386

Nuclear Instruments and Methods in Physics Research A249 (1986) 386-390North-Holland, Amsterdam

THE USE OF A VELOCITY FILTER IN a-DECAY STUDIES OF NIICROSECONDRADIOACTIVITIES

H.J . KIM, K.S . TOTH, M.N . RAO * and J.W. McCONNELLOak Ridge National Laboratory, Oak Ridge, TN37831, USA

Received 24 March 1986

A new method of studying microsecond radioactivities of evaporation residues produced by heavy-ion reactions is described .Forward recoiling products were separated from the beam particles by using a velocity filter and implanted into a Si(Au) detector .Their subsequent a decays were then studied with the same detector.

The use of velocity filters (or recoil separators) isbecoming more widespread in heavy-ion physics re-search. Essential parts of a modern velocity filter can beseen in fig. 1, where a schematic view of the Holifieldheavy ion research facility (HHIRF) velocity filter [1] isshown. These instruments are used to separate reactionproducts of a particular velocity from the incident beam(and other) particles, usually at forward angles. Thevelocity is selected by the strengths of the electric andmagnetic deflectors shown. A variety of investigationscurrently pursued with these devices is described in refs .[2-5]. A particularly good use for a velocity filter in-volves the study of radioactive nuclides produced infusion reactions induced by heavy ions . The uniquecapability of such a device to separate forward recoilingevaporation residues from the beam particles and to

* Permanent address : Instituto de Fisica, Departmento deFisica Nuclear, Universidade de Sâo Paulo, C.P. 20516, SâoPaulo, 01498, Brazil .

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focus these residues onto a detection system can beadvantageously exploited when investigating radioactivedecays . Earlier, Hofmann et al . [6] and DiRienzo et al .[7] have used velocity filters to study the a decay ofrelatively long-lived (ms and longer half-lives) evapora-tion residues . However, since transit (or holdup) timesof heavy-ion induced evaporation residues through avelocity filter are short (typically in the As range orless), the instrument can be used to study decays ofnuclides with half-lives that are too short for conven-tional isotope separators . Here we discuss one suchinvestigation performed recently at HHIRF.

The lifetime of 218Ra and the energies of the alphaparticles emitted in the 218Ra

21aRn , 210Po de-cay-chain were determined by implanting evaporationresidues from the fusion of 13C ions with 2°8Pb into aSi(Au) surface barrier detector and observing their sub-sequent a-particle decay. A thin layer ( " 100 Fig/cm2 )of enriched 208Pb deposited on a 20-Fig/cm2 carbon foilwas bombarded with 13C ions from the HHIRF tandemaccelerator . An energy of 68 MeV was chosen to opti-

QUADRUPOLE

DETECTOR

Fig. 1 . Schematic view of the HHIRF velocity filter ; E and B stand for electrostatic and magnetic deflectors.

H.J. Kim et al. / A velocity filter in a-decay studies

387

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mize 218Ra production via' the (13 C,3n) reaction .Evaporation residues recoiling at zero degrees were sep-arated from the beam particles using the HHIRF veloc-ity filter. The recoil energy of the evaporation residueswas calculated to be about 4 MeV; the correspondingrecoil speed is about 2 m/ps . And, as the flight paththrough the velocity filter is less than 3 m, the decayactivity of residues having half-lives of a few p.s couldbe studied following implantation.

Fig. 1 illustrates how the evaporation residues wereseparated from the beam . Because of their rigidity inci-dent ions are hardly deflected by the electrostatic de-flector and are therefore stopped by the first baffleencountered after the target. Evaporation residues un-dergo proper deflections and are selectively transmittedthrough the device. Details about the operation of thisapparatus can be found in ref. [1] . Transmitted evapora-tion residues were stopped by and implanted into aSi(Au) detector at the end of the velocity filter. Al-though the residues had an incident energy of 4 MeV,only - 259 of this energy appears as a detector pulsebecause of the large pulseheight defect for these slowmassive ions . Hence the arrival of each residue at thedetector is accompanied by a prompt pulse of - 1MeV. An alpha particle emitted by an implanted re-sidue, on the other hand, produces a delayed but gener-ally much large pulse. The time correlation betweenarrival (of a heavy residue) and emission (of an aparticle) can then be established by starting a time-to-amplitude converter (TAC) with a small detector pulseand stopping it with a subsequent larger one.

