The application of alpha spectrometry to the discovery of new elements heavy-ion beam bombardment

Nuclear h>trumentsand Methods in Physics Research 223(1984) 303 311 North-Holland, A m s t e r d a m

303

T H E A P P L I C A T I O N OF A L P H A S P E C T R O M E T R Y T O T H E D I S C O V E R Y OF N E W E L E M E N T S BY H E A V Y - I O N B E A M B O M B A R D M E N T * J.M. N I T S C H K E Nu(h'ar ,S'(len('e Dir i~ion, l~:wrence Berl,elev Lahoratoo', Untt,ersitr of ('ahfi~rnia, Berkeh'y, CA 9472(L USA

Starting with polonium in 1898, alpha spectrometry has played a decisive role in the discovery of new, heavy elements. For ex,en-even nuclei, alpha spectra have proven simple to interpret and exhibit systematic trends that allow extrapolation to unknown isotopes. The early discovery of the "natural" alpha decay series led to the very powerful method of "genetically" linking the decay of new elements to the well-established alpha emission of "'daughter" and "granddaughter" nuclei. This technique has been used for all recent discoveries of new elements, including Z = 109. Up to mendelevium (Z = 101), thin samples suitable for alpha spectrometry were prepared by chemical methods. With the advent of heavy-ion accelerators, new sample preparation methods emerged. These were based on the large momentum transfer associated with heavy-ion reactions, which produced energetic target recoils that, when ejected from1 the target, could he thermalized in helium gas. Subsequent electrical deposition or a helium jet technique yielded samples that wcre not only thin enough for alpha spectrometry, but also for alpha and beta-recoil experiments. Many variations of these methods ha,,e been developed and will be covered in this paper. For the synthesis of element 106. an aerosol-based recoil transport technique was devised. In this most recent experiments, alpha spectrometry has been coupled with the magnetic analysis of the recoils. The time from production to analysis of an isotope has thereby been reduced to 10 6 s, while it was 10 1 10 ° s for helium .jets and 101 103 s for rapid chemical separations. Experiments are now in progress to synthesize super heavy elements (SHE) and to analyse them with these latest techniques. Again, alpha spectrometry will play a major role, since the expected signature for the decay of a SHE is a sequence of alpha decay's followed by spontaneous fission.

1. Introduction In 1898 P. and S. Curie published a paper "'On a new radioactive substance contained in pitchblende" [1]. The last sentence of their paper reads, " P e r h a p s we may be permitted to remark that if the existence of a new element is confirmed, this discovery will be due solely to the new method of investigation which the Becquerel rays provide." Thus starts the story of the application of alpha spectrometry to the discovery of new elements. In 1898 the Curies reported the discovery of the two elements, polonium and radium. They also took the first steps from alpha spectroscopy to alpha spectrometry by developing the forerunner of the ionization chamber, which they called "parallel-plate apparatus". A few years later Rutherford determined the e / m ratio of the " a l p h a rays" and concluded that their mass was of the same order of magnitude as the hydrogen atom [2] and his magnetic measurements showed that their specific energy was 3.2 M e V / A . Another important discovery made by Rutherford and Soddy was the radioactive decay series. This method of linking known to unknown elements via their alpha decay sequences is still one of *

This work was supported by the Director, Office of Energ> P,esearch, Division of nuclear Physics of the Office of High Energy and Nuclear Physics of the US Department of Energy under Contract I)E-A('03-76SF00098.

0167-5087/84/$03.00 :," Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

the most powerful means for the discovery and identification of new elements.

2. The synthesis of new elements All elements between uranium and lead, with the exception of astatine were discovered as members of the radioactive decay series of thorium, actinium and u r a n i u m / r a d i u m . Neptunium was the first transuranic element of the actinide series to be synthesized in a nuclear reaction with neutrons on a uranium target. In the same year (1940), astatine was discovered in the b o m b a r d m e n t of bismuth with alpha particles. Subsequently, elements up to fermium ( Z - 1 0 0 ) were produced in neutron or light ion ( _< He) reactions on heav'y targets. All these discoveries involved steps to separate the new element from the target material by chemical methods based on the predicted chemical properties of the actinide series. The chemical procedures yie ded samples thin enough for off-line alpha spectrometry with grid chambers. A new element was in most cases characterized by a specific alpha energy (or energies) and a partial half-life for alpha decay. C o m p a r e d to alpha spectrometry, the study' of beta and gamma decay plays only a minor role in the discovery of new elements even today. There are several reasons for this: (1) alpha particles can be detected with 111. S P E C T R O M E T R Y

