ADVANCES XN SOLID STATE NUCLEAR TRACK DETECTORS

P. B. PRICE

Deparmrent of Physics Univ~i~ of California. Berkeley, CA 94720, USA

Abstract - Several types of commercially available phosphate glasses with a wide range of sensitivities have. recently been developed. Chief among these is BP-l, which has a high sensitivity and an unpmcedented charge resolution. CR- 39 has been found to respond differently at velocities below -10-2 c. where nuclear stopping begins to dominates than at high velocities, where ekctronic stopping dominates. Atomic force microscopy has been shown to be a powerful new tool for the study of the etching process, using recoil tracks in mica CR-39 has been used to set limits on cold fusion rates in strong conflict with rates claimed by some. elecuochemists. CR-39 and BP-1 have been used to study a number of topics in atomic and nuclear physics including charge pickup in relativistic nucleus-Nell reactions, nuclear and clitic spallation of relativistic heavy nuclei, the determination of electron attachment and stripping cross sections for relativistic heavy nuclei, and cluster radioactivity. CR-39 is being used to search for strangclcts in nature and in ultrarelativistic nucleus- nucleus interactions. Several studies of cosmic ray composition, including TREK, ANTIPODE, HIIS, and UHCRE. are obtaining new results using CR- 39, polycar~na~, and BP-l glass. An experiment to search for weakly interacting massive elementary particles called WIMPS will use mica to detect very short tracks of nuclei that recoil when struck elastically by the WIMPS.

1. INTRODUCTION

My research in the iast few years has greatly benefitted from collaborations with talented Chinese scientists who have worked in my laboratory for periods of one year or more. The first of these was Guo Shi- lun, who has so successfully organized this conference. The others am Ren Guoxiao, Wang Shicheng, Jing Guiru, and He Yudong. all from the Institute of High Energy Physics in Beijing. It is a great pleasure to dedicate this paper to them.

Since the last review with this &let, new pho~ha~ glasses tailored for specific ~p~~~s have been created=, the sensitivity of CR-39 has been increased enough by addition of certain antioxidants to make possible the detection of minimum-ionizing boron4, further evidence has been prusented that natnral mica crystals have recorded tracks of minimum-ionizing singly-charged particles~, the disordered cores of trrg&s in crystals have been imaged with atomic resolution using transmission electron microscopye, and atomic force microscopy has been used to study the motion of steps one unit cell high at a-recoil etchpits in mica’. In addition, detectors made of CR-39 and phosphate glass have led to a number of discoveries, especially in nuclear physics, and several cosmic ray experiments using CR-39, polycarbonate, and BP-1 glass stacks have flown in space*-1 l.

Tabk 1. Minimum detectable values of REL for Solid State NuclearTrack Detectors

Mica ~~,~ MeV/cm BP-l phosphate glass -20,000 MeV/cm Polycarbonate 43000 MeVAm CR-39 -100 MeVEcm _AgCl -30 Mev/cm Photoemulsion -2 Mev/cm __--****________I*L_--~~.~~~~~__________~_~~*~._____*_*___ Pe3G4 in mica -0.01 MeVfcm ??

The range of values of REL over which solid state nuclear track detectors can be used to determine the charge of nuclear particles is greater than for any other type of detector, Table 1 gives examples of approximate

10 P .B . PRICE

minimum detectable values of REL. For some detectors the maximum REL for particle identification is two orders of magnitude greater than the minimum. Below the dashed line is listed the extraordinarily low threshold claimed by Russell5 for Fe304 in natural muscovite mica, which he believes capable of imaging not only minimum-ionizing panicles with charge 1 but also ballistic phonons and other linear defects with even smaller energy deposition. Progress in understanding this system has been slow due to the inability up until now to do calibrations with panicles at accelerators.

2. STUDIES OF DETECTORS

Response of CR-39 in the Nuclear Stopping Regime (6 < 0.01)

Researchers in Japan 12 and in the MACRO collaboration 13 at the Gran Sasso Underground Laboratory in Italy have deployed very large arrays of CR-39 to search for tracks of supermassive magnetic monopoles whose existence is predicted by Grand Unification theories. In all such theories the typical monopole velocity is -10 .3 c. Assuming that the track etch rate for particles losing energy predominantly by nuclear collisions in CR-39 is the same as for panicles losing energy by electronic collisions (taking into account restricted energy loss for electronic collisions), I concluded some time ago 14 that CR-39 with good sensitivity could detect monopoles with velocity down to -3 × 10-5 c.

Fig. 1. Dependence of reduced track etch rate on the funcuon of REL and nuclear dE/dx that fits data for Be and Si ions at [~ < 10 .2 (steep curve) and data for various ions at [3 • 0.25 (ref. 15).

