This is a sel -a chi ed e sion o an o iginal a icle. This e sion
may di e om he o iginal in pagina ion and ypog aphic de ails.
Au ho (s):
Ti le:
Yea :
Ve sion:
Copy igh :
Righ s:
Righ s u l:
Please ci e he o iginal e sion:
CC BY 4.0
h ps://c ea i ecommons.o g/licenses/by/4.0/
Alpha spec oscopy o pu i ied beams o exo ic nuclei a he FRS Ion Ca che
© 2024 The Au ho s. Published by Else ie B.V.
Published e sion
To o elli, Naza ena; Rei e , Mo i z Pascal; Rink, Ann Ka h in; Pu usho haman,
Si aji; Aye , San And és Samuel; Be gmann, Julian; Dickel, Timo; Diwisch, Ma cel;
Ebe , Jens; Geissel, Hans; G eine , Flo ian; Hae ne , Emma; Ho nung, Ch is ine;
Kelic-Heil, Aleksand a; Knoebel, Ronja; Lippe , Wayne; Miskun, I an; Moo e, Iain
D.; Pie i, S ephane; Plaß, Wol gang R.; Pohjalainen, Ilkka; P ochazka, And ej;
Scheidenbe ge , Ch is oph; Takechi, Maya; Thi ol , Pe e G.; Weick, Helmu ;
Win ield, John; Xu, Xiaodong
To o elli, N., Rei e , M. P., Rink, A. K., Pu usho haman, S., Aye , S. A. S., Be gmann, J., Dickel, T.,
Diwisch, M., Ebe , J., Geissel, H., G eine , F., Hae ne , E., Ho nung, C., Kelic-Heil, A., Knoebel,
R., Lippe , W., Miskun, I., Moo e, I. D., Pie i, S., . . . Xu, X. (2025). Alpha spec oscopy o
pu i ied beams o exo ic nuclei a he FRS Ion Ca che . Nuclea Physics A, 1053, A icle 122967.
h ps://doi.o g/10.1016/j.nuclphysa.2024.122967
2025
Nucl. Phys. A 1053 (2025) 122967
A ailable online 3 Oc obe 2024
0375-9474/© 2024 The Au ho s. Published by Else ie B.V. This is an open access a icle unde he CC BY license
(h p://c ea i ecommons.o g/licenses/by/4.0/).
Con en s lis s a ailable a ScienceDi ec
Nuclea Physics A
jou nal homepage: www.else ie .com/loca e/nuclphysa
Alpha spec oscopy o pu ified beams o exo ic nuclei a he FRS
Ion Ca che
Naza ena To o elli a,c, ,∗, Mo i z Pascal Rei e b, , Ann Ka h in Rinkb,
Si aji Pu usho hamanc, Samuel Aye San And és c, Julian Be gmann b,
Timo Dickel c,b, Ma cel Diwisch b, Jens Ebe b, Hans Geissel c,b,1, Flo ian G eine b,
Emma Hae ne c, Ch is ine Ho nungb, Aleksand a Kelic-Heilc, Ronja Knoebelc,
Wayne Lippe b, I an Miskunb, Iain D. Moo e d, S ephane Pie i c,
Wol gang R. Plaßc,b, Ilkka Pohjalainend, And ej P ochazka c,
Ch is oph Scheidenbe ge c,b,e, Maya Takechi c, Pe e G. Thi ol a, Helmu Weick c,
John Winfieldc,1, Xiaodong Xuc
aLudwig-Maximilians-Uni e si ä München, 85748 Ga ching, Ge many
bII. Physikalisches Ins i u , Jus us-Liebig-Uni e si ä Gießen, 35392 Gießen, Ge many
cGSI Helmhol zzen um ü Schwe ionen o schung GmbH, 64291 Da ms ad , Ge many
dUni e si y o Jy äskylä, 40014 Jy äskylä, Finland
eHelmhol z Resea ch Academy Hesse o FAIR (HFHF), GSI Helmhol z Cen e o Hea y Ion Resea ch, Campus Gießen, 35392 Gießen, Ge many
Uni e si y o Edinbu gh, Uni ed Kingdom
A R T I C L E I N F O A B S T R A C T
Keywo ds:
FRS-IC
𝛼-spec oscopy
Nuclea s uc u e
Hal -li e
The FRS Ion Ca che (FRS-IC) is loca ed a he final ocal plane o he F agmen Sepa a o FRS
a GSI. The FRS-IC se up is well known o high-p ecision expe imen s wi h s opped exo ic nuclei
p oduced by p ojec ile agmen a ion and fission. The acili y consis s o he c yogenic gas-filled
s opping cell (CSC), an RFQ-based beamline (DISTRICT), and a mul iple- eflec ion ime-o -fligh
mass spec ome e (MR-TOF-MS). This pape illus a es how alpha spec oscopy pe o med a his
acili y has eme ged as a p omising ool o un eil he nuclea s uc u e o exo ic nuclei, i.e., hal -
li e and decay ene gy measu emen s. Fi s s udies o ha kind we e pe o med on he decay chains
o
218Rn,
219Rn,
221Ac,
220F , and
223,224Th p oduced by p ojec ile agmen a ion o
238U. The 𝛼
decay ene gy measu emen s pe o med and he deduced Q𝛼 alues confi m he known maximum
a N= 128 and he alues o Q𝛼a N= 132 − 133 ollow he p edic ed inc easing in Q𝛼 alues
compa ed o he alues o A iso opes a he same neu on numbe N. Fu he , he p oduc ion
a e a io o he isome o he g ound s a e o
211Po was measu ed. I allows an es ima e o he
angula momen um dis ibu ion o
211Po agmen s ollowing agmen a ion o
238U in a
9Be a ge
a ela i is ic ene gies. In addi ion, he po en ial o mass-selec ed decay spec oscopy behind he
MR-TOF-MS was demons a ed wi h sho -li ed
215Po ions (𝑡1∕2 =1.78 ms). This demons a es ha
he FRS-IC is a eliable se up o 𝛼spec oscopy s udies and ela ed nuclea s uc u e s udies.