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nearly 100% efficiency using iomzation c h a m b e r s or surface barrier detectors (SBD); (2) the detectors are essentially b a c k g r o u n d free which, c o m b i n e d with z~ 100% detection efficiency, results in high sensitivity: ~3} a SBD can function as a particle identifier if the sensittve depth is only slightly larger than the range of the most energeuc a l p h a particles: under these conditions the probability of a b s o r b i n g high energy beta or g a m m a rays is very small: (4) for alpha particles of 5 to 10 MeV energy, the resolution of m o d e r n icooled} SBDs approaches 10 keV or 0.1%. which is sufficienl to reveal fine details in the level structure of the residual (daughter) nucleus. On the other h a n d . fission fragments that are frequently observed in the decay of heavy elements are detected with the same efficiency as alpha particles. However. they rarely create confusion, since their energy is a b o u t one order of magnitude higher. The main p r o b l e m that stands in the way of the effective use of alpha spectrometry in heavy element research is the p r e p a r a t i o n of thin samples: this will be discussed in more detail in the nexl sections. An occasional p r o b l e m is the pile-up between beta and alpha particles due to the finite time resolution of the detection system. This is. however, quite rare a n d results in a high-energy tailing of an alpha-line, which is equivalem to a decrease in resolution Early in the study of the a l p h a decay systemaucs of the heavy elements, p r o n o u n c e d correlations were found between half-lives, energies. N. a n d Z values: We no~ know from theoretical models that for nuclei far from the magic n u m b e r s Z - 82 and N = 126. the shell corrections to the nuclear mass formula are smooth functions of Z a n d N. which, when added to the smooth b e h a v i o r of the liquid-drop or droplet model, give a simple relationship between the a l p h a decay Q-value (Q,O and the half-life (/1/2): l o g t l / 2 - A / ¢ Q , , ~B. where the c o n s t a n t s A and B depend weakly on the Z of the element The data for e v e n - e v e n nuclei are fitted well by the semi-empirical formula [3]. Iogtl/2 = 1.61( ZE,, 1/2

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Here E,~ is in MeV. Z refers to the d a u g h t e r nucleus and t~/2 is calculated in years. E, shows an approximately linear rise with increasing Z for a family of isobars, as can be deduced from semi-empirical mass formulae. This accounts for the fact that beyond uranium, the partial alpha-decay half-life of heavier elements becomes shorter, a n d s p o n t a n e o u s fission decay starts to b e c o m e more prevalent. Decreasing half-lives a n d decreasing p r o d u c t i o n cross sections are the two principal difficulties in synthesizing new elements and the reason that the relatively slow chemical procedures h a d to give way to purely physical identification methods, as will be discussed in the next section. A n o t h e r reason for the popularity of alpha spec-

trometrv i~ that the interpretation ol man~ alphu ~,pcc+ tra is straightforward: in p a m c u l a r for e~cn even emitters, where most of the transmol,~ strength goc: to the ground state of the d a u g h t e r nucleus and the b r a n c h i n g that occurs to the 2 - first rotational level Js a s m o o t h function of the 0 ~ - 2 ' level spacing. As the level spacing decreases because ~f a~ increase in the distortion of the nuclei farther ren3uxed from the ,x 126 neutron shell_ u significant amount of transition strength does. however, go to the first 2 excited slate of the rotational band. The spectra of odd nucleon alpha etmttcr~ arc m general much more complex and the t r a n s m o n to the g r o u n d state is hindered. It is. however, possible to establish some order through the conccpt of " f a v o r e d " a l p h a decay [4], in which the last odd particle is m the same Nilsson orbital in the daughte~ as in the parent nucleus. H i n d r a n c e factors for such transiuons are close to unity, since they resemble ground-~tate transitions m e v e n - e v e n nuclei The more complex the rearrangement of the nucleons is between alpha emitter and final nucleus, the larger, m general. ~s the hindrance factor for alpha decay: this becomes plausible considering the overlap integrals of the nuclear wave functions. The alpha particle, when emitted from a hear,. nucleus, carries away about 20{; of the n u c l e u s mass and imparts to the daughter nucleus a rcctnl energy of about 100 to 150 keV. A nucleus of this energy i.~ able to traverse 10 50 /zg cm 2 of material and in 5Oral of all d e c a ' ~ will therefore be " k i c k e d out" tff the sample tf il is thin enough. For ,err thin samples, even the recoil from beta decay is sufficient to ,4jeer the d a u g h t e r nucleus Extensive use has been made of these recoil effects to produce thin. secondary samples and to establish genetic relationships between ~uccessive members of an a l p h a decay chain.