I

4 !

°! 3

t i 0 ~0 I000 1500 2000

R~'~÷0~I~l'x I"~--IC~V

This assumption was based on very skimpy experimental data. Recently Snowden-lfft and Price 15 did detaded studies with beams of low-velocity Be and Si ions that showed, surprisingly, that nuclear stopping is only -20% as effective as electronic stopping at the same value of REL. Figure 1 shows that on the low- velocity side of the Bragg curve (6 < 0.01) the reduced track etch rate, s = VT/VG, is steeper and has a higher threshold than does s on the high-velocity side (J] • 0.1). In Fig. 1 the abscissa gives REL defined as (dE/dx)elec + (dE/dx)nuc. f, where f is determined to have the best-fit value f = 0.2. We concluded that the steep curve with a high threshold is the relevant one for supermassive monopoles with ~ < 0.01, with the con.~quence that CR-39 should not be capable of recording tracks of bare monopoles with ~ <, 0.I c. To further explore the response of CR-39 at low velocities, we are doing further calibrations with beams ranging from deuterons up to Fe.

Development of BP-I Phosphate Glass

Following exploratory work in which several of us discovered the remarkable properties of VG-13 phosphate glass as a detector 2.16, Shicheng Wang and we 3 made a systematic study, the outcome of which was the development of BP-I, the detector with the highest resolution and sensitivity of all glasses. Figure 2 shows the dependence of sensitivity on composition for the most promising group of oxides -- baria, silica, and sodia -- within a phosphate matrix. BP- l has the composition with peak senqitivity -- approximately 65 wt.% P205, 25 wt.% BaO, 5 wt.% Na20. and 5 wt.% SiO2.

One of the most useful features of BP-I is that its sensitivity can be tuned by proper choice of etcham, as shown in Fig. 3. For applications requiring the ability to detect low-energy carbon or relativistic tin, one uses 49% HBF4 solution at -600 C, at the other extreme, when one wants to pass an intense beam of

ADVANCES IN SSNTD 11

relativistic Au ions through BP- l and record products of nuclea~ ~ct ions wifli ~ higher than -90 but not to record the Au ions, one can use a 6.25 N NaOH etchant. Like other glasses, the sensitivity of BP-I is unaffected by the presence or absence of oxygen, so it may be exposed in space without a pressurized container. Furthermore, in contrast to CR-39 or polycarbonate, it shows an almost negligible regisu'ation temperature effecL

2S0

• IS0.

~ 1 ~ .

!-4 m

• o(ll l i ica) • s(sods)

)

l

10

!

Qo

1

. . . . , . . . . , . . . . , . . . . , . . . . • . . . . , . . . . , . . . .

5 lo i s 2o as so 3s 4o 0.1 50

OXIDE BF.mG VARIED (wt %)

/// 100 ~ 0

z~

Fig. 2. Dependence of glass sensitivity on composition. Fig. 3. Dependence of BP-1 sensitivity on e~hant

Study of Very Low-Velocity Ion Tracks in Mica using Atomic Force Microscopy

Price and Salamon 17 established the existence of a new class of fossil tracks, of length 0.25 to 1.6 pro, that result from interactions between alpha particles in the uranium and thorium decay chains and nuclei in the mica, leading to recoiling compound nuclei. Their accelerator calih'adons showed that alpha-interaction tracks result mainly from interactions of 8.79 MeV alphas from 2121)o in the 232Th decay chain with AI and Si. That brought to three the number of known sources of fossil tracks in mica, the others being spontaneous fission and recoiling nuclei in alpha decay. Fission tracks arc typically ~10 ~tm in length, whereas co-recoil nacks arc only ~0.02 ~tm long. Price and Salamon showed that a supermassive magnetic monopole would produce an etchable track of size and shape similar to an alpha-interaction track, provided it captured an aluminum nucleus in the earth's crust before reaching the mica. A monopole track could easily be distinguished from an alpha- interaction track because it would penetrate many km of rock, leaving a linear array of etchpits on mica surfaces. Searches for supermassive monopoles by the fossil track techniquel8.19 have given negative results that provide better limits than any other techmquc.

Mica can also he used to search for fossil tracks of nuclei that recoil elastically when struck by weakly interacting massive panicles (WIMPs), which will be discussed in Section 10. The normal constituents of micas -- Si, O, H, K, AI, Mg, and Fe -- are too light to produce easily detectable radiation damage when recoiling at a typical velocity o f -10 -3 c. Tests by D. Snowden-Ifft in our group have shown that only heavy atoms with Z >. 40 are suitable. In certain lepidolite micas, some of the Li atoms in the structure are replaced by Cs or Rb, leading to heavy element concentrations as high as 10-3. After etching, the recoils show up in phase contrast microscopy as shallow etchpits. The problem is that in old micas the etchpits due to a-recoils from the U and Th decay chains provide a large background against which a few recoiling Cs or Rb atoms must be sought.

