Nuclear astrophysics studies by recoil mass separators

Revista
Mexicana
de Física 43, SUl,lemento
1 {1997} 169-177
Nuclear astrophysics studies by recoil mass
separators
L.
M.
GIALANELLA,",d
C.
M.
MonONE,b
ROMOLI,d
G.
K.
I3RAND,"
OLIVlERO,b
S. SCIIMlDT,"
H.P.
"Institut
L.
CAMPAJOLA,b
A.ORDINE,d
W. H.
V.
SCHULTE,"
TRAUTVETTER,"
u.
A. D'ONOFRIO,c
ROCA,b
ANO D.
F.
C.
ROLFS,"
F.
STRIEDER,"
M.
GREIFE,"
ROMANO,b
TERRASl,"
ZAIINOW."
¡ü,. Experimentalphysik
lIl, Ruhr- Unive,'sitiit BochlllTl, Germany.
di Scienze Fisiche, Universitd Federico II
Pad. 20 Mostra d'Oltremare, 80125 Napoli and INFN, Napoli
eDipa7'!ilTlento di Fisica Teorica SMSA , Ullive,'sitá di Salemo
Via S. Allende, 84081 Baronissi, Salemo alld INFN, N'lpoli
d Istituto Nazionale di Fisica Nuclcare, Seziolle di Napoli
Pad. 20 Mostra d'Oltremm'c 80125 Napoli
eFacoltd di Scienze Ambientali, Seconda Ullive,'sitá di Napoli
Via Arena 22, 81100 Case,.ta and INFN, Napoli
b Dipartimento
ABSTRACT. It has been recenlly demollslrated thal an accelerator lIlass spectrometry
(..1.:\15)
system, used as a recoil separator in conjunction with a windowless gas target, can yield the
high suppression factor needed to dispersively analyze radiative capture rcsidues, wilh the aim
of measuring cross sections in the sub-microbarn rangc. An cxperimcllt is underway utilizing
a radioactive 7Be beam for the measuremcnt of the cross section of the astrophysically important rcaction 7De(!" ,)8D at a ceuter of mass energy ECM = 11\1eV. Preliminary results of this
experiment are presented.
The extcnsion of the method to another reactioll playillg a kcy role in stcllar evolution, i. e.
rcquircs an improvement of thc angle- ami mOIlH~ntum-acceptance of the recoil
separator, the use of a jet gas target and of a special1y designcd low-thrC'shold detector. The
solutions proposcd by a joint italian-german project are discuss(ld.
12C(n, ,)160,
RESUf\.IEN.
Se ha demostrado recientemente en un sistema de espectrometría de Illasas con aceleradores (A!\IS), que un separador electromagnético cn conjunto con un blanco de gas sin ventanas puede producir el factor de supresión necesario para analizar dispersivamente residuos de
la captura radiativa con el objetivo de medir secciones eficaces por abajo de los microbarns. Un
experimento está en marcha con un haz radioactivo de 'Be para lIledir la sección eficaz de importancia en astrofísica 7I3c(p,,)8D a la energía de centro de ma.sa ECM
1 ~leV. Se presentan
resultados preliminares.
=
La extensión del método a otras reacciones que juegan palwles import.antes en la evolución
estelar, i.c. 12C(o, ,)160, requiere del mejoramiento de la aceptación angular y cn momento del
separador, el uso de un blanco gaseoso sin ventanas y de UIldetector dI' bajo umbral especialmente
diseüado. Se discute la solución propuesta por un proyC'cto conjunto italo-gprmano.
PAC5, D5.30.C; 25AOL
170
L.
GIALANELLA
ET AL.
l. INTRODUCTION
Thc cross sections of several rcactions taking place in astrophysical cllvirOIllllcnts al
the relevant energies are still poorly (or not at all) known uI' to date. On the other
hand the corresponding [cactíon rates are fundamental qllantitics Ilceded lo pcrfofm
reliable predictions related to astroI'hysical problems sueh as the solar neutrino puzzle,
Ilucleosynthesis and tho associated energy productioll, the structural evoiutioll uf stars.
etc. [1].
.
