Abstract
Early tests are performed on a parallel plate proportional avalanche counter detector intended for accelerator mass spectrometry (AMS) use in a gas-filled Browne-Buechner spectrograph at the University of Notre Dame. A brief introduction to AMS and proportional avalanche counters is given.
The gas handling system (GHS) is described and a general procedure provided. Results are reported for early testing, including: generated pulse signals, voltage and pressure optimization. And reference is given for subsequent testing.
April 6, 2006

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1.1 Nuclear Astrophysics
1.2 Accelerator Mass Spectrometry
1.2.1 Basic Principles
1.3 The Spectrograph
1.4 History of the spectrograph at Notre Dame and Motivation
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2.1 Principles of Detector Function
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4.2.1 Optimal Pressure
4.2.2 Optimal Voltage
4.5
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Experimental set up: Electronics, GHS, and Detector (covered by a dark cloth)
Modified GHS and Detector designed for the addition of a mylar foil

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200,000 years after the Big Bang, the hydrogen, helium, and lithium soup of the universe began to collapse and coalesce into stellar bodies. Gravitational energy converted to thermal energy, allowing for the initiation of nuclear reactions. Four hydrogen nuclei fused to form helium. Helium burned and combined to form
12
C and
16
O. As each source of fuel became depleted, the stars collapsed further, burned hotter, and fused larger and larger elements. When a star has reduced its fuel to an iron core, the star unsustainably collapses, resulting in a supernova. This core collapse supernova ejects nuclei into interstellar space and generates conditions for further elemental synthesis. These new nuclei coalesced or were accreted by other stellar bodies, repeating the process, until the entire set of elements required for life had formed. [1]
Today, 14.5 billion years after the Big Bang, the process continues in our own sun, and ubiquitously throughout the universe. Although theoretically this complex process is thought to be understood, it is challenging to determine the required reaction rates at energies below the coulomb barrier using current laboratory methods. In addition, extrapolations of high energy reaction rates have uncertainties of one or more orders of magnitude. Thus, an experimental technique was sought to explore reaction rates at low energies (on the order of a few MeV, see Figure 1).
[1] One possible technique is accelerator mass spectrometry.
Figure 1. Temperature, density, and energy ranges of stellar nuclear synthesis events. [1]

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Accelerator mass spectrometry (AMS) is an isotope counting technique particularly useful for measuring low isotopic concentrations (10
-12
to 10
-15
) for isotopes with long, or poorly known, half-lives. AMS allows for the identification of isotopes based on their nuclear charge and mass, and is capable of determining the abundance of particular isotopes in the presence of large isobaric backgrounds. It is extensively used in environmental science to measure concentrations of:
26
Al,
36
Cl,
41
Ca,
129
I,
39
Ar,
10
Be, and
14
C, at natural levels. This powerful detection method also has many applications in nuclear astrophysics measuring low cross section reaction rates [1, 2]. In addition, AMS has been used to search for new isotopes, to determine the half-lives of
32
Si and
44
Ti, to study radionuclides produced by interaction of cosmic rays with materials, and to detect solar neutrinos [3].
AMS improves on the basic capabilities of mass spectrometry. In mass spectrometry, a sample is converted into an ion beam, which is then separated by mass in a low energy analyzing magnet (Figure 2). However, a mass spectrometer is not able to differentiate isobarically, ie between two particles with the same mass to charge ratio, for example: between abundant
14
N and rare
14
C. Through a combination of techniques,
L o w - e n e r g y a n a l y s i n g m a g n e t i o n d e t e c t o r
M a s s s p e c t r o m e t r y i o n s o u r c e
T a n d e m a c c e l e r a t o r
L o w - e n e r g y a n a l y s i n g m a g n e t
H i g h - e n e r g y a n a l y s i n g m a g n e t
S t r i p p e r
N e g a t i v e i o n s o u r c e
E l e c t r o s t a t i c a n a l y s e r
A c c e l e r a t o r m a s s s p e c t r o m e t r y
E
 E
Figure 2. Mass spectrometry (upper) and accelerator mass spectrometry (lower). [P. Collon].
C y c l o t r o n
L o w - e n e r g y a n a l y s i n g m a g n e t
D e t e c t o r s e t u p
P o s i t i v e i o n s o u r c e

