62.65(6) keV a 6002.35(9) keV, FWHM =147.5(2) keV, (2.5 %) 6—S. Rö ge e al.: Radon senso ne wo ks 214Po: 𝜎=64.64(9) keV a 7686.82(6) keV, FWHM =152.2(3) keV, (2.0 %) which is an excellen esolu ion o an ai filed pulse ioniza- ion chambe a his size o olume. I is mo e han enough o sepa a e he di e en iso opes o 222Rn and i s p ogeny. E en 210Po s a s o become p ominen a 5304.33(7) keV in Figu e 5 bu is no e alua ed so a , due o he delayed p o- duc ion, because o he long hal li e o 210Pb o T1∕2(210Pb) = 22.23(12) a in he decay chain. The a eas unde he peaks a e used as a measu e o de e mine/calib a e agains he 222Rn ac i i y concen a ion. 4.1 The e e ence o aceable calib a ion The aceable e e ence o he SI- aceable na ional e e - ence s anda d o PTB is gi en by an ac i e moni o ed 226Ra sou ce (IRSD) [23] and he co esponding aceable me e o he de e mina ion o he olume. The olume o he calib a ion (LLRRC) is desc ibed in Sec ion 3 and shown in Figu e 2 as z. The adium sou ce, which emana es he 222Rn is an implan ed Si-diode de ec o s o Mi ion PIPS® se ies wi h se ial numbe 41-125F1 wi h 226RaCl2deposi ed ia he mal-physical apou deposi ion ( PVD) di ec ly on o i s sensi i e su ace [24]. The SI- aceable ac i i y o 226Ra de e mined by he PTB na ional s anda d is A(226Ra) = 66.4(5) Bq. 4.1.1 De e mina ion o he Rn e e ence a mosphe e The s abili y o he IRSD is moni o ed du ing he whole cali- b a ion ime aking 𝛼-pa icle ene gy spec a wi h he IRSD i sel using a p e-amplifie o ype Ame ek O ec, Model 142B connec ed o a Mi ion Lynx®da a acquisi ion egis e ing 16 k channel spec a e e y 5000 s. Figu e 6 shows a wa e all plo o he spec a collec ed du ing his calib a ion. The x-axis ep esen s he ene gy o he 𝛼-pa icles in channel comp essed by a ac o o 2. The y-axis ep esen s he da e ime he spec um was s o ed. The colou codes he numbe o e en s eco ded o he espec i e channel. P ac ically no a ia ion abo e he single coun le el can be ound du ing he whole ime. The e o e all spec a a e summed up, o de e mine he bes peak shape and fi unc ion o he e alua ion o he ime se ies o 𝛼-spec a, see Figu e 7. The 𝛼-spec a o he IRSD whe e e alua ed fi ing he app op ia e nuclide 𝛼-ene gy peaks o he measu ed coun s. As a shape o he single peaks an exponen ially modified gaussian wi h se e al high and low ene gy ail o e a con- inuous s ep unc ion (sigmoid) is used wi h an op imisa- ion p ocess, ha p e e s simila peak shapes o all peaks Figu e 6: Wa e all plo o he 𝛼-spec a acqui ed wi h he IRSD du ing he calib a ion measu emen o he CIPIC. Gi en is om bo om o op he imely e olu ion o he 𝛼-ene gy-spec a o e he comp essed channels showing he coun s pe channel in colou coding. This kind o igu e is used o check he s abili y o he e iciency and he s abili y o he de ec o and ampli ica ion o he elec onics, which in his case is excellen o e such a long ime. Figu e 7: 𝛼-ene gy-spec um o he IRSD-41-125F1 calib a ion sou ce wi h a calib a ed 226Ra ac i i y o A(226Ra) =66.4(5) Bq agains he Ge man na ional s anda d. 226Ra and i s p ogeny wi h hei con ibu ion o he peaks a e shown in di e en colou s (see legend). The da k blue cu e is he sum o all peak a eas and is compa ed wi h he black do s which a e he esul s o he sum o he measu emen s o he IRSD. The ela i e di e ence be ween he sum o he peak a eas and he measu ed coun s is shown in he esidual below. 𝜒2=4.21 is a measu e o he goodness o he model and he i . bu allows o di e ences adding a penal y unc ion in he loss unc ion o he op imize o a ying shapes. Excel- len ag eemen be ween model and da a is und as shown in he esiduals o Figu e 7 wi h a small 𝜒2=4.21. This shape o he summed 𝛼-ene gy spec um is hen ans e ed o all he single spec a o p oduce a ime and iso ope esol ed se ies o coun s which is necessa y o he fil e ing p ocess. Using his ime esol ed da a i is possible o de e - mine he s abili y and unce ain y o he IRSD s anda d. S. Rö ge e al.: Radon senso ne wo ks —7 Figu e 8: F equency dis ibu ion o he ela i e di e ence o he a eas o he peak i s o he 226Ra decay ene gy om he IRSD o he 5000 s in e al measu emen s du ing he en i e calib a ion exe cise is shown in his igu e. The esidual dis ibu ion which is dis ibu ed well wi hin ±0.5 % demons a es he quali y, s abili y and e iciency o he IRSD used e en wi h such low, o al ac i i y as A(226Ra) =66.4(5) Bq. The e o e he 226Ra da a is used and plo ed as his og am o he ela i e di e ence be ween he a e age and he single alues – see Figu e 8. He e he quali y, e iciency and s abili y is demon- s a ed h ough a dis ibu ion ha is well wi hin ±0.5 % o a sou ce wi h an o al ac i i y o only A(226Ra) =66.4(5) Bq. The e e ence 222Rn ac i i y concen a ion in he cali- b a ion olume in non-s eady-s a e si ua ions is es ima ed using he Swi ching Linea Dynamical Sys em (SLDS) decon- olu ion app oach o he obse ed coun