The schematic of the electronics used, similar to thatutilized by Valli et al. [8], is shown in fig. 2. Both thestart and the stop pulses were derived from the samedetector. Due to the large dynamic range required, twolinear shaping amplifiers were used (one for a particlesand the other for residues) . They were operated with thesame shaping time constants but with about a factor of8 difference in gain . The start for the TAC was derivedfrom the higher gain amplifier; both the start and thestop amplifier outputs were digitized by analog-to-dig-ital converters (ADCs) having a fixed 5-Ls conversiontime, and a coincidence circuit detected two consecutivestarts without a stop . The HHIRF data acquisitionsystem [9] includes a front-end processor which continu-ously monitors the busy output of the TAC and thestart channel ADC. Events converted by this ADCwhich do not start the TAC were rejected . Once theTAC went busy, the processor checked for an additionalstart preceding a stop and rejected such events byresetting the TAC and all ADCs . Every time the TACwent not-busy, a check for valid data (presence of astart and a stop) was made ; if the data were valid, allADC outputs were stored event by event. The startdiscriminator was set to fire on pulses larger than 0.6MeV but smaller than 3.5 MeV while the stop dis-

HJ. Kim et al. / A velocity filter in a -decay studies

320

240

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TIME (v s)

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I

.JI

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10.0 150 200ENERGY (MeV)

10

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Fig. 3. Representative pulse-height and TAC spectra (see textfor details).

criminator was set to fire on larger pulses . Energy scalesfor the recoil and a-particle spectra were calibratedwith a 244 Cm source and a precision pulser ; the TACcalibration was accomplished using a commercial timecalibrator .A set of representative time and energy spectra,

illustrating the type of information obtained in thepresent study, is shown in figs. 3a, 3b, and 3c. Thesehistograms were generated from the event-by-event data :the evaporation residue spectrum (3a) was obtained bygating on all valid TAC start pulses, the a-particlespectrum (3b) was obtained by gating on TAC outputsbetween 10 and 90 Ws and the time spectrum (3c) wasobtained by gating on linear amplifier pulses larger than4.5 MeV. Pulses corresponding to implanted residuesare small and broadly distributed . Although the spreadin the residue's energy does contribute a little, the maincause of the broad distribution is the above-mentionedpulse-height defect . In contrast, the a-particle spectrumin fig . 3b is characterized by sharp peaks. Since thebombarding energy was optimized for 21sRa productionand the TAC range was selected to determine the half-life of this nuclide, a groups from the 218Ra -+ 214Rn

012 z1GPo decay chain dominate this spectrum. In ad-dition, a strong sumpeak should be present because the0.27-iAs half-life of 214Rn is much shorter than theamplifier time constant (about 1.1 us) and a consider-able amount of summing of the two a pulses takes

place in the linear amplifier . Indeed the energies ofthree strong peaks seen in fig . 3b are consistent with thevalues reported by refs . [8,10] for al and 02 and theirsum (a1 + a2) . However, much stronger evidence con-cerning the origin of these peaks is furnished by theprominent but unusual tails that accompany the threepeaks in fig . 3b . This is discussed in detail in theappendix. The a energies, determined in the mannerdescribed in appendix, are 8.48 and 9.15 MeV for a1and 02, respectively. These values are somewhat largerthan the literature values [8,10] . The precision of thepresent measurement is f 20 keV.

Having ascertained the origin of the peaks and hav-ing determined their energies, we now address the mea-surement of 218Ra half-life. The shapes of the TACspectra obtained by gating on the three peaks in fig. 3bare identical confirming the nature of the sum peak. Acomposite spectrum obtained by linearly combiningthese three time spectra is shown in fig. 3d . The 218 Rahalf-life determined (using the least squares method)from this spectrum is 25.6 ± 1 .1 us . This value is nearlytwice as large as the value available in the literature [8] .Our result alters the 218 Ra a reduced width signifi-cantly and removes one piece of evidence quoted for theexistence of a clusters in heavy elements (these pointsare discussed in ref. [11]) .