To study the etching behavior of low-velocity ion tracks as a function of velocity and charge, Snowden- lift formed a collaboration 7 that included myself and L. Nagahara (University of Tokyo), who is an expert at atomic force microscopy, a quantitative variant of scanning tunneling microscopy. Figures 4 to 7 illustrate the power of this new technique when applied to shallow etchpits in mica. Figure 4 is an image of an a-recoil etchpit showing individual steps that are measured to be exactly 2 nm high. Figure 5 is a profile one pixel wide across part of an a-rocoil etchpit. The arrows indicate the terraces between steps, each of which increases in height by 2 nm. Notice that in Fig. 4 there are two places where steps bunch together into a single high step. We think these bunches may occur where the nuclear stopping, (dFJdx)nuc, is at its maximum, at the point where an a-panicle is emitted. As the recoil slows, (d~dx)nuc decreases to zero at the end of its range, then jumps to a high value again when the next a-particle in the decay chain is emitted. Depending on where the mica is cleaved and which way the recoils navel, as many as eight such bunches could, in principle, be observed.

12 P . B . PRICE

+

Fig. 4. a-recoil etchpit seen by atomic force nucroscopy.

I I 1.500

-130

-60

-40

-20

Fig. 5. Profile across pan of an a-recoil etchpit, showing steps 2 nm high. Scale is in nm.

Fig. 6. 200 keV Ag etchpit showing I nm steps joining pairwise.

Fig. 7. Ag etchpit showing steps retarded at an insoluble particle on the surface.

Figs. 6 and 7 show etchpits of 200 keV Ag ions produced in an ion implantation accelerator. Because of the smaller value of (dE/dx)nec, the reduced track etch rate is also smaller, as a result of which these etchpits are much shallower than are those due to a-recoils. In fact, most of the steps have a height of exactly 1 nm instead of exactly 2 nm. The distance 1 nm is the distance, measured normal to the layer planes, between layers of K atoms, which connect the rather tightly bound Si-O-AI su'ucture with weak van der Waals binding. It is known that the planes of K atoms are planes of easy cleavage. It then occurs to ask the question, "Why are some steps 2 nm high instead of I nm high, which would seem to be the natural unit for dissolution as well as cleavage?" Observations by atomic-force microscopy of the step structure of etchpits due to nuclear recoils provide an important new tool to answer this question and to make possible a general study of the theory of crystal dissolution. In Fig. 6 one can see that the steps are joining each other pairwise. Along certain azimuthal directions the steps are all 1 nm hlgh; m other azimuthal direcuons they have paired and are 2 nm high. This pairing process is a consequence of the competition for fresh etchant, which can attack the mica only at steps on basal-plane terraces, and is predicted in a classic model of diffusion-controlled crystal dlssolutlon 20. To our knowledge this is the first direct example of the occurrence of the process. Finally, in Fig. 7 one can see that steps are being retarded by an insoluble panicle attached to the mica. Eventually the steps will flow around the pm~icle and reconnect.

The technique of atomic-force microscopy should, I believe, assist us in selecting etching and viewing conditions that will make it possible to scan for recoils of heavy impurity atoms in muscovite or lepidolite mica m a background of a-recoils in order to search for WIMPs.

ADVANCES IN SSNTD

3. S T R I ~ ( 3 E N T L I M I T S O N R A T E S OF O D L D F l J s | ( ~ k

13

Although the evidence for production of large thermal energy and excess neutrons from deuterated Pd and 13 is no longer taken seriously, to check on the reports of cold d-d fusion, we searched in 1989 for charged particle emission during the operation of a electrochemical cell21. Since the rate for d + d --* p(3.02 MeV) + t(1.01 MeV) is comparable to that for d + d -.~ 3He(0.82 MeV) + n(2.45 MeV), we designed the cell to permit the protons and tritons to be detected by CR-39 placed outside the cell, in contact with thin (2-~tm) Pd electrodes. Figure 8 shows our results: alpha panicles were detected from both the H20/LiOH cell and the D20/LiOD cell, but no protons or tritons were detected. The rate of alpha particles was the same from the Pd electrode even when the foil was kept separate from the cell. They originated in alpha decay of members of the U and Th decay chain present as trace impurities in the electrode. Our limit on the rate of cold fusion was -106 and 180 times lower than the rates reported by Fleischmann and Pons and by Jones and co-workers.

Fig. 8. Energetic panicles detected by CR-39 next to Pd electrodes in a H20 cell, in a D20 cell, and in a high-pressure D2 cell. The calibration was with alpha particles from a 252Cf source (ref. 21).