s ime-se ies, which is e alua ed om he IRSD spec ome ic ime-se ies using he me hods o he Sa i zky Golay Fil e [25] o smoo h- ing o he da a and ega ding he s a e o he sys em as s ochas ic di e en ial equa ion (SDE) in he I  o sense. Using ecu si e Bayesian es ima ion yield o a Kalman-Fil e o s a e-in eg a ing, disc e ized Gaussian p ocesses, as ealised in e . [21]. The esul s o he applica ion o such a fil e ing p ocess on he da a collec ed wi h he IRSD in he calib a ion sys em used, is summa ised in Figu e 9. Figu e 9 combines he measu ed and ime esol ed fi - ed da a o he IRSD ep esen ed in he blue do s wi h he le scale in he second and hi d ow o 222Rn and 226Ra, espec i ely. The esul s o he fil e ing p ocess is gi en in he fi s ou ows as black line wi h g ey unce ain y a eas Figu e 9: E alua ion o he IRSD measu emen s using a Kalmann il e sys em wo king on he ime and ene gy esol ed da a gene a ed wi h he IRSD and p e-e ealua ed i ing he peak a eas o he di e en nuclides using he i shown in Figu e 7. F om op o bo om: he i s ow shows he compa ison o he 222Rn peak a ea esul s (g een do s) de e mined o e ime o 1800 s ime in e als calib a ed o Bq m−3o e ime, compa ed o he esul s in e ed om he IRSD ha was used o pe o m he calib a ion. The black line ep esen s he alue and he g ey a ea he s anda d unce ain y (k=1) a his poin in ime and connec ed o he igh axis. The same sys ema ics is used o he uppe ou ows. The second ow shows he peak a eas o he 222Rn peaks in e ed om he IRSD o 5000 s in e als as blue do s wi h espec o he le scale. This is compa ed o he ac i i y o 222Rn measu ed in he sou ce o he IRSD. The hi d ow shows he same as he second bu wi h espec o 226Ra. The o h ow shows calcula ed numbe o emana ed 222Rn a oms pe second om he sou ce o he IRSD. The i h ow shows he ela i e humidi y in he calib a ion olume LLRRC ( eco ded wi h he AlphaGURAD 1950 de ec o p esen as a e e ence ins umen du ing all he calib a ion) shown he e because o he main dependency o he emana ion om changing humidi y. 8—S. Rö ge e al.: Radon senso ne wo ks (k=1) using he igh scales. The new esul calcula ed wi h he fil e is he ou h ow which shows he eleased 222Rn a oms pe second (𝜂/s−1). This shows some e y mino a i- a ion bu no significan co ela ion wi h he main en i on- men al pa ame e which is he humidi y. Fo compa ison he ela i e humidi y is gi en in he fi h ow. And he 222Rn ac i i y concen a ion du ing he calib a ion in he olume which is shown in he fi s ow (AV Rn/Bq m−3, igh scale). The fi s ow compa es his absolu e aceable e e ence alue wi h he calib a ed measu emen s om he CIPIC, which is de i ed om he calib a ion explained in Sec ion 5. 5 Resul s Taking he 222Rn ac i i y concen a ion de e mined wi h he IRSD as e e ence, i is possible o p epa e a co ela ion plo be ween he imely synch onised alues om he IRSD (on he x-axis) and he co esponding de e mined 222Rn a eas om he peak fi s o he spec a Figu e 4 (on he y-axis). This plo is shown in Figu e 10. ThebluelineinFigu e 10 is de e mined ia a linea fi o all he da a acqui ed du ing he calib a ion. Wi h he help o his calib a ion plo one can de e mine he esidual dis ibu ion o he CIPIC coun s a eas o 222Rn calib a ed o Bq m−3 o he linea fi . This dis ibu ion is shown in Figu e 11. Figu e 10: Calib a ion plo o he single spec a measu emen s wi h he CIPIC in e ed 222Rn 𝛼-peak a eas agains he in e ed esul s om he 𝛼-spec a o he IRSD. The blue line indica es he i ed linea calib a ion o all da a poin s du ing he calib a ion exe cise. The dis ibu ion o he di e ence be ween his de e mined calib a ion and he measu emen esul s o he CIPIC is gi en in Figu e 11. Figu e 11: F equency dis ibu ion o he de ia ion o he CIPIC measu emen s o he 222Rn 𝛼-peak a ea calib a ed o Bq m−3 om he calib a ion de e mined wi h he measu emen esul s in e ed om he IRSD. The dis ibu ion shows a well cen ed shape wi h a na ow dis ibu ion demons a ing he quali y o he CIPIC esul s. Figu e 12: G ouping all he measu emen s om Figu e 10 in equal dis ances o ac i i y concen a ion gi en by he IRSD, a dedica ed calib a ion plo om 2 Bq m−3 o 56 Bq m−3is gi en he e as boxed plo showing he unce ain y in e al o each g oup o ac i i y concen a ion. The s igh blue line is he de e mined calib a ion which is in good ag eemen wi h all de e mined unce ain y in e als. I shows a Gaussian shape wi h an app oxima e FWHM o abou 5 Bq m−3, which is a e y good esul ega ding he ac i i y concen a ion ange which is below 56 Bq m−3. G ouping o he da a allows o a mo e de ailed s a is- ical iew on he calib a ion, which is gi en in Figu e 12, showing he box plo o he calib a ion. The so de e mined linea calib a ion o he CIPIC is gi en by he o se o 2.677(9) coun s and he linea ac o o 5.958(2) coun s Bq−1. All he essen ial pa ame e s and ea u es o he used pulse ioniza ion cambe a e combined o Table 1.