In experimental nuclear astrophysics, it is not only necessary to produce reactions that occurred in the formation of the universe and still occur today in stars, but also to detect the products to analyze the results and determine what reactions occurred. At the University of Notre Dame, a new focal plane detector has been built for a Browne-Buechner mass spectrograph. The optimum settings of the detector, including the electronics used to process the signals from the detector, are discussed here. The results from first tests, initially with an alpha source and then with an accelerated beam of ions, are recorded and analyzed in this thesis.

Nuclear astrophysics attempts to explain stellar evolution and the origins of common elements, using concepts and models developed in nuclear physics. Advancements in experimental apparatus and techniques have resulted in a greater understanding of the reaction processes in the universe as well as the earth's place in it, but have also exposed more questions regarding the fundamental principles inherent in the universe. Much research concerning the sites and conditions required for stellar nucleosynthesis has been performed, which has identified crucial reactions and isotopes that cause stellar nucleosynthesis.
Experimental nuclear astrophysicists attempt to study the reactions that occurred during stellar evolution in order to understand the mechanisms involved. As a result of the need to identify products of reactions, and especially due to the relative rarity of some significant isotopes, an experimental method of differentiating nuclei and detecting those of interest had to be found. Accelerator Mass Spectrometry (AMS), a procedure developed in environmental science, is used in that field to isolate certain important nuclei like 14 C. The principles are the same in nuclear astrophysics, and so the technique has been extended and applied to reactions important to nuclear astrophysics. AMS combines a particle accelerator with a mass spectrograph to isolate particular nuclei from intense background, specifically isobaric background. Isobars are nuclei with the same number of nucleons (protons and neutrons), like 40 K and 40 Ca. The powerful techniques utilized by AMS allow very rare nuclei (as few as 1 per 10 14 nuclei) to be isolated and counted.
In the Nuclear Structure Laboratory at the University of Notre Dame, pictured in Figure 1, a Browne-
Buechner spectrograph, built in the 1970s but neglected during the past decade, has been renovated and updated in order to be used in combination with the FN Tandem Van de Graaff accelerator for AMS experiments. The primary focus of this paper is the implementation and testing of a new PPAC proportional counter in the focal plane of the spectrograph, including the support electronics used to process the signal. However, to understand the concepts and importance behind the work, relevant background information on the spectrograph and the accelerator, as well as the detector itself, must be included.
Figure 1: The setup of the Nuclear Structures Laboratory at the University of Notre Dame.

In a tandem Van de Graaff accelerator, as seen in Figure 2 , a negatively charged beam of ions (produced externally from the accelerator) travels in vacuum towards a terminal charged to a very high positive potential, on the order of several million volts. These negative ions are accelerated by the electric field produced by the terminal according to the equation F=ma=qE. As the beam approaches the electrode, it travels through a carbon stripping foil, so called because it "strips" electrons from the ions as it passes through. Therefore, as electrons are removed, the beam becomes positively charged and is accelerated away from the electrode. Because the beam is accelerated twice (towards the terminal when it is negative, and then away from it when it is positive), the accelerator is called a tandem Van de Graaff accelerator. After the beam has passed through the stripping foil, it is composed of many types of particles and nuclei with different charge states due to the interactions and number of collisions undergone with the foil. An analyzing magnet outside of the accelerator selects the desired ions and accelerates them toward an experimental apparatus (the spectrograph) by the equation F = ma = qv x B. The acceleration is perpendicular to the velocity, and is proportional to q/m. As a result, the magnet changes the trajectory of the beam by "bending" it, as shown in Figure 3 .
Figure 2: Tandem Van de Graaff Accelerator [1].
The ions travel down a beam line and enter the scattering chamber of the spectrograph, which houses a target ladder consisting of five targets. The ladder can be moved vertically so that any of the five targets can be exposed to the beam. Currently, two of the spots are gold transmission targets of thickness
200g/cm 2 , one is a 2 mm diameter hole, one is a 5 mm diameter hole, and one spot is empty to allow complete transmission of the beam. After the beam hits the target (or travels through the hole), it continues into the spectrograph, as shown in Figure 4. The spectrograph primarily consists of one large magnet, which bends charged particles into the focal plane as in Figure 3 ; therefore, the spectrograph magnet acts like the analyzing magnet in the previous section, although it uses a much stronger magnetic field.
Detecting the particles in the focal plane of the spectrograph results in the determination of the particles, and therefore the determination of what reactions occurred in the scattering chamber.