' - 10 . . . . . . . . , . . . . . . m • H20

o D 2 0 u.I alphas o • o • x D2 I"- . 0 ~1:I,

~_ .t..Oo • ~ b o

Irllolls " ~ P'~L "-~

protons

I

w

Q w n~

.1 10

MEAN RANGE (microns) 100

One suggested explanation of the apparent observation of cold fusion was that the lattice of a metal electrode might become embrittled enough by deuterafion to undergo microfracnmng and that deuterons might be accelerated in the transient electric fields across the cracks. In fact, Klyuev et al. 22 had claimed that neutrons were being produced during brittle fracture of dielectric crystals. I tested the idea of dynamic production of d-d fusion by cleaving a 15-cm 3 crystal of LiD into -100 thin plates while the bottom of the crystal was supported 0.9 nun in air above the surface of a CR-39 detector. The ranges of 3.03 MeV protons and 1.01 MeV tritons from d+ d fusion are much greater than 0.9 mm. My result 23, that no charged particles were emitted during cleavage, ruled out the conjecture that fusion can occur in deuterated metal electrodes during crack propagation.

4. CHARGE PICKUP IN RELATIVISTIC NUCLEUS-NUCLEUS REACTIONS

Relativistic nucleus-nucleus reactions in which the nuclear charge of the projectile changes by one unit with small momentum-u'ansfer are thought to take place by exchange of a charged pion between target and projectile or by excitation of a A-resonance. Charge pickup, in which the projectile charge increases by one unit in the collision, had until recently been studied only m light projectiles, for which the cross section is very small. To establish that a projectile nucleus has undergone a charge-pickup reaction is straightforward and reliable if one uses CR-39 or BP-I, which have extremely high charge resolution, as shown in Fig. 9.

Using stacks of CR-39 sheets downstream from several types of targets, Ren Guoxiao et al.24 found that the charge pickup cross section increases surprisingly rapidly with the projectile charge, obeying the empirical relation o÷I *, 1.7 × 10 .4 Ap 2 (Apl/3 + ATI/3 -I) to within a factor of two, as shown in Fig. I0. Notice that the cross section for charge pickup by uranium fails below the correlation line by at least an order of magnitude, as established by Wcstphal et al. 2s. They gave the plausible explanation for the low cross section that charge pickup by a uranium nucleus leads to an excited neptunium nucleus, which almost always fissions in flight rather than de-excites by neutron evaporation. To date, no good explanation of the very steep dependence of charge pickup cross section on Ap exists.

Recently He and Price 26 have studied charge pickup by 14.5 A GeV 28Si and by 11.4 A GeV 197Au at

14 P.B. PRICE

1 0 '

I I

I

0 I

l 01 I

46 48 50 52 54 58 58

mean charge of fragment (10 surfaces)

. . . . . . . . i

100 1000

PROJECTILE MASS (ainu)

Fig. 9. Fragmentation of 1.26 A GeV La beam in CR-39. Fig. 10. Cross sections for one-charge pickup Small peak at Z = 58 is due to charge pickup, and loss at -1 A GeV (ref. 25).

the Brookhaven AGS accelerator. They found that the cross sections were almost the same as at energies of 1 A GeV. In the case of Au, using BP-1 detectors, they studied charge pickup in seven targets ranging from H up to It) and found a target dependence of the cross section -AT 0.38.

5. FRAGMENTATION OF RELATIVISTIC HEAVY NUCLEI

Using CR-39 stacks interleaved with various targets, our group 27-32 and W. Heinrich's group 33-36 in Siegen have studied projectile fragmenlation at the Bevalac (0.7 to 2 A GeV), at the AGS (-15 A GeV), and at CERN (200 A GeV). With fully automated scanning and measuring systems, the two groups have been able to achieve extraordinarily high charge resolution, high spatial resolution, and high statistics. Some highlights of the studies are:

• The two groups set stringent upper limits on the cross sections for production of fractionally charged fragments. For any combination of beam and target, the upper limit on the fraction of fragmmts with fractional charge was always less than 10 "4. See refs. 27,28,31,36.

• The Berkeley group found that the transverse momentum distributions of the projectile fragments are approximately energy-independent, from 1 m 200 A GeV. See refs. 27 and 29.

• Using a series of thick targets, Price and He 29.30 produced projectile fragments at 14.5 A OeV at Brookhaven and ueated them as secondary beams whose interaction cross sections in subsequent targets they then studied. They found that the charge-loss cross sections for the secondary beams are higher by 10% to 30% than expected for primary beams. To explain the enhanced cross sections they suggested that some of the secondary particles were off-stability isotopes that are more resistant to nuclear and elecuomagnefic interactions than are stable primary beams.