* Co esponding au ho .
E-mail add esses: [email p o ec ed], [email p o ec ed] (N. To o elli).
1Deceased.
h ps://doi.o g/10.1016/j.nuclphysa.2024.122967
Recei ed 26 July 2024; Recei ed in e ised o m 30 Sep embe 2024; Accep ed 30 Sep embe 2024
Nuclea Physics, Sec ion A 1053 (2025) 122967
2
N. To o elli, M.P. Rei e , A.K. Rink e al.
1. In oduc ion
E e since he disco e y o he phenomenon in 1899 by Ru he o d [1], and he empi ical e idence o he ela ionship be ween
he hal -li es and decay ene gies in 1911 by Geige and Nu all [2], nuclea alpha decay has been used in a la ge numbe o ap-
plica ions. The pionee ing wo k by Ru he o d s udying he in e ac ion o alpha adia ion wi h ma e led o he disco e y o he
a omic nucleus. The ole o alpha spec oscopy as an impo an expe imen al nuclea physics echnique was s eng hened wi h he
heo e ical explana ion o he alpha decay mechanism using quan um-mechanical unneling by Gamo in 1928 [3]. S a ing om
he second hal o he las cen u y, alpha spec oscopy has been employed in a ious scena ios as a powe ul iden ifica ion me hod
o adioac i e iso opes [4], especially as a diagnos ic me hod in adioac i e ion beam acili ies. High- esolu ion alpha spec oscopy
can p o ide a unique finge p in o an alpha-decaying nuclide in e ms o he cha ac e is ic alpha ene gies and b anching a ios. The
iden ifica ion o long alpha chains has been hea ily used in he sea ch o new supe -hea y elemen s [5]and s udies o ac inides
[6]. The popula ion o di ec alpha decaying nuclea isome s has been employed in cha ac e izing he angula momen um in a ious
nuclea eac ions: agmen a ion [7], usion [8], and ecen ly mul i-nucleon ans e eac ions [9]. In addi ion, he cha ac e iza ion o
alpha-decaying nuclides has aided nuclea s uc u e s udies in he lead egion in combina ion wi h high- esolu ion lase spec oscopy
[10]o ime-o -fligh mass spec ome y [11]. This pape discusses he implemen a ion o an alpha spec oscopy se up a he FRS
Ion Ca che (FRS-IC) a GSI [12]. The FRS-IC has been designed o high-p ecision expe imen s wi h s opped exo ic nuclei p oduced
by p ojec ile agmen a ion and fission. The FRS Ion Ca che consis s o he c yogenic gas-filled s opping cell (CSC) [13–15], an RFQ
anspo and diagnos ics beam line (DISTRICT), and a mul iple- eflec ion ime-o -fligh mass spec ome e (MR-TOF-MS) [16–18].
Alpha spec oscopy o he s opped exo ic nuclei using silicon de ec o s has been employed as one o he key ools o physics measu e-
men s as well as a sys ema ic cha ac e iza ion ool o he FRS-IC. The fi s s udies we e pe o med by in es iga ing he decay chains
o
218Rn,
219Rn,
221Ac,
220F and
223,224Th p oduced by p ojec ile agmen a ion o
238U. Ano he key physics esul om he FRC-IC
employing alpha spec oscopy was he de e mina ion o he angula momen um dis ibu ion o
211Po ollowing he agmen a ion
eac ion
238U+ 9Be a ela i is ic ene gies [19]. Including he mass-selec ion me hod shown in ha pape , i is possible o pe o m
decay spec oscopy o sho -li ed nuclides ha ing a pa en ’s backg ound- ee decay, exemplified by he hal -li e measu emen o he
e y sho -li ed iso ope
215Po (𝑡1∕2 =1.78 ms) which we epo he e.
2. FRS Ion Ca che (FRS-IC)
The FRS-IC is loca ed a he final ocal plane o he FRagmen Sepa a o FRS a GSI [20]. I is an expe imen al acili y whe e he
ad an ages o ISOL and in-fligh sepa a o echniques a e elegan ly combined o enable low-ene gy, high-p ecision expe imen s o
exo ic ions p oduced a ela i is ic ene gies.