5 which allow for the determination of change in energy (dE) vs energy (E) and time-offlight, AMS presents the capability for determining the atomic and mass numbers (Z and
A), which in turn identify the isotope.
In AMS, the mass separated beam is accelerated (to increase its energy from keV to MeV, for accelerator details see Signoracci [4]). The accelerated beam is then subject to further sorting by a high energy analyzing magnet and electrostatic analyzers (Figure
2). A spectrograph is one high energy analyzing magnet.
A spectrograph is a vacuum chamber equipped with magnetic field capabilities and a detector. A mixed beam of accelerated ions enters the spectrograph chamber. Under the influence of the magnetic field, the trajectories of the ions curve toward the detector. These trajectories are sensitive to the mass to charge ratio of the ion and thus are used to sort isotopes according to the equation: F = m a = q v x B , as shown in Figure 3.
Figure 3. Spectrograph and beam path under the influence of a magnetic field. [original in [5], modified by P. Collon].

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Unique challenges are presented by heavy ions. Heavy ions enter the spectrograph in a range of charge states. As a result, these ions travel a range of trajectories (as seen in Figure 4, left), and are indistinguishable from isobaric ions which have the same charge state and energy. In order to distinguish between isobaric ions, the spectrograph is filled with a gas. Collisions with the gas result in a mean charge state, which depends on the atomic number (Z) and is proportional to the velocity (v) of the heavy ion. Consequently, the trajectories collapse around the trajectory dictated by the mass to mean charge ratio, which depends mainly on mass number (A) and atomic number. See Figure 4, right. As a result, isobaric isotopes may be identified by their unique A and Z. [6]
Figure 4. Schematic representation of heavy ion trajectories in a vacuum (left) and gas filled (right) magnetic field region. [6]
A typical example used to illustrate the usefulness of such a capability is separation of rare
39
Ar from abundant
39
K (several orders of magnitude more abundant). This is illustrated in Figure 5.
Figure 5. Gas filled Spectrograph separation of 39 Ar and 39 K. [P. Collon]

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A Browne-Buechner spectrograph (pictured in Figure 6) has been used in the
Nuclear Structure Laboratory at Notre Dame since the early 1970s [7]. However, it was abandoned in the early 1990s due to labor intensive detection methods. At the time, the detector was constructed of approximately 5 square feet of photoplates, which required analysis by a personnel member with a microscope. The decision to discontinue use was also influenced by a shift in the interest of the Physics community in the lab.
In 2003, a team lead by Dr. Philippe Collon began restoration of the abandoned spectrograph. They realigned the beam line, upgraded the cooling system, motorized the target chamber, replaced the safety railings, rewired electronics, installed a new vacuum system, fitted the
Figure 6. Browne-Buechner Spectrograph [5]. spectrograph with a gas handling system to allow for a gas filled mode function, and, developed and installed an electronic detector. This paper discusses the development of the new detector.
The detector, built by Steve Kurtz in collaboration with Argonne National
Laboratory, is a multiwire parallel plate proportional avalanche counter (similar to that described by Rehm and Wolfs, [8]). The detector is contained in an aluminum box, roughly two feet long, one foot tall, and four to six inches deep (depending on the configuration). It has five planes of 20 micron thick gold plated tungsten wires, separated by 1.28 mm, with 3g of tension. The first and last planes function as cathodes. Voltage

8 is applied to cathode planes as well as the center plane, which acts as an anode. The second and fourth planes are called the ‘x’ and ‘y’ planes due to their purpose of locating the incident particles. The wires in these two planes are strung vertically parallel (for x) or horizontally parallel (y), and the planes are layered, front to back. See Figure 7. All five planes are electrically isolated from one another.
Figure 7. A layered view of the detector’s five wire planes: (front to back) cathode, x, anode, y, cathode.
[P. Collon]
When a charged particle enters the detector chamber, it stimulates electron emission and ionizes the gas (isobutane) through which it passes. The ionized gas particles are accelerated by the electric field toward the cathode planes, while the electrons are drawn to the anode. The accelerated ions create additional ion pairs
(proportional in number to the energy of the instigating particle) through collisions as they are accelerated by the electric field. The small, high velocity electrons arrive at the anode first, and their arrival marks the occurrence of an ‘event’ and starts two clocks (x and y). See Figure 8.
The clouds of ionized gas pass through the x and y planes as they are accelerated toward the cathodes. This induces a current along localized x and y wires. This current