• Measurements of cross sections as a function of energy and target mass enabled the two groups to disentangle the contributions of nuclear and electromagnetic spallation. In the latter process the projectile sees the rapidly changing electromagnetic field of the approaching target nucleus as a distribution of virtual photons which interact with the projectile mainly by the giant dipole resonance and by mesonic resonance, dcponding on the energy of the virtual photon. For projectiles up to Fe, the electromagnetic spallation cross section was found to increase approximately as ZT !.8, in accord with theory. In the heaviest target, Pb, the elccwomaguetic contribution was much higher than the nuclear contribution. See rcfs. 28,29,32-35. The Siegen group has done an especially complete study of electromagnetic spallation.

Figure I 1 shows an example of some results of an ongoing experiment to measure the partial cross sections for fragmentation of 11.4 A OeV Au in seven different targets 32. The charge resolution achieved with the BP-1 detector stack was --0.1 charge unit. Electromagnetic spaliafion, which is much more important fora high-Z than a low-Z target, causes the partial cross sections for small AZ to be much larger in a Pb target than in a CH2 target. One intriguing result of this experiment is that the electmmagucfic spallafion cross section for the Au projectile increases with target mass only as -AT instead of as -AT 1"8.

ADVANCES IN SSNTD 15

Fig. 11. Partial cross sections for charge loss in interactions of 11.4 A GeV Au with CH2 and Pb targets (ref. 32).

g

I

l i ! r r , ~ !~ L

m

+ +

k ~ &

h t

I

~ F l I

11.4 A C~V

An Projectile

0 . 9 L O 3.0

tt I lT f i i ? , ',,,,

$.0 8.0 ."O.O

6. P~L~CTRONIC CHARGE STAI~_~

Figures 12 illuslrates the exlramdinarily high charge resolution of BP- I. A beam of 900 A MeV U ions has emerged from a sheet of BP-1 glass and has impinged on a second sheet. The three peaks corres~nd to the charge states +90, +91, and +92 and are resolved with a single etehpit with typical sampling distance of -20 ~ The relative areas in the peaks give the steady-state ionic charge-state distribution. Figure 13 compares data at 600 and 900 A MeV, obtained in our group 37, with calculated curves supplied to us by W. Meyerhof. One sees that there are some differences between the theory and the data. We have recently extended the ~ to include measurement of the cleclronic capture and loss cross section of 11.4 A GeV Au ions, which are almost always in charge state +79. Such data are of importance in the design of colliders in which circulating beams of heavy ions have to survive in non-perfect vacuum for many hours.

+ I L 14 15 16 17 18

msno¢ am (pro)

Fig. 12. Distribution of chm'ge states ofg00 A MeV U ions in BP- 1 etched in 6.25 N NaOH.

I

os g l

o6

Z - 0 4

~ 0 . 2

• , , , o !

~ o Y (A MeV)

Fig. 13. Measured and theoretical fractions of U in charge states +92, +91, and +90 in BP-I.

7. CLUSTER RADIOACTIVITY

Cluster radioactivity results from penetration of the Coulomb barrier around a heavy nucleus by a cluster heavier than an alpha panicle. Although the fh-st example, 223Ra -.~ 14C + 209Pb, was discovered by means of a AP.~ semiconductor detector system, the topic could only have grown to maturity by the exploitation of the unique properties of solid slate nuclear track detectors. The Immching ratio relative to alpha decay for emission of 14C by 223Ra is --6 × 10.10. Most of the subsequent decay modes have far smaller branching ratios, some even as low as 10-16. Table 2 gives the number of decay modes in which particular

16 P . B . PRICE

light and heavy clusters have been seen. The barrier is most easily penetrated when the Q value is highest, which occurs when the light daughter is neutron-rich and the heavy daughter is close to the doubly magic nuclide 208pb. All but two of the decay modes have been discovered by our group at Berkeley, by the Dubna group (led by S. P. Tretyakova), and by the Milano group (led by R. Bonetti). The work up to 1989 is reviewed in ref. 38; the Russian work is summarized in ref. 39; and all work up to 1991 is summarized in ref. 40.