Ions ex ac ed om he hea y-ion synch o on SIS-18 impinge on a p oduc ion a ge a he FRS o p oduce p ojec ile o fission
agmen s. The spa ial sepa a ion o he agmen s is pe o med by wo- old magne ic igidi y analysis in on o and behind a
mono-ene ge ic deg ade placed a he cen al ocal plane. The e y sho fligh ime o a ew hund ed nanoseconds h ough he
FRS gi es access o e y sho -li ed nuclides and allows iso opes o be iden ified using he in-fligh pa icle de ec o s o he FRS on
a ion-by-ion (e en -by-e en ) basis. The in-fligh sepa a ion echnique is pa icula ly challenging o nuclei p oduced a ela i is ic
ene gies because o he la ge longi udinal and ans e se emi ance o he ions a e p oduc ion and slowing down. The ions a e fi s
ange-bunched [21]using a dispe si e magne ic s age and a mono-ene ge ic deg ade sys em o educe he ange sp ead o he ions
by up o an o de o magni ude, which makes s opping acco dingly mo e efficien .
A e p oduc ion and sepa a ion, he ions a e hen slowed down and finally s opped and he malized in a gas-filled s opping cell.
Typically, he s opping cell is filled wi h helium gas a a p essu e on he o de o 100 mba a c yogenic empe a u e. The he malized
ions a e hen guided o he exi side o he cell using s a ic DC elec ic fields, ocused o an exi nozzle using an RF ca pe applying AC
and DC fields, and ex ac ed as an ion beam wi h low kine ic ene gy, see de ails in [13–15]. The ions ex ac ed om he s opping cell
may be p e-fil e ed o a ce ain mass-o e -cha ge ange o in e es ia an ex ac ion RF quad upole (RFQ) ope a ed as quad upole
mass fil e (QMF) [22]. The ions hen en e a low-ene gy (∼eV) RFQ beamline which p o ides fi s diagnos ics ools, calib a ion ion
sou ces, anspo , isola ion as well as cooling and apping; om he e on e e ed o as he DISTRICT module (DISTRICT, Diagnos ics,
Ion Sou ces, T anspo , Isola ion, Cooling and T apping module). The fi s alpha spec oscopy de ices o he FRS-IC a e si ua ed in
he fi s DISTRICT acuum chambe . DISTRICT is equipped wi h a emo e-con olled ansla ion s age ha accommoda es wo silicon
de ec o s, a channel on de ec o and an RFQ segmen such ha he ions can ei he be iden ified and quan ified by hei alpha-decay
ene gy and hal -li e o ansmi ed downs eam o he mul iple- eflec ion ime o fligh mass spec ome e (MR-TOF-MS). The MR-
TOF-MS i sel is a powe ul and uni e sal mass spec ome e wi h single-ion sensi i i y, a mass esol ing powe (FWHM) as high as
106and mass accu acy o low 10−8 [23]. In he MR-TOF-MS, he ions a e accumula ed, cooled by collisions wi h helium buffe gas
in an RF ap sys em, and hen injec ed as bunches in o he ime-o -fligh analyze , whe e hey fly o a ce ain numbe o u ns un il
hey exi he analyze and impinge on a ime-o -fligh de ec o o mass measu emen s o a B adbu y-Nielsen ga e o mass selec ion
ollowed by ano he silicon de ec o . This is he second alpha spec oscopy de ice o he FRS-IC. To illus a e his desc ip ion a ske ch
o his acili y is shown in Fig. 1. Howe e , an in-dep h desc ip ion o he expe imen al se up and he wo king p inciple o he ull
FRS-IC se up is no wi hin he scope o his wo k and is desc ibed in de ail elsewhe e [12,15]. The ollowing p esen a ion o he
expe imen al se up will ocus on he alpha spec oscopy sys ems o he FRS-IC.
Nuclea Physics, Sec ion A 1053 (2025) 122967
3
N. To o elli, M.P. Rei e , A.K. Rink e al.
Fig. 1. Schema ic layou o he agmen sepa a o FRS oge he wi h he FRS Ion Ca che .
3. FRS-IC alpha spec oscopy sys ems
The alpha spec oscopy se up ins alled in he fi s DISTRICT module con ains wo iden ical de ec o se ups on a emo e-con olled
ansla ion s age. This edundancy addi ionally acili a es as swi ching be ween de ec o s and educed decay backg ound by swi ch-
ing o a clean de ec o . The Si de ec o s used a e silicon ion-implan ed cha ged pa icle de ec o s manu ac u ed by ORTEC om he
ULTRA se ies wi h 100 μm deple ion dep h and 150 mm2ac i e a ea (O ec Ul a 𝐵𝑈 − 016 − 150 − 100 bakeable). The de ec o and
i s da a acquisi ion (DAQ) sys em (CAEN DT5780SDM) a e op imized o achie e op imum ene gy esolu ion, ypically sigma be ween
30 o 60 keV a e achie ed.