In the quiescent burning phases of the evolution of a star, reactions between light
Iluclci, cssentially
p amI n capture,
take pla.ce at. cllcrgics
far bclow
t}¡c Coulomb
har-
rier with extremely small eross seetions. Apart fmm selected cases in whieh one can
aceess directly the reaetioll of interest-taking advilntag;e either of the rc<illctioll of cosmic
background in underground laboratories or of the inere'L",d phase space in the so-called
indirect
Iuethods,
olle ha..')lo cxtrapolate
lo astrophysical
cllcrgies
thc data mea .."'iurcd al
higher energy in a range a.s wide as possiblc.
The exI'losive burning (::L'eis characterized by rmetion energies clase to tbe COlllomb
barrier, giving rise to sizeablc cross sections. 011 the othcr hand OIle of lile rcactioIl
partllcrs is typically a short lifctime radioactivc Iluclclls, so that laboratory iuvestigations
of such rcactions ¡mpIy the use of radioactive ion beallls Bsually of relat.ively low illt.cnsity,
1
ami thc final rcsnIt is aga.ill a low reaction rateo
To [aee this sitllation a. great cffort is presentIy beillg dcvoted to devclop IlCW efficicllt and selective dctcctioIl apparatuses
lo be tlsed in nuclear a.."itrophysics studics.
Amollg these, first results [2,3] show that rol' radiative capture reactioll mCSlUC1UCllts
an interesting tool is a reeoil seI'arator capable of identifying and cOllnting with high
cfficicncy the heavy rcsiducs of thc reaction. Sllch a device is characlerizcd by angular
alld mornentmll acccptallcc large enough to accolIlodate aH rcaction prodllcts amI by a
very high rejecting power of events directly or inr1ireetly related to the trausport al the
bcam inducing thc rcactiotl. An italian-gcrtll<lll collaboration
has uccn workinp; for a fcw
years deInonstrating
that joining the cquipnlellt all(\ kllOW-how derivc(i frOlIl accclcratar
n"L," spectrometry
(AMS), on one side, and windowle" gas targets, ou the other ,ide,
it is possible to íu:hicvc [01' the first timc, the ~ood pcrformances
nccdcd to obtain in
singles a cross scctiOIl tIIca,"illrcmcnt \....ith high accuraey. The aim of this research projcct
wa.."i to determine \vith a direct mcthad thc eross s('d.ioll 01' the astrophysicalIy
important
1
reaetion 713e(p,,.)8 13,which is to date a¡fected by large uncertaintics.
In the present papel' we \vill prcscllt the l"('sults 01' thc characterizatioll
uf tlw recoil
ma.ss scparator ctnploycd. 'rhe acceptancc awl tlw sl1ppreSSioll factor of t.lw recoil lIlass
seaparator
(RMS) wherc tcstcd Illeasurill~ in illV(~rS(~kitwmatics t.hc cross scction of a
rcactioIl, 12C(P, ')') ¡:J N al ECM = 0.841 ke V 1 aIready IIlcasurcd in direct kitleIllatics [5]'
giving an excellent agreement with the literature value. 1'reliminary result, 01 the experimcllt ainling to determine the cross scction uf tlw 7Uc(p;)ysI3 rpactioll al. Ec~, =
1 MeV are a!so given. In Sect. 3 we will discuss ti", peculiarities 01 Ihe '"C(n,,.)IGO
n~action which hamper tite use of the same B!vIS for the cross sectioll lIIca."'mrcment at
a.s low a.'i possible cncrgies, pn~sclltillg possible SOlllt.iolls which han! \('ad f,o th(' project
of a t1CW R:0.1S to lHl insl.allcd al. t.he n:llhr-Ulliv(~rsitilt.
Bodllllll.
Part.icular (~lllphíL"iis
Itas becn givcII (lll llpgrading of angular alld IIIOII)('IIt.1I1I1
an:ept,flIlC(', sllpprl'ssioll fa{'t.or
NUCLEAR
ASTROPIIYSICS
STUDIES
BY ItECOIL
MASS
SEPARATORS
171
and elimination of machine background. The results of a test measurement performed
in Naples are aIso presented. Finally, in Sect. 4 we draw our conelusions and discuss the
potentialities of such an apparatus, also in Yiewof its possible utilization with radioactive
ion beams.
2.