Table 2. Number of Radioactive Decay Modes Observed to Lead to Clusters

light c lus~ no. of c a s e s h e a v y cluster no. of cases

12C 1 211Bi 1 14C 5 208pb 9 200 1 209pb 2 23F 1 210pb 2

24Ne 5 212pb 3 25Ne 1 206Hg 3 26Ne 1 20~TI I 28Mg 3 102Sn 1 30Mg 1 32Si 1 34Si 1

The typical energy of the emitted cluster is 2 to 2.5 McV/nucleon, and the range is comparable to the range of the alpha particles comprising the enormous background that must be discriminated against. The Berkeley and Milano groups use one of the phosphate glasses, BP-1, PSK-50, and LG-750, depending on the charge of the cluster being sought. By a judicious choice of etchant and etching time, three independent messurements of the reduced track etch rate VT/V O can be made. They utilize the semi-minor axis of the etchpit mouth, the average cone angle, and the time to reach end of range. In some cases several modes of decay can occur in the same nuclide. Fur example, 234U is observed to decay by alpha particle emission, spontaneous fission, 24Ne emission, and 28Mg emission. From Fig. 14, which shows measurements 51 of VT/VO as a function of range for 234U, one sees that the Ixanching ratios for emission of Nc and MS ions are comparable. Figure 15 compares measured and calculated halflife for the 25 cases studied to date. For clarity, calculations of only two of the many models are shown. The symbols labeled × give predictions of the superasymnmric fission model of Pnenaru et al.41, and open circles give predictions of the cluster model of Blendowske et al.42 The former is a macroscopic model, which can be adjusted to fit alpha decay and the special case of cold spontaneous fission; the latter is a microscopic model, which takes into account the probability of finding the appropriate number of neutrons and protons in an incipient cluster inside the parent.

I - -

Na Mg "~. ~|

y:.\" " F

• i . , . , . | , I , I . I - ' - ' - ' -

2 , s e ~o ~= ~, ~s ~s 20 22

R a n g e (JJm)

'I - 1o msm,

s . . . . . . . . . . . . . . . . , , , , , : ,, , • • : ; , ~

' ~ " c " ° o " F N , MO !1

Fig. 14. Distribution of VT/V G and range for Ne and MS clusters emitted by 2-UU (ref. 51).

Fig. 15. Comparison ofmeuurodandcalcula~ halflives for cluster radio,wtivity.

In the case of even-even parents, most of the models agree with measured halflives to within better than an order of magnitude, and in some cases better than a factor three. The challenge comes in trying to predict

ADVANCES IN SSNTD 17

the halflives of odd-A parents, some of which decay at a far smaller rate than predicted with even-even systematics. The explanation favored by recent data is that decays of odd-A parents are strongly inhibited if there is poor overlap of the singie-particle wave function of the parent and of the ground state of the heavy daughter. One of the most challenging and interesting cases is 231pa, which the Milano group and I recently studied43. The ~ s ~ c t ~ h o t h ~ ~ ~ F s ~ h e ~ with Ne being favored over F by one or two orders of magnitude. We observed only one F out of 1347 Ne ions, making 231pa the first example of a nuclide that emits an edd-Z cluster.

8. ULTRAHEAVY COSMIC RAYS

One of the most important goals in cosmic ray astrophysics is to det~,,~ne the charge composition in the region 50 ~ Z ~ 96, for which previous measurements have yielded very tittle information, both because of the very low fluxes and the difficulty of resolving adjacent elements. Electronic detectors on satelfites have been able to see even-Z peaks up to about charge 60 but were not able to say anything about abundances of odd-Z elements~, 45. At higher charges the resolution was not sufficient to resolve with certainty the major peak at platinum from the peak at lead. Only an upper limit has been set on the abundance of the actinide elements (2 >83).

By far the largest body of d~_m on ullraheavy cosmic rays has been collected in the UHCI~ expaimmt (but still remains to be processed) by researchers at the Dublin Institute of Advanced $tudins 9, who exposed CR-39 and polycarbonate stacks on NASA's Long-Duration Exposure Facility (LDEF), which was recovaed from space after a 5.8-year exposure. Based on an analysis of a small number of stacks containing -100 events, they believe they have a total of nearly 3000 events with Z > 65. In the small sample, they have found two events with Z > 90, which leads them to conclude that they should ultimately see several tens of actinides.

Table 3 compares the numbers of events seen on HEAO-3 (ref. 44) and Ariel-6 (ref. 45) with the numbers of events expected in an experiment 10 called TREK that is now flying on the Russian space station Mir. The expected number of actinides is least certain; the estimate of 10 to 30 is based on the HEAO-3 and Ariel-6 results and brackets the UHCRE result based on 2 events. The last column gives the detection efficiency for TREK when account is taken of the distribution of zenith angles and energies and the requlnms~t that the component of track etch rate normal to the surface be greater than the general etch rate.