The presence of other nuclides with different life-times is suggested by the shape of the time spectrumshown in fig . 3c. Accordingly, when gated on other timeintervals, a number of weaker groups become morevisible in the pulse-height spectrum, confirming thatseveral isotopes are present among the implanted re-sidues . For example, the relative intensity of the 8.99-MeV a group from 1 .7-ps 217Ra, the ( 13C,4n) product,becomes stronger when gated on a 3 to 15 gs timeinterval (see fig. 4) .

n0

1000

100

10

0800 900 1000

PULSE HEIGHT (CHANNELS)

Fig . 4. Portion of an a-particle spectrum obtained by gating ona 3 to 15 ps time interval .

H.J. Kim et al. /A velocity filter in a -decay studies

Acknowledgements

Appendix

P(E) =-

(E)[ _jR

(E)l

_ Ro _dE -1

P(E)

2R2(E) IdR (E),

389

In conclusion, we present a novel method of measur-ing half-lives and decay energies of short-lived radio-nuclides produced in heavy-ion induced fusion reactionswith the use of a single detector . The virtue of thismethod is illustrated by our measurement of the half-lifeand the energy of 218Ra nuclideproduced in the (13 C, 3n)reaction on a 208Pb target at an incident energy of 68MeV. The prerequisite conditions for applying thismethod are : (1) the half-life be compatible with theminimum holdup time and (2) the implanting pulsesmust be distinguishable from the decay pulses. As theseconditions can be generally satisfied by evaporationresidues produced by fusion of such light ions as C, O,F, etc. on heavy targets, the study of a variety ofparticle decays, inaccessible to isotope separators, be-comes possible.

The authors wish to thank J.A . Biggerstaff for pro-ductive comments . One of us (MNR) acknowledges atravel grant from the Fundacâo de Amparo à Pesquisado Estado de Sâo Paulo, Sâ Paulo, Brazil . Oak RidgeNational Laboratory is operated by Martin MariettaEnergy Systems, Inc. under contract DE-AC05-840R21400 with the US Department of Energy.

The range of a 4-MeV 218 Ra ion in silicon is about 1ttm, but the range of a typical a particle is nearly 100times longer . Thus an a particle from an implant sitehas about a 50% chance of escaping from the detectorand depositing a varying fraction of its total energy ;this fraction depends on the path length traversed bythe particle on its way out of the detector . Since emis-sion is isotropic the probability that an energy E isdeposited by a particle is

where dl is the solid angle, dE/dR(E) is the specificenergy loss and R(E) is the traversed range. Assumingthe initial emission direction is preserved, we obtain

where Ro is the minimum range traversed (originalimplant depth, i.e., -1 Itm for the present case) .Qualitatively, the P(E) vs E curve resembles that of ameniscus of a very wettable liquid in a tube, havinglarge values at both bounding energies but diminishingrapidly away from the bounds . Because of pulse sum-

390

ming the observed intensity distribution I(E) becomes

I(E) = I PI(E- e)PZ(E) de .

This combined distribution gives the characteristic tailsobserved experimentally. Qualitatively the height of thesum peak is p2 while the two single peak heights arep/2, where p is the probability that an a particledeposits its full energy in the detector. For the present

case p is about 0.5, and explains why the three peaksare of nearly equal intensity.

Several terms contribute to the peak energies E'shown in fig. 3b :

Ei = E., +e2 + ER1 +ER2 ,

E' =E., + et + ER1 +ER2,

and, for thesumpeak

E's = E�,+EaZ+ER1+ER2,

where e are the minimum energy deposited by thealpha particles escaping the detector and ER are thenuclear recoil energies associated with the alpha emis-sion. Making the simplifying approximation e =e1 = e2and ER =ER, = ER2, one obtains the following rela-tionship to determine a-particle energies :

E~,= E, - S/2 - ER/2,

where

S

is the difference El' + E2' - Es,

ER =Imo/2][Ea1/m 214 + E.2/m2IOU, and f=0.25 is thefactor that accounts for the pulse-height defect (mstand for nuclear masses).

H.J. Kim et al. /A velocity Jilt& in a -decay studies

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