THE
7Be(p"YB
EXPERIMENT
The absolute cross section of the 7Be(1', ,)8B reaction is a fundamental parameter for
the predictions of the standard solar model [6], in connection with the solar neutrino
problem. In particular, the astrophysical S factor at solar energies [S(E) = a(E) . E .
e2~"l is an important input parameter influencing sensitively the calclllated flux of high
energy nelltrinos. The present knowledge of such a parameter is still uncertain, due
to big discrepancies in the available data, which agree in the energy behaviour of the
excitation function, but show differences up to a factor of tluee iu the absolute value.
The cross section has been to date measured by means of indirect ¡;lethods, due to
the radioactivity of the 7Be targel. We report here on preliminary results of a new
eross section IIlcasurement in the Ilon-resonant regíoll, i.e. around ECM = 1 MeV. The
reaction is studied in inverse kinematics, 1'(7Be,,)8B, i.c. a radioactive 7Be beam at
Elab = 8 MeV from the 3 MV TTT-3 accelerator in Naples is gllided into a windowless
gas target system filled with H2 gas at a pressure of 5mbar, thus avoiding the problems of
target stoichiometry. The system allows to observe the kinematically focused 8B recoils
using an efficient recoil separator.
Details about the production of 7Be activated Li20 cathods used in the sputtering
saurce of the accelerator and acceleration and purification of an 8 McV 7Be beam are
given elsewhere [7]. Here we concentrate on the use of the combined system gas targetrecoil separator and its utilization for the extraction of the desired cross section. The
interaction of 7Be ions with H2 target atoms takes place in the windowless dilferentially pumped gas target, characterized by a central region, defined by two high gas-flow
impedance of 6.7 and 5 mm diameter, in which the density profile is almost constant. The
total effective length was determined to be 376 :I: 8mm, while the central part is viewed
by a series of Si detectors, placed in a disk shaped chamber at several angles with respect
to the direction of the beam. Accurate values of angles and solid angles were mea.sured,
as well as the effective length of the gas target seen by each of the detectors [41.
In this way, the cross section of the reaction can be rclated to the ratio of the number
of detected recoil nuelei to the nUlnber of elastically scattered recoil protons mca.sured
concurrclltly, in a way which is illdependent of ion heanl current, beam induced heating
in the target and also of gas pressure.
In particular, the number of 8B nuelides of I'(Be,,)8B prodllced in the gas target
chamber per unit of time, 18" is given by the expression:
(1)
where Nb is the
oC 7Bc projcctiles pcr ullit of time, Np is the dcnsity of the 112
target gas, leffn is the effectivc target lcngth alollg tite bcam axis produdllg 8B 1l1lclidcs,
ami a(Eerrl is the absolute cross section of1'(Be,,)8B at the effective energy Eerr (takcn
1l11lllber
172
L.
GIALA:-IELLA ET AL.
,~, the energy at the eenter of the gas target). In order to optimize the healll s\lppression
factor, 813 ions iu their lIlaxim\lm eharge state ('1 = 5+) are deteeted; the lIl11nher of 813
recoils ohserved in the !lE-E telescope of the recoil ,eparator is
(2)
where <1>5 is the probahility of the 813 recoils elllergiug from the gas target with a 5+
charge state and €r reprcscnts thc transmissioll of the 8I35+ rccoils throllgh the rceoil
separator.
The p + 713e el"sti<: seatteriug yield (i. c. of t.he prot.on reeoils), l." ohserved with a
detector placed at 0lab, is given by
(3)
where lellp is the effeetive target leugth seeu hy tI", deteetors, OCM,,(O, E.lf) is the e¡astic
scattering cross sectioll al the associatcd angle 0CM'alld OCM/Olab is the ratio of ccnlerof-lIlass solid angle to lahoratory solid augle. The ratio of the observed yields 18 aud 1,
theu leads to the expression
<1>5
".lelfn a(Eelrl
lefTpnlahaC~t,$( 0CM,
The IlIcasurenlent
Edf)
?!~~~
related lo ga..s t.arget alld silicon dctcctors has hecn
of (1).')\Va.'=' pcrforrncd llsing
1113 hC<lms with
charge states 4+ aIHI 5+ and the same veiocity as tite 8E recoils. Thc trallsmission of
the separator was fO\lnd to he 100% to within :¡% (s('e also helow) \lsiug different beams.