Figure 3: Motion of a beam of particles in a magnetic field, where the field is out of the page, showing the separation of ions by charge state.
The detector, built by Steve Kurtz in collaboration with Argonne National Laboratory and pictured in
Figure 5, is a gas-filled parallel-plate avalanche counter (PPAC) detector. The parallel plate refers to the geometry of the anode and cathode, which are planar wires parallel to one another. The cathode is split into two sections, with the anode in between the two. Furthermore, the anode is split into right and left sections.
Voltages are applied to the anode and cathode (350-420 V on the anode and -200 V on the cathode), which create an electric field between the plates. Negatively charged particles, like electrons, are accelerated by the field towards the anode, while positively charged particles are accelerated towards the cathode. The incoming radiation is very energetic, and as it moves through the detector it interacts with the gas contained inside it. Ion pairs are created when the gas molecules ionize by freeing an electron with a large kinetic energy, and the components of this ion pair (free electron and positively charged gas molecule) accelerate in opposite directions due to the electric field. These ion pairs then gain energy and cause more collisions, resulting in the generation of more ion pairs and stronger signals than from one incoming particle.
However, the applied voltage is in the range of true proportionality, which means that the collected charge is proportional to the energy of the incoming radiation [2]. The size of the pulse is proportional to the number of ion pairs created within the detector, and therefore is also proportional to the original number of ion pairs. As a result, information is not lost concerning the original strength of the incident radiation. The entrance to the detector from the spectrograph is an opening in the front plate of the detector. Behind this is a 350 μg/cm 2 thick Mylar window, and then the position, anode, and cathode wire grids. The particles lose energy from interactions with the window and with the gas particles between the entrance to the detector and the wire grids. The loss of energy can be determined through SRIM calculations if the initial energy is known. SRIM, The Stopping and Range of Ions in Matter, is a software program that calculates the loss of energy of an ion through a gas or solid [3] .
In these experiments, the alpha particles (or oxygen beam) travel through both a Mylar window and then isobutane gas, generally with a pressure of 3.000 Torr. As shown in Figure 6, the particles first reach the cathode, a vertical set of wires, followed by the x-position wires, also vertical, then the anode wires, also vertical, then the y-position wires, which are horizontal wires, and finally a second set of vertical cathode wires. Each wire is connected to a common delay line that is used to read out the signals.

Figure 4: Diagram of the spectrograph.
Figure 5: Picture of the detector.

Figure 6: Diagram of the wire grids of the detector.
Figure 7 depicts a signal traveling down the delay line for the x-left position signal, but which applies to any of the wire grids. When a current is induced in a wire due to a moving particle, the signal travels along the wire to a delay line, and then proceeds in both directions away from the wire. The sum of the time it takes for the signal to reach the two ends of the wire is equal to the total delay length of the delay wire for all true signals. For instance, the x-left position signals have a delay line 200 ns long. Any deviation from the set delay time in the wires corresponds to simultaneous events, which are ignored. This can be caused, for instance, by two electrons approaching the wire at nearly the same time (less than the delay time of the wires themselves). However, for single signals, the relative times indicate the position where the signal was generated by the electron. As a result, an accurate reading of the position can be determined by the timing of the signals. The anode signals are used as a start signal by the data acquisition system, while the position signals are used as a stop signal. Therefore, the position signals are additionally delayed by a fixed amount in order to ensure that they reach the data acquisition system after the anode signals. Position signals have a y-component and an x-component, where the x-component can either be x-left or x-right since the detector is split into left and right halves. For each of these three components (y, x-left, and xright), there are two readouts, one at each end of the wire to detect the relative times. Therefore, there are a total of six position readouts on the detector: x-left left (LL), x-left right (LR), x-right left (RL), x-right right (RR), y-up (UP), and y-down (DOWN). The anode is split into two halves, left and right. Because voltage must be applied and signals must also be read out, both anode signals go through a preamplifier.
The preamplifier has three inputs and two outputs: inputs for each of the anode halves and for the voltage, outputs for the two anode signals. A resistor is connected in series from the high voltage input to each of the anode inputs, while a capacitor is connected from each anode input to the corresponding anode output.