Table 3. Number of Events Seen or Expected on Various Space Experiments

Z ~ On ~¢n o n Ar~-6

expected for TREK; 52°:, fraction of full L 2 m 2 ~ anmw,_~z.~,~

51-52 50 64 56 0.13 53-54 34 47 37 0.15 55-56 54 61 80 0.17 57-58 34 39 29 0.19 59-60 14 21 26 0.21 62-69 48 62 194 0.44 70-73 15 21 121 1 74-80 36 40 344 1 81-83 8 15 102 1 >83 17 3? 10to30 1

TREK is a collaborative experiment between our group and the Russian Space Research Ia~tute. The main goal is to measure the composition of both odd-Z and evon-Z ultraheavy cosmic rays with much higher charge resolution than was stlaisw, d on HEAO-3 and Ariel-6 and that can be ~tt~ine~4 with the LDRF plastic slacks. Panels holding an array of BP-I glass 1.2 m2in area and 16 plates thick are mounted outside the Kvant-2 module on Mir. Each of the 2400 glass plates measures 9 cm x 9 cra x 0.15 cm. Heaters and relays regulate the temperature of the glass at 25o :t: 5o C. Table 3 gives the number of events expected, if all the ~ i s are r e c o v e ~ The p ~ it m remm o n e - ~ ~ ~ p ~ s ~ an u I ~ shock-sbsor~ I cspsule after 2.5 years and to retorn the remaining plates from the Mir station at a later dme.

18 P.B. PRICE

9. COSMIC RAYS IN THE IRON GROUP (24 < Z ~; 30)

A secondary goal of TREK is to measure the isotopic composition of Fe-group nuclei. The present state of knowledge of isotopic composition is still very limited for elements in the Fe group, due both to poor statistics and to poor mass resolution. To attack this problem, which is of considerable astrophysical importance, an array of BP-I glass 0.09 m 2 in area and 32 plates thick has been attached to the inside wall of the Soyuz spacecraft (attached to the Mir station). Each of the 1152 small plates measures 5 cm × 5 crn × 0.07 cm. One-third of the plates were returned to earth after a six-month exposure; one-third have recendy been returned after a one-year exposure; and the remainder will be rctumod in spring, 1993. From a quick look at the density of tracks of Fe-group nuclei, we expect to collect a total of ~I04 Fe and at least 300 Ni nuclei.

An earlier, balloon-borne experiment called ANTIPODE, discussed in the Ph.D. thesis of A. J. Westphal ~ , has given results that encourage us to believe that the interior detectors on TREK will provide good enough statistics and mass resolution to answer several questions associated with the astrophysics of the Fe-group isotopes in the cosmic rays. Westphal used BP-1 glass plates the same size as those exposed inside the Mir station, but his stacks were carried on a circumpolar balloon launched in Antarctica and kept aloft for nine days 46. To date, Westphal has analyzed only 18% of his stacks and has studied -100 Fe ions and ~10 Ni ions. He finds that the abundance of the Fe isotopes is roughly consistent with the values for tron in the sun but that the ratio of 6~Ni to all Ni is about twice as large as the solar value. Due to his limited statistics, he can say only that the 60Ni abundance is inconsistent with the solar value at the 90% confidence level.

Adams and his colleagues at the Naval Research Laboratory8 are obtaining interesting results in their analysis, still at an early stage, of the composition and en~gy specu'um of the Fe-group cosmic rays using CR- 39 stacks exposed on LDEF for 5.8 years. Their experiment is called Heavy Ions in Space (HHS). During this exposure, in October, 1989, there was one spectacular solar flare, the largest since the giant one in February, 1956. They find that this flare resulted in a large enhancement of the flux of Fe-group nuclei at energies up to at least -600 A MeV. One mystery is to explain how particles at these energies could poneume the earth's field at the LDEF orbit of 28 o. Their tentative explanation is to conjecture that the particles are only partially ionized and thus have a higher magnetic rigidity than do fully slripped ions at the same velocity. This could occur ff they were accelerated in a partially ionized state in the giant solar flare at -8 solar radii where the matter density is too low to change the ionization state. At the highest energy they have studied, -950 A MeV, the solar-flare particles are much less abundant than the galactic cosmic rays.

At energies below -200 A MeV, Adams et al. have found what may be a thtrd component of Fe-group nuclei, characterized by a high abundance ratio of sub-Fe to Fe (sub-Fe is defined as the nuclei with 21 < Z ~; 25), which may have resulted from spallation of Fe in space. Their observations support the claims of the group at the Tam Institute of Fundamental Research 11, who have analyzed CR-39 stacks in an experiment called Anuradha, which was exposed on Spacelab. The origin of this component of the cosmic rays is not yet known.