In o de o cha ac e ize he pe o mance and op imize he sys em offline, an in e nal sou ce emi ing alpha-decaying nuclei is
needed. Fo his pu pose, se e al ion sou ces ha e been ins alled, a
223Ra alpha ecoil sou ce wi h a hal -li e o ≈11days has been
used ini ially, whe eas mo e ecen ly a
228Th sou ce wi h a hal -li e o ≈2yea s was ins alled a he en ance side o he CSC. As
alpha ecoils a e ypically emi ed wi h ≤100 keV ene gy, hey can easily be s opped in he CSC and deli e ed o he DISTRICT se up.
In addi ion a 3-line alpha sou ce con aining
239Pu,
241Am and
244Cm is ins alled di ec ly in DISTRICT a an off-axis loca ion.
Ions om he in e nal sou ces and hose om online p oduc ion a e anspo ed o he se up and he ene gy o he alpha pa icles
emi ed can be measu ed. Howe e , in o de o no accumula e long-li ed ac i i y di ec ly on he de ec o su ace, an aluminum
collec ion oil wi h hickness o 100 μg/cm2o 200 μg/cm2is ins alled in on o each de ec o . The oils a e placed 5mm in on o
he de ec o su ace, which limi s he maximum solid angle co e ed by he Si de ec o . In ou case he de ec o co e s a solid angle
o 𝑑Ω∕4𝜋=0.21, assuming a poin -like implan a ion spo a he cen e o he oil. Simula ions sugges an implan a ion spo size o
less han 1 mm; hus, he poin -like spo is a alid assump ion.
The pe o mance o he alpha de ec o pe manen ly ins alled as pa o DISTRICT will be epo ed in he ollowing pa ag aph.
The alpha de ec ion sys em loca ed a he end o he MR-TOF-MS will be discussed in Sec. 5.
3.1. Pe o mance cha ac e iza ion
Recoiling ions om he
223Ra sou ce we e ex ac ed and collec ed on he oil in on o one o he Si de ec o s. Fig. 2shows ypical
alpha ene gy spec a as ob ained wi h DISTRICT unde h ee diffe en scena ios: no oil ins alled, 100 μg/cm2and 200 μg/cm2 oils
ins alled. Wi hou a oil ins alled, he se up eaches an alpha ene gy esolu ion o abou 14 keV in sigma, abou a ac o o wo lowe
compa ed o he manu ac u e ’s specifica ions. Howe e , wi h a oil ins alled he ene gy esolu ion o he alpha spec oscopy de ec o s
is dominan ly limi ed by he collec ion oils hemsel es, as hose cause addi ional peak b oadening due o wo effec s. Fi s ly, due o
di ec ene gy s aggling 𝜎𝑑𝑖𝑟𝑒𝑐𝑡 o he alpha pa icles pene a ing h ough he oil and, secondly, due o peak b oadening 𝜎𝑎𝑛𝑔𝑙𝑒 caused
by alpha pa icles pene a ing he oil unde diffe en angles gi en by he solid angle co e age o he de ec o , which esul s in a
a ia ion o effec i e hickness when pene a ing he oil [24]. As shown in Fig. 2, a peak wid h o 𝜎𝑒𝑥𝑝 =26keV and 𝜎𝑒𝑥𝑝 =48keV
could be achie ed o 100 μg/cm2and 200 μg/cm2 hick aluminum oils, espec i ely. The peak b oadening by he oil se up was
u he es ima ed ia SRIM ene gy loss calcula ions. In Table 1, he expe imen al esolu ion is compa ed o he es ima es. The 𝜎𝑑𝑖𝑟𝑒𝑐𝑡
and 𝜎𝑎𝑛𝑔𝑙𝑒 alues a e quad a ically added o he in insic de ec o esolu ion o 14 keV achie ed wi hou oil, which gi es an es ima e
o he expec ed o al peak wid h 𝜎𝑡𝑜𝑡𝑎𝑙 o be ob ained when using collec ion oils. The calcula ed peak wid hs a e in good ag eemen
wi h he expe imen al esul s and confi m he limi s se by he oils; no measu emen was pe o med o a oil hickness o 50 μg/cm2.
I is u he possible o employ e en hinne oils, such as e.g. 10 μg/cm2as ecen ly demons a ed in [25].
Nuclea Physics, Sec ion A 1053 (2025) 122967
4
N. To o elli, M.P. Rei e , A.K. Rink e al.
Fig. 2. Alpha decay spec um o
219Rn, o igina ing om he in e nal
223Ra sou ce and measu ed using he silicon de ec o o he DISTRICT wi h Al collec ion oil
hicknesses o 200 μg/cm2and 100 μg/cm2, espec i ely, placed in on o he de ec o su ace in compa ison o he esolu ion achie able wi h he ba e de ec o . The
measu emen s a e pe o med unde diffe en cleanliness and ex ac ion condi ions, which esul s in diffe en elemen s o be ex ac ed and anspo ed wi h diffe en
efficiencies. The h ee da ase s we e no acqui ed du ing iden ical acquisi ion imes and no no maliza ion was applied, such ha he numbe o coun s is plo ed in
a bi a y uni s.
