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.