3. Techniques developed for alpha spectrometD' in conjunction with heavy-ion reactions All elements above e i n s t e m m m ~ Z = 99~ were synthesized with heavy ion beams. There are distinct advantages to heavy ions c o m p a r e d to alpha, deutermm. proton or n e u t r o n projectiles, as used in earlier expertments. The heavy-ion fusion product acquires a large forward m o m e n t u m , which can propel it through target thicknesses of the order of 1 m g cm 2. While in earlier experiments the target material had to be dissolved to isolate the reaction products, the recoil nuclei can now be stopped separately in a catcher foil. A n o t h e r advantage of the heavy-ion recoil method is that verx, exotic targets can be used. since the material is not lost to the chemisl after each b o m b a r d m e n t . The discover,¢ of element 101 mendelevium, illustrates this point. Starting with p l u t o n i u m placed inside ;, high flux reactor. a total of 109 atoms of 25SEs with a half-life of 2t) d

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J.M. Nitschke // Application of alpha ~pectrometrv was produced. A tiny electroplated target was formcd and bombarded by a helium beam. The recoils were caught in a gold catcher foil, which was subsequently dissolved and element 101 was isolated by a chemical procedure, which in turn yielded thin samples for alpha and spontaneous fission counting [5] In general, the unslowed recoil products are imbedded so deeply in the catcher foil (several hundred/tg cm 2) that these samples are not suitable for direct alpha spectrometry. The problem of sample thickness can, however, be solved elegantly by the use of a stopping gas [6]. As the recoils are slowed to thermal velocities in a noble gas (or N 2) at about 1 arm pressure, a large percentage retains a positive charge and can be guided electrostatically to a catcher [7-9]. Samples obtained in this fashion are often as thin as an atomic monolayer and make excellent sources for alpha spectrometry. A modern version of this technique has been developed by a French group [10] and is shown in fig. 1. The thermalized recoils are guided along the electric field lines and are deposited directly on a SBD. The total transport time is 10 ms. For the discovery of nobelium (No, Z = 102), Ghiorso and co-workers refined the target recoil method into a "double recoil" technique [11]. The nobelium atoms produced in the reaction of 12C ions with a curium target were stopped in helium gas and attracted electrically to a moving, metallic belt that in turn passed near a negatively charged foil. The 25°Fm from the alpha decay of 254102 was "kicked off" the belt with about 50% probability and captured by the foil. The discovery of element 102 then was based on observing the 30 rain alpha activity of 2s0 Fm on the foil and thereby linking the new e l e m e n t /ai s o2 5t0o p e to(~a known one via the genetic sequence 254102 --+ Fm --* (246Cf). 30 nun

An accidental observation that was made during experiments with helium as a stopping gas led to the development of a new technique [12], which was used extensively in the discoveries of elements 102, 104, 105 and 106 and several of their isotopes. Fig. 2 shows an early version of what was later called a helium jet system. The heavy-ion beam from the accelerator passes through a set of degrader foils to adjust its energy to the peak of the excitation function of the isotope to be studied. It impinges on the target and the recoiling fusion products are thermalized in a gas chamber that is filled with helium of about 1 atm pressure. A small orifice lets some of the gas escape into a vacuum chamber. The rapidly moving helium entrains with it the radioactive recoils and deposits them on the periphery of a wheel. Since the velocity of the helium gas is in the sonic regime, the recoils do not penetrate the metal of the wheel and thus form a thin sample ideally suited for alpha spectrometry. The wheel is rotated by a stepping motor so that the irradiated spots face several SBDs in