Table 1
Compa ison o he peak b oadening induced by aluminum
oils placed in on o he silicon de ec o su ace o p e-
en long-li ed ac i i y deposi ion on he de ec o . The sili-
con de ec o se up has an in insic alpha ene gy esolu ion o
14 keV. 𝜎𝑑𝑖𝑟𝑒𝑐𝑡 e e s o he con ibu ion due o di ec ene gy
s aggling o he alpha pa icles pene a ing h ough he oil,
while 𝜎𝑎𝑛𝑔𝑙𝑒 akes in o accoun he peak b oadening con ibu-
ion caused by he dependency o he alpha pa icles on he
ene gy om he pene a ion angle in o he oil. The quad a ic
sum o hese wo con ibu ions is gi en as 𝜎𝑡𝑜𝑡𝑎𝑙 . 𝜎𝑒𝑥𝑝 e e s
o he expe imen ally measu ed ene gy esolu ion.
Al oil 𝜎𝑑𝑖𝑟𝑒𝑐𝑡 𝜎𝑎𝑛𝑔𝑙𝑒 𝜎𝑡𝑜𝑡𝑎𝑙 𝜎𝑒𝑥𝑝
μg/cm2(keV) (keV) (keV) (keV)
200 8.837 40.948
100 6.118 24.226
50 4.311.319.3-
Howe e , hicke oils a e ypically conside ed mo e s able and he eliabili y o he se up in DISTRICT, a s anda d commissioning
and cha ac e iza ion ool, was conside ed o be mo e impo an . As such, oils wi h a hickness o 100 μg/cm2we e chosen o
s anda d ope a ion and we e used du ing all measu emen s epo ed in his pape . Unde hese condi ions alpha spec oscopy o
sepa a ed seconda y beams can be pe o med wi h ene gy esolu ions o abou 60 keV sigma.
4. Op imiza ion o he FRS-IC pe o mance
The ollowing sec ion is de o ed o offline and online p ocedu es and echnical de elopmen s o op imize he pe o mance o he
FRS Ion Ca che se up.
4.1. Offline op imiza ion o he FRS-IC
To p epa e he se up o anspo and hen s udy he ions o in e es in a ce ain mass ange, p epa a o y offline measu emen s
a e equi ed o cha ac e ize and op imize indi idual subsys ems. The FRS-IC acili y consis s o mul iple fil e ing and analysis s ages
composed o de ec o s and mass fil e s [22]. Offline cha ac e iza ion o he ion ex ac ion and de ec ion pe o mance om he gas
cell is enabled by he in e nal alpha ecoil ion sou ce ins alled in he en ance egion (a ached o he fi s elec ode ing) o he DC
elec ode sys em o he CSC. Ions a e sing he CSC on off-axis ajec o ies a e guided o he ex ac ion nozzle a he exi side by he
RF ca pe . The RF ca pe p o ides a DC g adien owa ds i s axial cen e and a epelling RF po en ial. Thus, i guides ions owa ds
he exi nozzle, whe e hey a e d agged ia a supe sonic gas je ou o he CSC in o a subsequen RFQ, ac ing as an ion guide and
mass fil e , wi h a mass esol ing powe o 10 (using FWHM), while achie ing a ansmission efficiency o abou 80% [26].
The mass fil e allows us o selec he mass egion o in e es o a ce ain cha ge s a e and o b eak apa he molecula ions
du ing he anspo . Depending on he se ing o he RFQ as an ion guide o mass fil e , he ex ac ed ions a i e a DISTRICT ei he

Nuclea Physics, Sec ion A 1053 (2025) 122967
5
N. To o elli, M.P. Rei e , A.K. Rink e al.
Fig. 3. S opping ange dis ibu ion o
223Th ions in he c yogenic s opping cell de e mined as a unc ion o he a iable deg ade hickness. The black solid cu e
shows a Gaussian fi o he expe imen al no malized o al efficiency. The expec ed ange dis ibu ion o a monoene ge ic beam in ed is shown wi h s aggling in he
a iable deg ade . The ha ched egion indica es he a eal densi y (AD) co e ed by he s opping cell and i is a bi a y placed in his plo o indica e he CSC hickness
in ela ion o he momen um sp ead o he incoming beam. (Fo in e p e a ion o he colo s in he figu e(s), he eade is e e ed o he web e sion o his a icle.)
in a wide o na ow mass ange. He e, alpha spec oscopy can be pe o med using he silicon de ec o s p e iously in oduced. The
alpha spec oscopy is o p ima y impo ance, as i enables sys em op imiza ion h ough he u iliza ion o adioac i e a omic species
due o he p ecise knowledge we possess ega ding he gene a ing sou ces and in insic p ope ies o adioac i e species. Using his
de ec o , a scan o he mass-o e -cha ge ange o a ce ain species ex ac ed om he CSC can be done. This allows he iden ifica ion
and quan ifica ion o he cha ge-s a e dis ibu ion and as such helps o p epa e op imum mass-o e -cha ge se ings o he downs eam
beam line. Fu he mo e, in he case when he e is no alpha emi e in he mass egion o in e es o he measu emen , he iden ifica ion
o masses is done a he MR-TOF-MS once he calib a ion o he mass fil e and he ex ac ion is done wi h he in e nal alpha ecoil
ion sou ce.