9 travels approximately instantaneously to the edge of the plane. At the edge of the plane, there is a series of delay chips through which the currents must pass. The time interval between the start signal received from the anode and the delayed signal from the position wire is used to determine the position of the wire on which the signal was inducted.
Therefore, for each incident particle, there are four Δt measurements: right and left (for x) and up and down (for y). These Δt measurements are used to determine the position through which the incident particle passed, by a calibration between the number of delay chips through which a signal must pass from a certain location.
+
- stop
1
2
2
1
2
40 ns 160 ns
2 ns 2 ns 2 ns 2 ns 2 ns 2 ns 2 ns 2 ns p
Figure 8. A side view of the detector’s five wire planes with illustrated path of the incident charged particle and formation of a position signal (top). A schematic of the x wire plane illustrating the basic
relationship between time delay and event position (bottom).

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It is important to note that each clock records only the first signal to reach it. In addition, the delay times are monitored in order to avoid the error of combining the locations of two simultaneous events. The total sum of delays along x and y are respectively 200 ns and 100 ns. Thus, the recorded times of the split signals of individual events must sum to a constant 200 ns (for x signals) or 100 ns (for y). A sum greater or less than this indicates signals from multiple events. Only data from single events is kept for analysis. The vertical wire plane (x) is divided in two, a right and a left half, in order to decrease the necessary delay time (in which other events may occur), thus increasing the amount of recordable data.
The detector described above requires a constant supply and removal of isobutane in order to allow for the formation of new ions by incident particles. This circulation of gas must be regulated so that it does not interfere with event location measurements. The detector is also designed to be connected to the spectrograph, separated by a thin mylar sheet. Though surprisingly strong and flexible for 10 mil (350 ug/cm
2
), this mylar does snap if the pressure gradient between its two sides is too large ( dP max
= 15 torr ). In the case that the mylar window breaks and needs replacement, it is necessary to vent and reevacuate the spectrograph in order to remove the detector from the focal plane. After replacement the whole system must be pumped down again. This is a major effort.
Therefore, a system was built to carefully monitor and control the relative gas pressure and rate of flow.
The gas handling system allows controlled introduction of gas into the system as well as controlled venting. See Figure 9. It includes a flow meter and a pressure transducer for measuring the relative pressure between the detector and a reference point
(either the pump or the spectrograph). Additional capabilities allow for a constant relative pressure to be set with observable of flow rate.
For procedure details, see Appendix.

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<<GHS diagram, cut for email purpose>>
Figure 9. Schematic of the Gas Handling System.
4.1 Initial Tests at Argonne
As the final stage in its construction at Argonne National Laboratory, a simple test was performed on the detector. A mask with a distinct pattern was mounted on the detector (see Figure 10) before incorporation into an established AMS system.
The results (shown in Figure
11) indicate a functioning detector.
Figure 10. Masked detector for initial testing at Argonne
National Lab.

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Figure 11. Results of masked detector test at Argonne National Lab, indicating a functioning detector by the close correlation between mask design and recorded data.

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In order to test the basic functioning of the detector, the first test sought to verify that a generated signal could pass through the system unchanged. During this phase, a generated signal was sent along each pair of wires, RR-RL, U-D, LR-LL (where the right and left outputs of the right half of the x-position plane are noted as RR and RL, similarly for the left half, LR and LL, and the up and down ends of the y-position wires are denoted U and D). See Figure 12. The generated signal was split so that the signal
Figure 12. Right and left division of the detector planes. [P. Collon] entered both the detector and an oscilloscope. The output signal was also sent to the oscilloscope and compared with the generated signals. Figures 13, 14, and 15 show that the signal may be read unchanged on the RR-RL and U-D pairs (except for the time delay corresponding to the length of the wire). However, the disparity between LL and LR indicated a bad connection. This connection was reattached and the disparity disappeared.

Figure 13. Oscilloscope image of generated signal (‘in’) to output signal (‘out’) for
U-D, with 100 ns per major grid line. The indicated full delay line width corresponds to 100ns.
Figure 14. Oscilloscope image of generated signal (‘in’) and output signal (‘out’) for
RL-RR, with 200 ns delay evident (scale =100 ns per major grid line).
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