A first mn performed \lsing a low activity pill [71 and au average current of 18 pA and
[;~,ting ahout 5 h yielded t.he matrix showu iu Fig. 1 in the final det.eet.or. In auot.her
rlll1 threc additional evcllts wcrc ousel'ved. T}¡e f¡ve x[3 events givell t}¡c experimental
con<iitions, arc compatible with a cross scction ill !.Iw expcct.cd rangc, ir (lile aSSllmes the
e"~,t.ic eross section for aCM,S(OCM'
Eell) me,~",red withiu 15% for IIlab = 30° ami 45° by
tlsillg an H2-Ar mixture alle! normalized the dastic scattering cross scction of 7Li on lh
mea.'5uredin a scparatc rUIl.
The characterizatioll of lhe pcrformanccs of the rccoil separator, and in particular its
angular and Inomcntulll acccplallcc and the primary heam supprcssioll had prcviously
heeu checked hy studyiug-wit.h
t.he same lIIethod outliuet! above-t.he
llC(]!,"()'3N at.
E"II = 0.841 MeV, i.c. lI MeV iucidcut euergy iu Ih" lahoratory syst.em. The ideutificalion matrices collccled wilh lhc se par atar t.ullcd with a pilot beam [:3] show{~d dearly tIte
7+ 13N recoils cOllllts, very well separatcd by tlw rclatively intensc pcaks arisillg frolU
the ¡:le "lcaki' beams. Tlle sllppressioll factor (jf thc illtellsity oi"the laltcr with respect
tu the incident heam wa...•fOlllld lo he :::::::
10-1U, \vhile the s{'paratiOlI of the loci 011 the
matrix allowed au addilioual suppr"ssiou of at. 1"'L,t 10-".
\Ve have investigated both lhe acceptance 01" thc separator alld the effpct of the
diffcrent cncrgy losscs of the incidcnt ueam and thc recoil in the ga."jtarget, pCrfOrIlling
scveral rUlls with sli,l!;htlydiffcrcllt valucs of tlle lIIagllet alld \Vicn filter tlllling. The
discussed
of aH qllflntitics
(4)
in [4]' while
the tllca.e;¡l1rcment
1
({UCLEARASTROPIIYSICS
STUDIESBY RECOIL"ASS SEPARATORS
88 recoils
3072
U> 2560
a;
e
e
ro
re
2048
173
-...•~
.-"'~
"Leaky" 7Be beams
~ 1536
w
<l
1024
512
~... -
. ..
.!-., . ~ .
~~~.,
..
Eres
(channels)
FIGURE 1. tJ.E-E plot obtaiued \Vith a 18p..l. 'Be beam at 8!>leV onll,
s('parator tllned by the pilot beam.
at 5mbar aud the recoil
results are displayed in Fig.2,
where thc experimental acceptallces are illdicated, and
sprcad nI' the recoilillg 1:1N ious. Thc arraws
marked "T" in the figure refer to the tuniug values derived by the pilot beam method,
without taking into accollllt that, say, for a reactiou taking place at the center of the gas
larget the sum of the energy loss of the beam in the first half and the recoil in the seeond
one is not exaetly the same as the energy loso in the whole target thiekness of the pilot
beam. This effeet can be caleulated, and tllms out to be in agreement with experimental
findings (arrows e) . ..1.11tunings we are presently IIsing take into account this eorreetion.
In any case one can see that the complete ph'L'" spaee of reaetion produets is properly
transported to the final deteelor.
The appropriatc chargc state correction to he applied WH •. also studicd, with thi.s
reaction, Ineasuring tite Ilormalized 13N yields both in reactions induccd by 5+ and G+ 12C
healIls, and comparing thcir ratio to the ratio hetwccn tbe average values-over a rauge
of prcssures betweeu O and .5mbar-of tbe !:iN charge states, Illeasllred (L~ previously
deseribed for 8n, both for 5+ amI 6+ ineidellt eharge states [3]. The results illdieate that
at the moment of the reactioll the capture products keep the charge state of the ineident
cOIuparcd
with the predicted
momentulIl
"i
ion.
In any case, our results show that the presellt flMS is ahle to aecurately measure radiative capture cross sections when the angular and momentum spreads of recoil products
are uot larger thall 0.35° alld 2%, respeetively.