4.2. FRS-IC as alpha agging se up
In case he ion o in e es s ems om a seconda y beam wi h cha ge 𝑍and mass 𝐴being a away om he p ima y beam, he
in-fligh iden ifica ion based on measu ing 𝐴∕𝑄 s 𝑍may become difficul . A non-negligible diffe ence be ween calib a ion and
ope a ional se ings may eme ge due o he la ge diffe ence in ene gy loss be ween he p ima y beam and seconda y agmen s,
ende ing he possibili y o a w ong assignmen o 𝑍. In such a scena io, he FRS-IC can co ec he calib a ion by iden i ying a single
nuclide close in Z o he ion o in e es . This can be done ei he ia “𝛾- agging” ([27], [28]), ia “mass agging” using p ecision mass
spec ome y wi h he MR-TOF-MS [29]o as “alpha agging” using alpha spec oscopy. A he FRS-IC wo sepa a e se ups ha e been
used o alpha agging, a single DSSD de ec o placed in on o he CSC and he silicon de ec o s wi hin DISTRICT.
In la e me hod, he ions a e he malized in he s opping cell and measu ed using a Si de ec o in DISTRICT. Based on a ailable
alpha decay ene gies he s opped ions can be iden ified and he in-fligh iden ifica ion can be upda ed. The me hod was used du ing
mul iple online expe imen s and demons a es he easibili y o using he FRS-IC as a agge o alpha decaying nuclides wi h hal -li es
om ew milliseconds ex ended o ew minu es.
4.3. Online op imiza ion o he FRS-IC
A e comple ing he iso ope iden ifica ion a he FRS, he ollowing s eps ha e o be aken o maximize he amoun o he beam
s opped in he CSC so ha he maximum beam can hen be anspo ed o downs eam se ups o example o he MR-TOF-MS. The
app oach in ol es op imizing he homogeneous deg ade o a iable hickness ( a iable deg ade ) ins alled jus be o e he CSC, see
Fig. 1. By adjus ing he hickness o he deg ade , i is possible o cen e he mean ange o he beam in he middle o he s opping
cell. The ideal hickness depends on he cha ac e is ics o he species o be s opped (mainly ene gy and a omic numbe Z) and on he
densi y o he helium gas filling he CSC. Depending on he specific expe imen , he CSC is filled wi h helium gas wi h an a eal densi y
om 2 o 9mg/cm2. Ions we e injec ed in o he CSC, ex ac ed and subsequen ly sen o DISTRICT, whe e hey we e collec ed on he
Si de ec o se up. He e, hey we e spec oscopically iden ified ia hei cha ac e is ic alpha decay ene gies. To ob ain he numbe o
ex ac ed ions o he nuclide o in e es , he numbe o egis e ed decay e en s, aking he solid angle o he de ec o in o accoun ,
we e compa ed o he numbe o iso opes iden ified In-Fligh ia he FRS de ec o s.
This op imiza ion p ocedu e allows o se he deg ade o a hickness ha maximizes he numbe o ions s opped in he CSC.
Fig. 3illus a es such a s opping ange dis ibu ion (no malized) o
223Th ions e sus he a eal hickness o he a iable deg ade a
a CSC a eal densi y o 4.9mg/cm2. The black cu e ep esen s a Gaussian fi o he expe imen al da a poin s; he ed cu e shows a
dis ibu ion expec ed o a monoene ge ic beam, while he ha ched egion indica es he a eal densi y co e ed by he s opping cell.
Nuclea Physics, Sec ion A 1053 (2025) 122967
6
N. To o elli, M.P. Rei e , A.K. Rink e al.
Fig. 4. Alpha-ene gy spec a o
221,220Ra p oduced by
238U p ojec ile agmen a ion a 1000 MeV/u a he FRS Ion Ca che , measu ed wi h he Si de ec o in he
DISTRICT.
Once he op imal se ings o s opping in he CSC ha e been de e mined, i is possible o scale he deg ade hickness o s op a bi a y
ions o in e es , which indeed migh ha e a diffe en ene gy and Z.
5. Resul s
In he ollowing sec ion se e al offline and online expe imen s will be discussed based on da a acqui ed using he alpha spec-
oscopy se ups.
5.1. Alpha spec oscopy o
238U p ojec ile agmen s
The alpha agging me hod explained in 4.2 has been applied du ing a s udy o sho -li ed nuclides p oduced by
238U p ojec ile
agmen a ion a 1000 MeV/u. Combining he sepa a ion o he FRS and he selec i e s opping in he CSC, he decay chains o se e al
decay chains could be measu ed wi h he Si de ec o s in DISTRICT. In Fig. 4, he alpha decay lines o
221Ra and
220Ra and hei
daugh e s a e shown.