307

succession and spectra similar to those shown in fig. 3 are obtained. The half life of a new isotope is calculated from the observed decay rate at each counting station. In the early days of gas-jet systems, the recoil transport mechanism was not well understood. It was occasionally observed that a very clean system gave poor yields, while gas that was contaminated with diffusion pump oil had a good transport efficiency. Now it is known that the charged recoils are attached via Van der Waals forces to aerosol particles, which can remain suspended in a gas stream if they are of the correct size [13]. Worth mentioning is that as early as 1907, Madame P. Curie had already observed that the "'dust" in a vial seemed to become radioactive and settle on the bottom under the influence of gravity [14]. Once the crucial role of aerosols was discovered, a wide variety of them was studied: water, acetone, sodium chloride, potassium chloride, carbon tetrachloride, oil, ethylene, trichlorethylene, ethanol, ethylenglycol and others. To initiate the formation of aerosols, condensation nuclei of silver chloride or silver iodide have been used, as well as irradiation with ultraviolet light. The aerosol technique has allowed the transport of radioactive species over tens of meters in a few seconds. It has, however, a serious drawback for high-resolution alpha spectrometry: since a non-negligible amount of aerosol material is deposited together with the radioactive substance, the sample becomes progressively thicker and long-lived alpha activities exhibit poor energy resolution due to this "'cemetery effect". Fig. 4 shows the experimental arrangement that was used in the discovery of element 106. It incorporates many of the experimental developments that occurred before 1974 [15]. The intense heavy-ion beam from the SuperHilac enters the target chamber through a gascooled window. The 263106 atoms, together with other transmutation products, recoil from the target into a stopping gas chamber and are swept by a flow of helium (containing NaCI aerosols) through teflon tubing into an adjoining counting area. Here the recoils are deposited onto the periphery of a wheel with seven detector stations which examines the samples for alpha and spontaneous fission decay. The main difference to the wheel system shown in fig. 2 is that each detector station consists of two fixed and two movable SBDs instead of one fixed detector (cf. inset of fig. 4). This permits, t ~ obser~c~tion o f the decay sequence 26~106 ~ 104 --, No ---, (2SlFm) using a double 0.9 s

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subsequent detectors facing the wheel. In the event that a 259104 daughter recoils from the wheel onto a detector and the alpha decay of this daughter is later observed with this detector in the off-wheel position, this detector is not returned to the on-wheel position until t0 mm have elapsed. This time period permits an adequate opportunity for detecting the subsequent decay of the 3 min granddaughter. 255N0. A total of 22 atoms of recoil-transferred 255Rf were observed to decay in the off-wheel detectors. Shortly after these daughter events. granddaughter 255N0 alpha particles were detected from four off-wheel daughter decays, thus establishing a genetic link from 245No to the new nuclide 263106. Starting with element 101 (mendelevium) and culminating with the discovery of element 109. new elements have been synthesized and identified one atom at a time. This has been possible to a large extent because of the high detection efficiency, low background and good resolution achieved in alpha spectrometry.

4. The combination of alpha spectrometry with other analytical tools The discovery of a new element requires the unambiguous determination of its Z value [16]. In the previ-

ous sectton, the method of genetic linking via alpha decay chains to determine Z was discussed. This method has w o r k e d w e l l up to the latest discoveries of elements 107 [17] and I09 [18]. The classical way of determining the Z value of a new element has, however, been through chemica! experiments: Z ~ 104 is the last element with which this was possible [19]. The short half-life of z~1104 (65 sl later required the development of the computerized, fast chemistry system shown in fig. 5 [20]. This system allows the study of the anionic-chloride complexes of element I04 on a "'one atom a time" basis. T h e final samples, obtained after chemical processing, were examined by twenty SBDs for the characteristic alpha group of element 104 at 8.28 MeV. A major drawback of the chemical apparatus shown in fig. 5 is its batch processing mode. Continuous gas-chemical methods that yield samples thin enough for alpha spectrometry have been developed at GSI [21 ]. Gas chromatographic methods for elements up to 107 [22] were developed at Dubna. These methods do not yet lend themselves to on-line alpha-spectrometry and are only applicable to spontaneous fission emitters: They are, however, about one to two orders of magnitude faster than rapid solution chemistry. A very powerful method of Z identification is based

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on an a - X - r a y coincidence technique [23]. The parent nucleus decays by a l p h a emission to an excited state n the daughter nucleus, which subsequently de-excites bx the internal conversion process. This process )ields characteristic K series X-rays of the daughter element m coincidence with alpha particles from the parent and provides an u n a m b i g u o u s Z identification for both elements. While this m e t h o d is very elegant. ,t suffers ['rom the reduced efficiency of most coincidence experiment~. Being aware of the difficulties of Z d e t e r m i n a t i o n s by chemical m e a n s and X-rays, experimentalists have designed a n entire class of i n s t r u m e n t s for the somewhat easier task of d e t e r m i n i n g the mass of u n k n o w n isotopes. A m o n g these are: (1) alpha recoil time-of-flight mass spectrometers [24], (2) helium-jets coupled with ion sources and magnetic mass spectrometers [25]. (3~ direct mass analysis of singly charged, thermalized recoils [26] (4) conventional on-line isotope separators. (51 gas-filled magnetic spectrometers [27] and (6) velocitv filters. While all these i n s t r u m e n t s have been more or less useful in the study of new isotopes, so far only the velocity filter S H I P [28] has c o n t r i b u t e d to the discovery of new elements. Details will be covered in a n o t h e r c o n t r i b u t i o n to this conference. Relevant to the subject of a l p h a spectrometry and the discoverv of new elem e n t s is. however, a technique that was developed at GSI in c o n n e c t i o n with S H I P [29] and has also been used in Berkeley with the gas-filled separator SASSY [30] in the search for super heavy elements. The principle' is shown in fig. 6. The evaporation residues are i m p l a n t e d several t t m deep m a position-sensitive surface barrier detector. In cases where the evaporation residues are alpha- or s p o n t a n e o u s fission emitters, the deca'~ sequences of m o t h e r - d a u g h t e r and g r a n d d a u g h t e r nuclei can be observed in > 2vr geometry, whereby the positum