:3.
EXTENSION
12C(a.,)160
OF TllE RECOIL SEPARATOR TECllNIQUE
REACTION
TO THE STUDY OF THE
In Fig. 3 we report él eOlllparisou of the silllulaled linear IIl0mentUIIlalld anglc distribution
of reeoiling Iluelei for the tWIIreactions illdicaled at the topo The simulatiolls inelude allgle
straggling, 8nergy losscs iJl the gas targeL hcalll cmittance and recoil of capture residues
174
L.
GIALANELLA
ET AL.
3.0
2'
p("C,y)13N
<1>
E1ab •• 11.0 MeV
c~ 2.5
"1"2'77Z22nl7.22/';7?7~21
(.c\pJp)
N •• 1.1
%
> 2.0
.~
~
o
1.5
..J
W
>;
z
::'
1.0
(tl,pIPlSM •• 1 9%
0.5
0.0
T
C
1
1
3050
3100
SWITCHING MAGNET 8(G)
3150
30
2'
'c 2.5
~
<1>
E¡ab. 11.0 MeV
.~
2.0
~
1.5
;¡;
p(12C,y)13N
O
..J
W
>;:
1.0
::'
0.5
z
0.0
(tlplP)WF" 26%.
720
730
740
WIEN FILTER 8(G)
pe
2
FIGURE 2. Acceptance of the recoil separatar investigated by means of the
C,,) reaction at
lll\leV: 13N yield vs. swilching magnel field (upper panel) and Wien filler magnelic field (lower
panel).
due lo ¡-emission. As one can see lhe silualion is very di!ferenl for lhe pel3e,¡)8B case
wilh respecl lo n(.12C;y)160. In order lo maleh lhe laller emillanee lo lhe aeeeplanee
of lhe recoil separalor, major modifiealions are required. A jel gas largel, wilh mueh
smaller lenglh, will replaee the extended one, so thal lhe maxilllulll angle defined by lhe
apertures contains the angular spread of rccoils and, moreover a focusillg quadrupole
lriplel wilh larger gap can be plaeed doser lo lhe inleraelion region.
Then an RMS wilh larger momenlum aceeplanee has been designed lrying di!ferenl
solutions by magnetic optics ealculatioIls. The gap of aH transport clement was increased
to 4" and thc position of the Wicn filler alld magnetic allalyzcr will he inverted lo
minimize lIlultiple scallering e!feels. The 30° lIlagnel will be replaeed by a 90° one ami
a speeial D.E-E lelescope wilb very lhin enlranee window for low lhreshold delection
will be developed. The lower panel in Fig. 4 sbows lhe resulls of oplics ealculalions for
lhe lrajeclories of a 2.8 MeV 12C3+ beam ineidenl on lhe gas largel and lhree 1603+
1
1
NUCLEAR
ASTROPHYSICS
STUDIES
,,-----------,
3000 E--1f-~1.3% ~
'"
'ii2000
o
BY RECOIL
MASS SEPARATORS
175
1200
f-
800
E-
O
400
1000
6
6.5
7
6.5
7
7.5
ERecoil(MeV)
ERecoil(MeV)
12000
4000
'"
~8000
o
O
2000
4000
';/
O
.
0.5
.
..
1
°rec(O)
O
0.5
1
8,ece)
FIGURE 3. Comparison of energy and anglc sprcad of reactioIl products for thc reactions pelle,,)
aud pe'C,'¡).
recoils with D.P/ P = 0%, +3.6% atul -:1.6% with respcct to the paraxial bealll with an
enüttancc of 27r.1.5.35 nun Insr 3.t the ga.~target cluplacCIncllt. The desigll spceifications
01'the reeoil separator are an angular acceptancc of :1:40 lllrad, a beaIn spot of :i: 1.5 mIll,
an image size of :1:10 Imn, a momcntulll a.eccptance of :i: 4%. Our ainl is to rcach a
suppression factor of the prilllary bearn of the order of 10-IS.