The silicon de ec o s we e ini ially calib a ed using he
223Rn decay chain om he in e nal sou ce inside he CSC and alpha
lines om he 3-line alpha sou ce ins alled a DISTRICT. This showed ha , a linea calib a ion unc ion wi h only a small quad a ic
co ec ion, con ibu ing o sys ema ic shi o abou <2keV, could be used. In o de o de e mine he alpha ene gies o he p ojec ile
agmen s all peaks we e fi ed wi h Gaussian and exponen ially modified Gaussian (Hype EMG) line shapes [30]. As du ing he
online da a collec ion decays o long-li ed
211Bi, p e iously collec ed on he oil, we e also eco ded, hose we e used o e-calib a e
and adjus o gain d i s o he p e-amplifie s h ough ou he expe imen . To he unce ain y ob ained ia he fi ing and calib a ion
p ocedu e a sys ema ic unce ain y o 5keV was added accoun ing o he diffe ence in ene gy loss in he aluminum oil be ween
diffe en iso opes and alpha ene gies. The unce ain y due o he ecoil dis ibu ion o he alpha decaying daugh e s has no been
added since he ene gy diffe ence was negligible. The new measu ed Q-alpha alues a e lis ed in Table 2.
Fig. 5shows he de ia ion be ween he measu ed Q-alpha alues om
238U p ojec ile agmen s and p e ious li e a u e. The
measu ed Q-alpha alues ep oduce known li e a u e alues well. The de ia ion is cha ac e ized by a no mal dis ibu ion, wi h a
Bi ge a io o 0.5, and as a consequence no u he addi ional sys ema ic unce ain ies we e conside ed.
No ewo hy is he measu emen o
221Ac, whose 𝑄𝛼measu emen was done in he pas as pa o he decay chain o
225 Pa
[6]. The e, wo close-lying, abou 50 keV apa , peaks we e iden ified in he ene gy spec um and due o he limi ed p e-sepa a ion
capabili ies (using a He-je echnique wi hou mass esolu ion and mass iden ifica ion) i could no be uled ou which peak should
belong o he
221Ac g ound s a e o migh belong o diffe en species. Mo eo e , also he e alua o o NNDC [31]pu s he exis ence o
his second alpha line as belonging o he decay o
225 Pa in doub . Howe e , al eady in [32], and mo e ecen ly in [33], sui ing mass
sepa a ed samples, he gs-gs alpha decay was epo ed wi h a good p ecision and he exis ence o he spu ious peak, as belonging o he
decay o
225 Pa, was e u ed. In he wo k p esen ed he e, he di ec In-Fligh p oduc ion and sepa a ion combined wi h he selec i e
s opping in he CSC has been applied and his s ongly supp esses neighbo ing iso opes [34]. In ou case he e is no iden ifica ion
o a seconda y peak in ou spec um. Fu he , we we e able o ule ou possible long-li ed con amina ion ia a ime- esol ed alpha
measu emen , see Sec. 5.2. Ou 𝑄𝛼ene gy o 7778(10) keV is in good ag eemen wi h he 𝑄𝛼ene gy epo ed in [32]and [33]
p o iding a confi ma ion o he eliabili y o he FRSIC da a.
The 𝑄𝛼 alues o he iso opic chains o Th, Ac, Ra, F , Rn, A and Po a e shown in Fig. 6as a unc ion o he neu on numbe 𝑁
using open symbols. Values o iso opes de e mined in his wo k a e shown as solid symbols. The well known cha ac e is ic maximum
o he 𝑄𝛼 alues a ound 𝑁= 128 can be seen. The known c ossings o he 𝑄𝛼 alues o he as a ine and adon chains a 𝑁= 131
and adium chains a 𝑁= 133 a e ep oduced in ou measu emen s, whe e he open symbol (li e a u e alues) a e o e lapped by he
Nuclea Physics, Sec ion A 1053 (2025) 122967
7
N. To o elli, M.P. Rei e , A.K. Rink e al.
Table 2
Alpha decay ene gies (𝐸𝛼) and Q-alpha alues (𝑄𝛼)
o adioac i e nuclei measu ed using he FRS Ion
Ca che p oduced online ia
238U p ojec ile agmen-
a ion a 1000 MeV/u. ‘Le el𝑙𝑖𝑡’ e e s o alpha an-
si ions in o exci ed s a es, ene gy alues we e aken
om li e a u e [31].
Iso ope Le el𝑙𝑖𝑡 𝐸𝛼𝑄𝛼
𝐴X (keV) (keV) (keV)
211Po 0 7450 (7) 7595 (7)
211𝑚Po-1 1633 7273(5) 9047 (5)
211𝑚Po-2 0 8877 (13)
211𝑚Po-3 897.8 78000 (28)
212Po 0 8801 (13) 8970 (13)
213Po 0 8387 (46) 8548 (47)
214Po 0 7706 (37) 7853 (38)
216A 0 7792 (16) 7941 (16)
215Rn 0 8692 (19) 8849 (19)
216Rn 0 8055 (15) 8207 (15)
217Rn 0 7750 (26) 7896 (26)
218Rn 0 7143 (26) 7277 (26)
217F 0 8313 (10) 8469 (10)
219Ra-1 0 7990 (15) 8135 (11)
219Ra-2 315.8 7674 (13)
220Ra 0 7457 (9) 7595 (9)
221Ac-1 0 7640 (11) 7778 (10)
221Ac-2 209 7432 (13)
223Th 140.0 7297 (11) 7571 (11)
224Th-1 0 7164 (13) 7296 (12)
224Th-2 178.4 6996 (20)
Fig. 5. 𝑄𝛼 alues o
238U agmen a ion p oduc s measu ed a he FRS Ion Ca che in compa ison o hei li e a u e alues, aken om [31]. An upda ed alue o
221Ac was ecen ly epo ed in [33], indica ed by he g een da a poin .
solid symbol (measu ed alues). A de ailed discussion is beyond he scope o his pape , bu i demons a es he po en ial o FRS-IC
alpha spec oscopy sys em o nuclea s uc u e s udies o ac inides.