Figure 7: As radiation approaches the grid, a current I is induced in both directions of the delay line, away from the wire that was struck by the radiation. As can be seen from the diagram, the values of t1 and t2 determine the position of the incoming radiation.
Since the detector is an avalanche counter, the gas contains many ionized particles after radiation has entered through the window. Over time, as more radiation enters the detector, the concentration of ionized gas particles, which are less reactive with the incoming radiation, will increase. To ensure that the gas remains reactive, it is kept almost entirely neutral by continuously circulating new gas through the detector by a gas handling system . New gas molecules are constantly introduced at the same rate as the reacted molecules are pumped out, thereby keeping the pressure constant. The gas handling system enables control of the pressure within the detector as well as the rate of flow of the gas in and out of the detector. For a more detailed explanation of the gas handling system, as well as the procedure to pump down the detector and fill it with isobutane gas, see Patricia Engel’s report [4].
Before the detector could be used in experiments involving the FN accelerator, the optimum operating conditions and responsiveness of the detector had to be determined offline. The detector and gas handling system were removed from the spectrograph and attached to a roughing pump.

A pulse-generated signal was split into two sections from the output of the pulse generator: one section went straight to an oscilloscope, while the other went to the detector and then to the second channel of the oscilloscope. The purpose of this test was twofold. First, the functionality of the detector was checked by comparing the size and shape of the two signals, which should be identical. Furthermore, the second channel, which travels through the detector, should be delayed from the first signal by the delay length of the detector wires. Initially, the pressure was set to 3.000 Torr, the flow was approximately 255 cm 3 /sec, the anode voltage was 330 V, and the cathode voltage was -211 V. Three different configurations were used: pulse in through LL and read out by LR, pulse in through RL and out through RR, and pulse in through UP and out through DOWN. In the RL-RR and UP-DOWN configurations, the generated pulse, viewed in Channel 1 of the oscilloscope, had an identical size and shape as the signal read from the detector in Channel 2. The only difference was a shift to the right in the channel 2 signal. For RL-RR, the shift was
200 ns, which corresponds to the delay line in the x-right wires in the detector. For UP-DOWN, the total shift was 100 ns, corresponding to the total delay time in the y-position wires. Initial tests of the LL-LR setup had no Channel 2 signal, resulting from a faulty connection on the detector. This connection was repaired, and after this correction, the signals were identical to the RL-RR signals. Therefore, the x-position signals have a total delay of 200 ns, while the y-position signals have a total delay of 100 ns. Engel’s paper shows further details and graphs pertaining to these tests [4].
At this point, signals could be read out from all the position wires, but the optimal settings of the detector parameters had not been determined. Using 241 Am, a closed source placed inside the detector, constant radiation hit the detector and produced signals. 241 Am decays into 241 Np by emitting an α particle ( 4 He nucleus) of energy E
α
= 5.638 MeV [5] .
The output from the anode was read on the oscilloscope. The optimum voltages and pressures, where the signal was sharpest and largest, were then found by varying the voltage and pressure. Tables 1 and 2 show the data obtained by measuring the size of the signals over a range of pressures and voltages. For constant voltage, the strongest signals were for 3.0 Torr and lower, but the noise was greater as the pressure was decreased. Therefore, the optimum pressure was determined to be 3.0 Torr. Then, at 3.0 Torr, the anode and cathode voltages were both set at 200 V. The anode voltage was increased in 20 V increments. At 200 V, there was no signal from the anode. As the voltage was increased, the size of both the signal and the noise increased, until the detector sparked at 392 V on the anode. These voltage measurements suggest that the optimum setting of the anode voltage is the maximum before sparking, which is approximately 420 V for α particles when the voltage is slowly increased.
However, the value of the optimum voltage depends on the incoming radiation; some radiation will leave the region of true proportionality for this voltage. Therefore, the voltage tests should be done for each source of radiation.
Pressure (Torr) Signal Height (mV) Noise (mV)
2.2
2.4
410
510
500
360
2.6
2.8
3.0
3.2
510
510
490
490
340
340
300
300
3.4
3.6
3.8
4.0
460
400
350
310
300
240
150
80
Table 1: Determining optimum pressure for signals with an α source.
Anode voltage is 330 V, cathode voltage is -211 V.