Another inlportant diffcrcncc bctween thc reactioIls studied so far and the reactioll
a(l2e,,)t60
is the stability ofthe finalnucleus. Even ifno 160 accelerated beam matches
thc rigidity of the 12e bealll inducing the reaction, charge exchange processes in the high
energy tube of the accelerator give rise to a white spectrurn of 160 ions, a portion of which
is ulldcrneath thc Inain bcmll, even ir ,,,,'ith much lower intcnsity. However, thesc ions,
with the SaIne linear IUOIucutum of rcaction products, may aSSllUle the selected charge
state of the recoil nuclei and could be ,iriven directly to the final detector, giving rise to
a ba.ckground undistinguishiblc from reaction pro<1ucts. For this purpose, installation of
an additional \Vicll filter is planned bctwecn thc accelcrator and the analyzing magnet
which will virtually eliminate this sonrce of background.
A test of this procednre has heen performed wit.h the present RMS in Naples, exploiting the fact that the ECM = 3.2 MeV 4+ resonance decays strongly to the 2+ 6.92 MeV
state [8]' giving rise, with its reduced ,-recoil eHcct, to IGO recoils falling within the
acceptance of the RMS. An additional Wien filter (only 28cm long dile to mechanical
constraints) was installed bctwecn thc high energy quadrupole and the entran ce slits of
the anlyzing magneto When cOInparing the ident.ificat.ion D.E-E maps collect.ed with the
176
L.
GIALANELLA El' AL.
s""'ldee
'2C
t>e.....,
dump
MQDS
'6Q reco,ls
~I
p
RECOIL SEPARATOR
Fe
'
~
p
El ó,.,
lelescope
krllMaoT IlwF I
2
HOrizontal
Plane
1603+
1
O
.1
"''"
oC
U
Ver1lcal Plane
.~
03•
16
O
.1
.2
O
"
'"
'"en
ro
N
O
'"
N
en
"
N
N
'""'
N
inches
FIGURE.1. Lay-out of the proposed recoil separator to be installed al Ilochutll (upper pauel) ami
optics calculatiou (see text).
filter off and on a huge difference was noticed. In Fig. 5 we display the matrix obtained in
a mn with the filter on and 5 mbar of He gas in the target chambeL In order to estimate
the residual background not associated to the reaction (expected to be not negligible because of the bad quality of the filter) we perfoflned an identical run filling the gas target
with H2, amI correcting the 160 yields by the ratio of the nllmbers of the cla.stic recoils
(a and p, respectively) nOfmalized to the respective elastie eross sections. The residual
background tumed out to be 20% of the total, ami after sllbtraction an estimate of the
cross section on tal' of the resonance yielded a value of 3.5:l: 0.9 mb, to be compared with
the value of 4.7mb qlloted in [8J.
4.
CONCLUSIONS
In summary, based on the experience gained with the Naples reeoil separator in the study
oC low angic-and momcntum-emittancc capture reactions, wc have shoWI1, also with
preliminary IueasurenlCuts OIl lllore complicated cases, that RMS's can be designed and
NUCLEARASTROPIIYSICS
STUDIESBY RECOILMASSSEPARATORS
177
'He("C,y)"O
1000
V>
a;
e
e
ro
.c
u
W
800
600
400
<l
"O
"',:,.,..
,,;,;
••....•.
l'~1,,_
"Leaky' beam
200
,
200
400
600
800 1000
Eres(channels)
FIGURE 5. t.E-E plot obtained from the a(12C,'Y) reaetion at 4+ resonanee (E
CM
= 3.2 MeV).
built to address measurements which willlargely expand the range of knowledge of nuclear
reactions of astrophysical interest. More precise detailed studies and extrapolations
for
the 0'(12C,/)160 will be possihle. In addition the potentialities of such a device hecome
more evident ir Qne forcsees opcration in conjunctioIl with radioactivc hcavy ion bcmns.
In comparison to the up to now performed measurements of the hot/explosive burning
phases, the efficiency can be inereased hy at least two arders of magnitude.
Of course an internationally joined big experimental elfort will he needed, taking also
into account that one wants no restrictions on the select.ed charge st.at.e of t.he recoils.
After t.he pioneering work in this field per!()[lned [91, which anyway required recoil-/
coincidences to achieve a sufficient beam suppression fact.or lowering significantly t.he
total detect.ion efliciency, modificat.ions of existing recoil separators or design of newly
designed ones will allow in a few years t.o reach the selectivity and sensit.ivity needed to
make a significant. st.ep forward beyond t.he current. st.at.e-of-t.he-art.
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