5.2. Hal -li e measu emen o
221Ac and
223Th
Combined alpha and hal -li e measu emen s o
221Ac and
223Th we e p e o med. To do so, he DISTRICT da a acquisi ion sys em
was se o eco d ene gy and ime o each e en , whe eas he s a signal o he clock was gi en by a igge signal om he FRS da a
acquisi ions. The spill leng h / bunch leng h o he
238U p ima y beam was se o less hen 10 ms. Unde hose condi ions, only he
ex ac ion ime om he CSC o he silicon de ec o se up causes addi ional delays and limi s he echnique o nuclei wi h hal -li es
o abo e a ew ens o ms. In he analysis one can hen ga e on a specific alpha ene gy ange and e alua e he empo al e olu ion o
hose decays.
In Fig. 7, he esul s o he ime- esol ed decays cu es o
223Th and
221Ac a e plo ed a e ga ing on hei cha ac e is ic dominan
alpha decay b anch. The obse ed pa e n can be well desc ibed by a Gaussian peak con olu ed wi h an exponen ial decay unc ion.
Nuclea Physics, Sec ion A 1053 (2025) 122967
8
N. To o elli, M.P. Rei e , A.K. Rink e al.
Fig. 6. The 𝑄𝛼 alues o he decay o he adio-iso opes measu ed a he FRS Ion Ca che a e hei p oduc ion ia p ojec ile agmen a ion o
238U a 1 GeV/u a e
plo ed as a unc ion o he neu on numbe N. The 𝑄𝛼 alues a e ep esen ed as solid symbols. The li e a u e alues a e he open symbols.
Fig. 7. Hal -li e measu emen o
223Th and
221Ac. The numbe o
223Th (221 Ac) decay e en s is plo ed as a unc ion o he ime elapsed a e he injec ion o he ions
in o he CSC. The fi unc ion consis s o a Gaussian plus an exponen ial decay unc ion used o de e mine he hal -li e.
I conside s he ini ial ac i i y inc ease domina ed by he ion ex ac ion om he CSC and he exponen ial decay domina ing a e all
ions ha e been ex ac ed and collec ed on he oil in on o he silicon de ec o . The solid line ep esen s he fi o he da a.
The hal -li es o
223Th and
221Ac can hen be de e mined in wo ways: (i) by ga ing di ec ly on he alpha decay line o he iso ope
as shown in Fig. 7o (ii) by ga ing on he alpha decay ene gies o much sho e li ed di ec daugh e iso opes. The me hod o fil e ing
on he nuclide daugh e s o igina es om he need o alida e he hal -li e and iden ifica ion o
221Ac.
Fo
223Th, i was possible o de e mine i s hal -li e by fil e ing on he alpha decay o
223Th and, in addi ion, fil e ing he alpha
decay o i s daugh e nuclides
219Ra and
215Rn. Since he daugh e nuclides
219Ra and
215Rn ha e a e y sho hal -li e compa ed
o ha o
223Th he e olu ion o hei decays is domina ed by he slowe decay a e o
223Th. Simila ly, o
221Ac, i was possible o
de i e i s hal -li e by fil e ing he alpha decay o he daugh e nuclides
217F and
213A . As o he case abo e, hese daugh e nuclides
ha e significan ly sho e hal -li es han
221Ac. This indi ec me hod ag ees well wi h di ec de e mina ion o he hal -li e o
221Ac. In
he case o
221Ac an abundance o e en s beyond 300 − 400 ms is no well desc ibed by he fi and may ha e sugges ed he p esence
o a long-li ed con aminan . Howe e , a simila long-li ed componen did no appea in he decay pa e ns o
217F and
213A and as
such could be uled ou .
The hal -li es o
223Th and
221Ac de e mined he e, a e lis ed in Table 3. The hal -li es o
221Ac is in good ag eemen wi h he
known li e a u e alues om he ENSDF [31] compila ion and wi h a ecen esul o [33]. Ou esul o
229Th u he ein o ce a
hal -li e o a ound 600 ms as sugges ed by [35]and [36], which had been in conflic wi h a hal -li e o 900(10) ms as ob ained by
[37].
5.3. Angula momen um dis ibu ion o
211Po
Measu ing he ac ion o he nuclei p oduced in hei g ound s a e and isome ic s a es can p o ide help ul in o ma ion abou he
angula momen um dis ibu ion popula ed du ing he agmen a ion p ocess. A he FRC-IC,
211Po was chosen as a sui able candida e
o be s udied in his con ex .
211Po has a ich le el scheme, including mul iple high-spin isome ic s a es. I s fi s isome ic s a e wi h