10. SEARCH FOR WEAKLY INTERACTING MASSIVE PARTICLES (WIMPs)

Morn than 90% of all the matter in the universe is m an unknown form, exerting gravitational effects but not emimng hght It is becoming increasingly likely that this matter is in the form of non-beryonic eleruentary particles called WIMPs, which interact so weakly that they can penetrate deeply into the earth, only occasionally colhdmg elastically with nuclei in rocks. Depending on the W1MP velocity (expected to be -4).001 to 0.002 c) and mass per panicle (which is unknown but may be comparable to the masses of some atoms), WIMP recoils might be detectable in mica despite a background of or-recoils. In his Ph.D. thesis, Snowden- Ifft 47 proposed two methods of searching for WIMP recoils. In the first method, one looks for etchpits due to fossil WIMP-recoil tracks in an old mica from a mine at least 100 m deep (to screen out recoils due to cosmic- ray muons). An electron probe is used to select a mica with a high concentration of atoms with Z • 45, and the track-retenuon age of the mica is determined by fisston-track dating. For an age of at least 5 × 108 yr and a heavy element fraction of -10 "3, a 1 cm 2 area would be sufficient to provide a useful limit on the flux of WIMPs. To make the method quantitative, one has to establish with accelerator calibrations the recoil euefllY interval over which a WIMP recoil (for example, a Cs atom) could be detectable in a background of or-recoils. Although scanmng a large area with an atomic force microscope is probably impractical, one might use this technique to examine in detail candidates for WIMP recoils that were found by scanning in phase contrast microscopy.

ADVANCES IN SSNTD 19

In his second mothod48 one looks for WIMP-recoils after a one.year exposure in an underground location of a sandwich consisting of a foil of a target such as AS held between two sheets of muscovite mica that was previously annealed to erase fossil tracks. The sandwich is continuously oriented by a motorized drive so that its plane faces normal to the direction of motion of the earth through the Galaxy. Because WIMPs have a thermal speed distribution with mean value comparable to the speed of the sun through the Galaxy, far more WIMPs would have head-on collisions with the sandwich than would catch up with it from the rear. Thus, an unmistakable signal pointing to the existence of WIMPs would be an asymmetry in the number of recoils in the two mica sheets: at least one order of magnitude more recoil tracks should be found in the walling mica than in the leading mica. The advantage of this method over the in,st method would be the positive signal in the form of a large backward/forward ratio; the disadvantage would be the necessity of scanning several m 2 of mica in order to achieve a collecting power comparable to that achievable in the fossil WIMP-recoil search.

11. FUTURE DIRECTIONS

A fascinating aspect of research with nuclear tracks in solids is that there are still so many possibilities to pursue. We are limited only by our imagination. In terms of improvements in techniques, it helps to have access to automated systems for scanning and measurements, which are now commercially available, and to the new methods of observation such as atomic force microscopy and confocal microscopy. Within the family of phosphate glasses, there are now commercially available glasses that range in sensitivity from the very insensitive LG-750 (made by Schott), which has been used 49 to detect energetic Mg and Si clusters in a background of -1014 alphas cm "2, to the very sensitive BP-1, which can detect 14C at an energy up to -1 MeV/nucleon. There seems to be no inherent barrier preventing the future development of a glass detector capable of detecting alpha particles. Such a detector could he used in vacuum and in sunlight and would have better surface quality than CR-39.

A number of interesting problems require detectors capable of recording particles at very low velocities. In order to use CR-39 to search for supermassive monopoles and hypothetical panicles called strangelets 50 (Jumps of strange-quark-matter, possibly left over from the early universe) traveling with velocity -10-3 c, we need to do further accelerator calibrations that will map out the response of CR-39 to nuclear particles at such velocity, where nuclear stopping dominates over electronic stopping. The same can be said about mica. In addinon to more accelerator calibrations, it would be very useful to develop an etchant with greater sensitivity. The full range of a WIMP-recoil is likely to be greater than the range of an a-recoil, even with all members of the radioactive decay chain taken into account, but this advantage is now lost because most of the length of the WIMP-recoil is etched away due to its small VT/VO. With a more sensitive etchant, deeper WIMP-recoil etchpits could be produced, and they could more easily be recogmzed in a background of a-recoils.

Some previously interesting topics, such as cold fusion, are now dead issues. Other topics, still timely, will continue to be explored using solid state nuclear track detectors. These include, but are not limited to, cluster radioactivity, nucleus-nucleus interactions at ever increasing energies, searches for hypothetical particles such as monopoles and strangelets, and the isotopic and elemental composition of very heavy cosmic rays.

ACKNOWLEDGMENTS

In addition to the Chinese scientists who have contributed so much, I would like to thank my students and former students Yudong He, Douglas Lowder, Dan Snowden-Ifft, Andrew Weslphal, and Win Williams for their contributions to my recent research using solid state nuclear track detectors. I have also benefitted from ongoing collaborations with K. J. Moody, R. Bonetti, C. Migliorino, A. Gugfielmelti, C. Chiesa, and R. Mathenud. The U. S. Department of Energy, the National Science Foundation, and NASA have supported our work.

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