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" ' b a c k g r o u n d " counts. This very powerful evolution ,fl conventional alpha- spectrometry has been a ke\ comp o n e n t m the discover~ of elements !07 a n d 109 and has also been used to stud~ several new ~sotopes of know]q h e a ~ elements, as will be dis~-uxsed m anofl~er c o n t r i b u t i o n to this c o n f e r e n c e

5. Conclusion and outlook The previous sections have given a very limited o,mrview of the role alpha- spectrometry has played in the discovery of new elements up to 107 and 109 In 1978 a t t e m p t s were made in D u b n a to synthesize element 108 in the r e a c u o n 226Ra(4~Ca.xn)274-'108 {31]. Only upper limits for a l p h a a n d s p o n t a n e o u s fission decay down to 1 ms half-life could be obtained. The S H I P velocity filter at GSI and the gas-filled spectrometer SASSY in Berkeley could in the future extend the detectable halflife range for element 108 down to the region of microseconds (see Note a d d e d in proof). A discussion of the discovery of new elements wouht be incomplete without m e n t i o n of super heavy elements (SHE). M a n y a t t e m p t s have been made at their discovery in nature and their synthesis in heavy-ion b o m b a r d m e n t s : and an entire conference was dedicated to this subject [32]. At the time of writing (April 1983k a n intense effort is u n d e r way to tW again to synthesize SHEs in the reaction 248cmIaSCa.xn) 2% '116. A collaboration of scienusts from LBL and GSI recenth performed a large n u m b e r of experiments at both

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Fig. 6. Implantation of evaporation residues in a posiuon-sensitire surface barrier detector and alpha decay of the implanted nuclei {from ref. [29]).

Fig. 7. Radioactive decay of the hypotheticalcompound nucleus 294t16 formed in the reaction ,-_4gCm(4XCa,2n). H•If-life predictions and alpha energies, which are shown in parentheses, were taken from ref. [351. Percentage figures a~e branching ratios for alpha decay and electron-capture (EC) decay

,I. M. Nitsehke / 4pplication o[ alpha ~peetrometo" htboratories using S H I P , SASSY a n d chemical m e t h o d s to find an alpha decay signature that would indicate the existence of e l e m e n t s b e y o n d the k n o w n region. A possible decay s e q u e n c e for e l e m e n t 116 f o r m e d in the a b o v e cold fusion reaction with x = 2 is s h o w n in fig. 7. Again, alpha- s p e c t r o m e t r y will play a decisive role in identifying a S H E "'event". Closer inspection of fig. 7 s h o w s thai the e x p e c t e d alpha- energies are not extraordinarily high a n d the o b s e r v a t i o n of alpha particles with say 10 MeV energy and a few s e c o n d s half-life w o u l d by ttsel/ not indicate an unusual event, since it c o u l d belong to a high-spin isomer like 212mpo or a n o t h e r isotope near the target that was f o r m e d in a t r a n s f e r reaction. T h e o b s e r v a t i o n of two correlated alpha- particles and a c o r r e l a t e d s p o n t a n e o u s fission event with high total kinetic energy would, however, be an indication that the decay o f a heavy p a r e n t nucleus was observed. Very p r e l i m i n a r y results from several m o n t h s of e x p e r i m e n t s indicate that such an event was seen neither in the physical nor in the chemical experim e n t s in either laboratory. This work was s u p p o r t e d by the Director, Office of Energy Research, Division of N u c l e a r physics of the Office o f High Energy and N u c l e a r Physics of the US D e p a r t m e n t of Energy u n d e r C o n t r a c t D E - A C 0 3 76SF00098. Note added in proof: The SHIP group at GSI (G. Mt~nzenberg et al.) has just announced the discovery of element 108. I'hree (!) atoms of 265108 were detected which decayed by alpha emission with a half-life of about 3 ms. The alpha energy was 10.4 MeV. In one case four consecutive alpha decays were observed linking the new nuclide 265108 to the known isotope 24~ Fro.

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II1, SPECTROMETRY