Anode Voltage (V) Signal Height (mV)
200
220
0
40
240
260
280
300
75
125
175
300
320
340
360
380
500
700
1000
1500
Table 2: Determining optimum anode voltage for signals with an α source.
Cathode voltage is 200 V, pressure is 3.00 Torr.
In these tests, an undesirable amount of noise was encountered. To reduce the noise, the detector, preamplifier, and NIM bin, which houses the electronics, were grounded. Also, the roughing pump was placed on rubber padding to reduce the vibrations that it created and imparted to the detector. The vibrations needed to be reduced because they induced waves in the detector that in some cases were larger than the signal from the α source, which made detection of the desired signal difficult. These waves are induced because the detector is made of long thin wires that respond to their surroundings. Small perturbations can result in large output signals. Furthermore, it was observed that the natural light from the room might also induce noise, so the detector required shielding from light. The measures taken to correct these difficulties considerably reduced the noise, by a factor of 2.4. For further information on early steps taken, especially regarding tests with a Mylar window and based on the position of the source, again see
Engel's report [4].
The last procedural step before installing the detector in the spectrograph and using the FN accelerator to produce a beam of particles to test the detector was to set up the electronics through a data acquisition system. The setup is shown in Figure 8. These tests were performed with a 350 μg/cm 2 thick Mylar foil and a 241 Am alpha emitter. As seen in Figure 9, the outputs from the detector are as follows: the six position signals travel by BNC cables to a fast amplifier. The rest of the electronics are connected by
LEMO cables. From the fast amplifier, the signals go through constant fraction discriminators (CFDs), and two output signals are sent to the data acquisition system. One of these signals is read as a scalar signal, which counts the number of events on each position location. The other output is read as a stop signal for the data acquisition system. The two anode signals go from the detector to the preamplifier by BNC cables, and then to a fast amplifier by BNC cables. The rest of the connections are LEMO cables, from the fast amplifier to CFDs, and then to the data acquisition system. The two halves of the anode (left and right) go to a logic unit, connected by an OR gate. This logic unit is the start signal for the time to digital converter
(TDC) on the data acquisition system. If either half of the anode triggers, the signal is accepted and all other events are vetoed until a position (stop) signal is received. The fast amplifier is used to multiply the signal by 200, with less than a 1 ns rise time and less than 20 μV of noise, for a clearer and stronger signal.
The CFDs are used to convert the signal to a rectangular pulse with a more defined front edge, making it easier to be used in conjunction with the data acquisition system. Furthermore, as seen in Figure 10, the
CFD ensures that peaks centered at the same time result in the same timing information in the output regardless of the initial crossing of the threshold. Although information of the size of the peak is lost, more accuracy is gained with respect to timing. Since only the timing is relevant to determine the position of the radiation, no important information is lost by using the CFDs.